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Chapter 6· Java Foundations

Conditions in Java

44 min read16 diagrams

Java Master Course — Chapter 6 of 50

Conditions allow a Java program to make decisions. Instead of always executing the same code, a program can check a condition and choose what to do.

1. What Are Conditions?#

A program often needs to make decisions.

For example:

Output
If marks are 40 or more
    → Student passes

Otherwise
    → Student fails

Or:

Output
If age >= 18
    → Eligible to vote

Otherwise
    → Not eligible

This is called decision making or conditional execution.

The basic idea is:

Diagram
             Condition
                 │
          ┌──────┴──────┐
          │             │
        true          false
          │             │
          ▼             ▼
       Action 1      Action 2

Java provides several ways to make decisions:

Output
if
if-else
else-if
nested if
switch
switch expressions
ternary operator

You already learned the ternary operator in Chapter 4. Here we will understand conditional programming properly.

2. Boolean Values#

Conditions are based on boolean values.

A boolean can have only two values:

Java
true
false

Example:

Java
boolean isJavaEasy = true;
boolean isRainy = false;

A condition usually produces a boolean result.

For example:

Java
10 > 5

produces:

Output
true

And:

Java
10 < 5

produces:

Output
false

3. Relational Operators#

The most common operators used to create conditions are:

Operator Meaning
> greater than
< less than
>= greater than or equal to
<= less than or equal to
== equal to
!= not equal to

Examples:

Java
10 > 5
10 < 5
10 >= 10
10 <= 20
10 == 10
10 != 20

Their results are:

Output
true
false
true
true
true
true

4. The if Statement#

The simplest decision-making statement is:

Java
if

Syntax:

Java
if (condition) {
    // code
}

Example:

Java
public class Main {
    public static void main(String[] args) {
        int age = 20;

        if (age >= 18) {
            System.out.println("You are an adult.");
        }
    }
}

Output:

Output
You are an adult.

The code inside the if block executes only when the condition is true.

5. How if Works#

Consider:

Java
int age = 20;

if (age >= 18) {
    System.out.println("Adult");
}

Java evaluates:

Diagram
age >= 18
20 >= 18
   ↓
 true

Because the result is true, Java executes:

Java
System.out.println("Adult");

Flow:

Start
  ↓
age = 20
  ↓
age >= 18 ?
  ↓
 true
  ↓
Print "Adult"
  ↓
End

6. When the Condition Is False#

Example:

Java
int age = 15;

if (age >= 18) {
    System.out.println("Adult");
}

Condition:

15 >= 18
    ↓
  false

Therefore the body is skipped.

Output:

Output

There is no output.

This is an important property of if:

Output
true  → execute block
false → skip block

7. Basic if Example#

Java
public class Main {
    public static void main(String[] args) {
        int marks = 80;

        if (marks >= 40) {
            System.out.println("Pass");
        }
    }
}

Output:

Output
Pass

If:

Java
int marks = 30;

there is no output because the condition is false.

8. if with Input#

Conditions become much more useful when combined with user input.

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter your age: ");
        int age = sc.nextInt();

        if (age >= 18) {
            System.out.println("You are eligible.");
        }

        sc.close();
    }
}

Input:

Output
20

Output:

Output
You are eligible.

Input:

Output
15

Output:

Output

9. if-else#

Usually, we want one action when a condition is true and another action when it is false.

For this we use:

Java
if-else

Syntax:

Java
if (condition) {
    // true case
} else {
    // false case
}

Example:

Java
int age = 15;

if (age >= 18) {
    System.out.println("Adult");
} else {
    System.out.println("Minor");
}

Output:

Output
Minor

10. How if-else Works#

Diagram
              Condition
                  │
          ┌───────┴───────┐
          │               │
        true            false
          │               │
          ▼               ▼
      if block        else block
          │               │
          └───────┬───────┘
                  ↓
                Continue

Exactly one of the two blocks executes.

11. Example — Even or Odd#

Java
int number = 10;

if (number % 2 == 0) {
    System.out.println("Even");
} else {
    System.out.println("Odd");
}

Output:

Output
Even

Why?

10 % 2
   ↓
0

Therefore:

Java
number % 2 == 0

is true.

12. Even/Odd with User Input#

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter a number: ");
        int number = sc.nextInt();

        if (number % 2 == 0) {
            System.out.println("Even number");
        } else {
            System.out.println("Odd number");
        }

        sc.close();
    }
}

Input:

Output
25

Output:

Output
Odd number

13. Positive, Negative, or Zero#

There are three possible cases:

Output
positive
negative
zero

We need more than two branches.

Example:

Java
int number = -10;

if (number > 0) {
    System.out.println("Positive");
} else if (number < 0) {
    System.out.println("Negative");
} else {
    System.out.println("Zero");
}

Output:

Output
Negative

14. else-if#

The general structure is:

Java
if (condition1) {
    // case 1
} else if (condition2) {
    // case 2
} else if (condition3) {
    // case 3
} else {
    // default case
}

Java checks conditions from top to bottom.

The first true condition wins.

15. Example — Marks#

Suppose:

Output
90–100 → A
80–89  → B
70–79  → C
60–69  → D
below 60 → F

Program:

Java
int marks = 85;

if (marks >= 90) {
    System.out.println("A");
} else if (marks >= 80) {
    System.out.println("B");
} else if (marks >= 70) {
    System.out.println("C");
} else if (marks >= 60) {
    System.out.println("D");
} else {
    System.out.println("F");
}

Output:

Output
B

16. Why Order Matters#

Consider this:

Java
int marks = 95;

if (marks >= 40) {
    System.out.println("Pass");
} else if (marks >= 90) {
    System.out.println("A");
}

Output:

Output
Pass

Why didn't it print A?

Because Java checks:

Diagram
marks >= 40
95 >= 40
   ↓
 true

The first condition is already true.

Java executes that block and skips the remaining else-if conditions.

Therefore, more specific conditions often need to come before broader conditions.

Correct:

Java
if (marks >= 90) {
    System.out.println("A");
} else if (marks >= 40) {
    System.out.println("Pass");
}

17. Important Rule: First Matching Branch#

In an if-else-if chain:

Java
if (...)
else if (...)
else if (...)
else

Java evaluates from top to bottom.

condition 1
    ↓
true? ── yes → execute and stop
    │
    no
    ↓
condition 2
    ↓
true? ── yes → execute and stop
    │
    no
    ↓
condition 3

Only one branch from the chain executes.

18. Multiple Independent if Statements#

Compare:

Java
if (condition1) {
}
if (condition2) {
}

with:

Java
if (condition1) {
} else if (condition2) {
}

They are not the same.

Example:

Java
int number = 10;

if (number > 0) {
    System.out.println("Positive");
}

if (number % 2 == 0) {
    System.out.println("Even");
}

Output:

Output
Positive
Even

Both conditions are independently checked.

19. if-else-if Is Different#

Java
int number = 10;

if (number > 0) {
    System.out.println("Positive");
} else if (number % 2 == 0) {
    System.out.println("Even");
}

Output:

Output
Positive

The second condition is not checked because the first one was true.

Remember:

Output
multiple if → multiple blocks can execute

if-else-if → at most one branch executes

20. Nested if#

An if statement can exist inside another if.

This is called a:

Output
nested if

Example:

Java
int age = 20;
boolean hasId = true;

if (age >= 18) {
    if (hasId) {
        System.out.println("Entry allowed");
    }
}

Output:

Output
Entry allowed

The inner condition is checked only if the outer condition is true.

21. Nested if Flow#

Diagram
        age >= 18?
          │
     ┌────┴────┐
    no        yes
    │           │
   stop      hasId?
                │
           ┌────┴────┐
          no        yes
          │           │
         stop     Entry allowed

Nested if is useful when one decision depends on another.

22. Nested if Example — Login#

Java
String username = "admin";
String password = "1234";

if (username.equals("admin")) {
    if (password.equals("1234")) {
        System.out.println("Login successful");
    }
}

Output:

Output
Login successful

This is only a learning example. Real applications should not store passwords like this.

23. Adding else to Nested if#

Java
int age = 20;
boolean hasId = false;

if (age >= 18) {
    if (hasId) {
        System.out.println("Entry allowed");
    } else {
        System.out.println("ID required");
    }
} else {
    System.out.println("Underage");
}

Output:

Output
ID required

24. Logical Operators in Conditions#

You learned these in Chapter 4:

Java
&&
||
!

They become extremely useful with if.

25. && — AND#

Both conditions must be true.

Example:

Java
int age = 20;
boolean hasId = true;

if (age >= 18 && hasId) {
    System.out.println("Allowed");
}

Both:

Output
age >= 18 → true
hasId     → true

Therefore:

true && true
     ↓
    true

Output:

Output
Allowed

26. Example — Login#

Java
String username = "admin";
String password = "1234";

if (username.equals("admin") && password.equals("1234")) {
    System.out.println("Login successful");
} else {
    System.out.println("Invalid credentials");
}

The user must satisfy both conditions.

27. || — OR#

At least one condition must be true.

Example:

Java
int age = 65;

if (age < 18 || age >= 60) {
    System.out.println("Special category");
}

Here:

Diagram
age < 18  → false
age >= 60 → true

false || true
     ↓
   true

Output:

Output
Special category

28. ! — NOT#

! reverses a boolean value.

Java
boolean loggedIn = false;

if (!loggedIn) {
    System.out.println("Please log in");
}

Since:

Output
loggedIn = false
!loggedIn = true

Output:

Output
Please log in

29. Combining Conditions#

Example:

Java
int age = 25;
boolean citizen = true;

if (age >= 18 && citizen) {
    System.out.println("Eligible");
}

Another example:

Java
int marks = 85;

if (marks >= 40 && marks <= 100) {
    System.out.println("Valid passing marks");
}

30. Parentheses Make Conditions Clear#

Consider:

Java
if (age >= 18 && citizen || specialPermission)

This may be difficult to read.

Prefer:

Java
if ((age >= 18 && citizen) || specialPermission)

Parentheses make the intended logic obvious.

31. Short-Circuit Evaluation#

Java's:

Java
&&
||

operators use short-circuit evaluation.

For &&:

Output
false && anything

is always false.

So Java may not evaluate the right side.

For ||:

Output
true || anything

is always true.

So Java may not evaluate the right side.

32. Useful Short-Circuit Example#

Suppose:

Java
String name = null;

This is dangerous:

Java
if (name != null && name.length() > 0) {
    System.out.println("Not empty");
}

Why is this safe?

Java checks:

Java
name != null

first.

It is false.

Because the left side of && is false, Java does not evaluate:

Java
name.length()

So there is no null dereference from that expression.

33. Dangerous Order#

This can fail:

Java
if (name.length() > 0 && name != null) {
}

Java tries:

Java
name.length()

before checking whether name is null.

So the order matters.

Good:

Java
name != null && name.length() > 0

34. Conditions with Strings#

Do not compare Strings using:

Java
==

when you mean content equality.

Use:

Java
.equals()

Example:

Java
String language = "Java";

if (language.equals("Java")) {
    System.out.println("Correct");
}

Output:

Output
Correct

For a potentially null String, a safer pattern can be:

Java
if ("Java".equals(language)) {
    System.out.println("Correct");
}

This avoids calling .equals() on a null variable.

35. == vs .equals() for Strings#

Consider:

Java
String a = new String("Java");
String b = new String("Java");

Then:

Java
a == b

checks whether the references refer to the same object.

It is not the correct general way to ask whether their contents are equal.

Use:

Java
a.equals(b)

to compare String contents.

Output conceptually:

Output
a == b       → false
a.equals(b)  → true

You will study references and object equality more deeply in the OOP chapters.

36. switch#

When you need to choose among discrete values, Java provides:

Java
switch

Example:

Java
int day = 2;

switch (day) {
    case 1:
        System.out.println("Monday");
        break;

    case 2:
        System.out.println("Tuesday");
        break;

    case 3:
        System.out.println("Wednesday");
        break;

    default:
        System.out.println("Invalid day");
}

Output:

Output
Tuesday

37. How Traditional switch Works#

Suppose:

Java
day = 2;

Java evaluates:

Output
switch(day)

Then searches for:

Output
case 2

It finds:

Java
case 2:
    System.out.println("Tuesday");

Then break exits the switch.

38. Why break Matters#

Consider:

Java
int day = 2;

switch (day) {
    case 1:
        System.out.println("Monday");

    case 2:
        System.out.println("Tuesday");

    case 3:
        System.out.println("Wednesday");
}

Because there are no break statements, execution can fall through after the matching case.

For day = 2, output can be:

Output
Tuesday
Wednesday

This behavior is called:

Output
fall-through

39. Traditional Switch with break#

Java
switch (day) {
    case 1:
        System.out.println("Monday");
        break;

    case 2:
        System.out.println("Tuesday");
        break;

    case 3:
        System.out.println("Wednesday");
        break;

    default:
        System.out.println("Invalid");
}

Now only the matching branch executes.

40. default#

The default branch executes when no case matches.

Example:

Java
int day = 10;

switch (day) {
    case 1:
        System.out.println("Monday");
        break;

    case 2:
        System.out.println("Tuesday");
        break;

    default:
        System.out.println("Invalid day");
}

Output:

Output
Invalid day

Think of default as:

Output
otherwise

41. Multiple Cases#

You can make multiple cases execute the same code.

Traditional syntax:

Java
int day = 6;

switch (day) {
    case 6:
    case 7:
        System.out.println("Weekend");
        break;

    default:
        System.out.println("Weekday");
}

Output:

Output
Weekend

Both 6 and 7 lead to the same block.

42. switch with String#

Modern Java allows switching on Strings.

Java
String language = "Java";

switch (language) {
    case "Java":
        System.out.println("Java selected");
        break;

    case "Python":
        System.out.println("Python selected");
        break;

    default:
        System.out.println("Unknown language");
}

Output:

Output
Java selected

String matching in a switch is based on String content semantics, not reference identity.

43. switch with char#

Example:

Java
char grade = 'A';

switch (grade) {
    case 'A':
        System.out.println("Excellent");
        break;

    case 'B':
        System.out.println("Good");
        break;

    case 'C':
        System.out.println("Average");
        break;

    default:
        System.out.println("Invalid grade");
}

Output:

Output
Excellent

44. switch with enum#

Enums are covered in detail later, but Java can switch on enum values.

Example:

Java
enum Day {
    MONDAY,
    TUESDAY,
    WEDNESDAY
}

Then:

Java
Day day = Day.MONDAY;

switch (day) {
    case MONDAY:
        System.out.println("Start of week");
        break;

    case TUESDAY:
        System.out.println("Tuesday");
        break;

    case WEDNESDAY:
        System.out.println("Wednesday");
        break;
}

45. Modern Switch Expressions#

Modern Java provides a more concise switch form.

Example:

Java
int day = 2;

String result = switch (day) {
    case 1 -> "Monday";
    case 2 -> "Tuesday";
    case 3 -> "Wednesday";
    default -> "Invalid";
};

System.out.println(result);

Output:

Output
Tuesday

This is called a:

Output
switch expression

because it produces a value.

46. Switch Statement vs Switch Expression#

Traditional switch:

Java
switch (day) {
    case 1:
        System.out.println("Monday");
        break;
}

Modern switch expression:

Java
String name = switch (day) {
    case 1 -> "Monday";
    default -> "Unknown";
};

The second form can directly produce a value.

47. Arrow Syntax#

Modern switch supports:

Java
case value -> result;

Example:

Java
int number = 1;

switch (number) {
    case 1 -> System.out.println("One");
    case 2 -> System.out.println("Two");
    default -> System.out.println("Other");
}

The arrow form does not have the accidental fall-through behavior of traditional case statements.

48. Multiple Labels in Modern Switch#

You can group multiple values.

Java
int day = 6;

String type = switch (day) {
    case 6, 7 -> "Weekend";
    default -> "Weekday";
};

System.out.println(type);

Output:

Output
Weekend

This is much cleaner than repeating code.

49. Switch Expression with yield#

Sometimes a switch branch needs multiple statements.

Example:

Java
int marks = 85;

String grade = switch (marks / 10) {
    case 10, 9 -> "A";
    case 8 -> "B";
    case 7 -> "C";
    case 6 -> "D";
    default -> {
        if (marks >= 40) {
            yield "E";
        } else {
            yield "F";
        }
    }
};

System.out.println(grade);

yield provides the value produced by that switch-expression block.

50. if vs switch#

Use if when conditions involve ranges or complex boolean expressions.

Example:

Java
if (marks >= 90) {
    ...
}

Use switch when you are selecting among specific discrete values.

Example:

Java
switch (day) {
    case 1 -> ...
    case 2 -> ...
}

Simple mental model:

Output
if
→ ranges, comparisons, complex logic

switch
→ specific choices/values

This is a guideline, not an absolute rule.

51. Ternary Operator#

You already learned this in Chapter 4.

Syntax:

Java
condition ? valueIfTrue : valueIfFalse

Example:

Java
int age = 20;

String result = age >= 18 ? "Adult" : "Minor";

System.out.println(result);

Output:

Output
Adult

It is essentially a compact expression for choosing between two values.

52. if-else vs Ternary#

This:

Java
if (age >= 18) {
    result = "Adult";
} else {
    result = "Minor";
}

can often become:

Java
result = age >= 18 ? "Adult" : "Minor";

Use ternary for short, simple value selection.

Do not turn complicated logic into a giant nested ternary expression.

53. Nested Ternary#

Technically possible:

Java
String result =
    marks >= 90 ? "A" :
    marks >= 80 ? "B" :
    marks >= 70 ? "C" :
    "F";

This works, but can become hard to read.

For complicated logic, prefer:

Java
if
else-if

Readable code is usually better than clever code.

54. Conditions and Scope#

Variables declared inside an if block are local to that block.

Example:

Java
if (true) {
    int x = 10;
    System.out.println(x);
}

This works.

But:

Java
if (true) {
    int x = 10;
}

System.out.println(x);

does not compile because x is outside its scope.

Think:

Output
{
    variable lives here
}

55. Declaring a Variable Before the Condition#

Suppose you want to use a value after the conditional.

You may write:

Java
String result;

if (marks >= 40) {
    result = "Pass";
} else {
    result = "Fail";
}

System.out.println(result);

This works because both branches assign result.

The compiler performs definite-assignment analysis to make sure local variables are initialized before use.

56. Definite Assignment Example#

This is problematic:

Java
String result;

if (marks >= 40) {
    result = "Pass";
}

System.out.println(result);

Why?

If:

Output
marks < 40

the assignment never happens.

Java therefore does not allow the compiler to assume result has a value.

A correct version:

Java
String result;

if (marks >= 40) {
    result = "Pass";
} else {
    result = "Fail";
}

57. Conditions with final#

A final variable can still be used in conditions.

Java
final int PASS_MARKS = 40;

int marks = 55;

if (marks >= PASS_MARKS) {
    System.out.println("Pass");
}

Output:

Output
Pass

final means the variable cannot be reassigned.

58. Comparing Characters#

Characters can be compared.

Java
char ch = 'A';

if (ch == 'A') {
    System.out.println("Character is A");
}

Output:

Output
Character is A

You can also compare character values:

Java
if (ch >= 'A' && ch <= 'Z') {
    System.out.println("Uppercase letter");
}

This works because char participates in numeric comparisons according to Java's character/integer conversion rules.

59. Checking a Lowercase Character#

Java
char ch = 'g';

if (ch >= 'a' && ch <= 'z') {
    System.out.println("Lowercase");
}

Output:

Output
Lowercase

For production code, Java also provides character utility methods such as:

Java
Character.isUpperCase(ch)
Character.isLowerCase(ch)
Character.isDigit(ch)
Character.isLetter(ch)

60. Character Utility Example#

Java
char ch = '7';

if (Character.isDigit(ch)) {
    System.out.println("Digit");
} else {
    System.out.println("Not a digit");
}

Output:

Output
Digit

This is often clearer than manually checking ranges.

61. Conditions with Floating-Point Values#

Be careful when comparing floating-point values for exact equality.

Example:

Java
double x = 0.1 + 0.2;

if (x == 0.3) {
    System.out.println("Equal");
} else {
    System.out.println("Not exactly equal");
}

The result may be:

Output
Not exactly equal

because binary floating-point numbers cannot represent many decimal fractions exactly.

For numerical code, an acceptable tolerance is often used:

Java
double epsilon = 0.000001;

if (Math.abs(x - 0.3) < epsilon) {
    System.out.println("Approximately equal");
}

This is an important real-world consideration.

62. Range Checking#

A common task is checking whether a value lies inside a range.

Example:

Java
int marks = 75;

if (marks >= 0 && marks <= 100) {
    System.out.println("Valid marks");
} else {
    System.out.println("Invalid marks");
}

The condition means:

Output
marks is at least 0
AND
marks is at most 100

63. Range Checking with if-else-if#

Java
int marks = 83;

if (marks < 0 || marks > 100) {
    System.out.println("Invalid marks");
} else if (marks >= 90) {
    System.out.println("A");
} else if (marks >= 80) {
    System.out.println("B");
} else if (marks >= 70) {
    System.out.println("C");
} else if (marks >= 60) {
    System.out.println("D");
} else if (marks >= 40) {
    System.out.println("E");
} else {
    System.out.println("F");
}

Output:

Output
B

Notice that validation comes before grading.

64. Practical Program — Largest of Two Numbers#

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter first number: ");
        int a = sc.nextInt();

        System.out.print("Enter second number: ");
        int b = sc.nextInt();

        if (a > b) {
            System.out.println("First number is larger");
        } else if (b > a) {
            System.out.println("Second number is larger");
        } else {
            System.out.println("Both numbers are equal");
        }

        sc.close();
    }
}

65. Practical Program — Largest of Three Numbers#

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter a: ");
        int a = sc.nextInt();

        System.out.print("Enter b: ");
        int b = sc.nextInt();

        System.out.print("Enter c: ");
        int c = sc.nextInt();

        if (a >= b && a >= c) {
            System.out.println("Largest = " + a);
        } else if (b >= a && b >= c) {
            System.out.println("Largest = " + b);
        } else {
            System.out.println("Largest = " + c);
        }

        sc.close();
    }
}

66. Practical Program — Leap Year#

A common leap-year rule is:

Output
A year is a leap year if:

divisible by 400
OR
divisible by 4 AND not divisible by 100

Java:

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter year: ");
        int year = sc.nextInt();

        if (year % 400 == 0 ||
            (year % 4 == 0 && year % 100 != 0)) {
            System.out.println("Leap year");
        } else {
            System.out.println("Not a leap year");
        }

        sc.close();
    }
}

Examples:

Output
2000 → Leap year
1900 → Not a leap year
2024 → Leap year
2023 → Not a leap year

This is a good example of combining:

Output
%
&&
||
!=

inside one condition.

67. Practical Program — Vowel or Consonant#

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter a character: ");
        char ch = sc.next().charAt(0);

        if (ch == 'a' || ch == 'e' ||
            ch == 'i' || ch == 'o' ||
            ch == 'u' ||
            ch == 'A' || ch == 'E' ||
            ch == 'I' || ch == 'O' ||
            ch == 'U') {
            System.out.println("Vowel");
        } else {
            System.out.println("Consonant");
        }

        sc.close();
    }
}

For a production-quality program, you would also validate that the input is actually a letter.

68. Practical Program — Character Classification#

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter a character: ");
        char ch = sc.next().charAt(0);

        if (Character.isLetter(ch)) {
            System.out.println("Letter");
        } else if (Character.isDigit(ch)) {
            System.out.println("Digit");
        } else {
            System.out.println("Special character");
        }

        sc.close();
    }
}

69. Practical Program — Calculator with switch#

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter first number: ");
        double a = sc.nextDouble();

        System.out.print("Enter operator (+, -, *, /): ");
        char operator = sc.next().charAt(0);

        System.out.print("Enter second number: ");
        double b = sc.nextDouble();

        switch (operator) {
            case '+':
                System.out.println("Result = " + (a + b));
                break;

            case '-':
                System.out.println("Result = " + (a - b));
                break;

            case '*':
                System.out.println("Result = " + (a * b));
                break;

            case '/':
                if (b != 0) {
                    System.out.println("Result = " + (a / b));
                } else {
                    System.out.println("Cannot divide by zero");
                }
                break;

            default:
                System.out.println("Invalid operator");
        }

        sc.close();
    }
}

Example:

Output
Enter first number: 20
Enter operator (+, -, *, /): *
Enter second number: 5
Result = 100.0

This demonstrates that conditions can exist inside switch branches.

70. Practical Program — Day of Week#

Using a modern switch expression:

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter day number (1-7): ");
        int day = sc.nextInt();

        String dayName = switch (day) {
            case 1 -> "Monday";
            case 2 -> "Tuesday";
            case 3 -> "Wednesday";
            case 4 -> "Thursday";
            case 5 -> "Friday";
            case 6 -> "Saturday";
            case 7 -> "Sunday";
            default -> "Invalid day";
        };

        System.out.println(dayName);

        sc.close();
    }
}

Input:

Output
5

Output:

Output
Friday

71. Practical Program — Electricity Bill#

For learning, suppose:

Output
0–100 units    → ₹5/unit
101–200 units  → ₹7/unit
above 200      → ₹10/unit

A simple slab example:

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter units: ");
        int units = sc.nextInt();

        double bill;

        if (units <= 100) {
            bill = units * 5;
        } else if (units <= 200) {
            bill = 100 * 5 + (units - 100) * 7;
        } else {
            bill = 100 * 5
                 + 100 * 7
                 + (units - 200) * 10;
        }

        System.out.printf("Bill = ₹%.2f%n", bill);

        sc.close();
    }
}

This is an excellent example of using ranges and else-if.

72. Practical Program — Admission Eligibility#

Suppose a student needs:

Output
marks >= 60
AND
attendance >= 75

Program:

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter marks: ");
        int marks = sc.nextInt();

        System.out.print("Enter attendance percentage: ");
        double attendance = sc.nextDouble();

        if (marks >= 60 && attendance >= 75) {
            System.out.println("Eligible for admission");
        } else {
            System.out.println("Not eligible");
        }

        sc.close();
    }
}

73. Practical Program — Login Decision#

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Username: ");
        String username = sc.nextLine();

        System.out.print("Password: ");
        String password = sc.nextLine();

        if ("admin".equals(username) && "1234".equals(password)) {
            System.out.println("Login successful");
        } else {
            System.out.println("Invalid username or password");
        }

        sc.close();
    }
}

Notice the style:

Java
"admin".equals(username)

instead of:

Java
username.equals("admin")

The first form remains safe if username is null.

74. Practical Program — Age Category#

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter age: ");
        int age = sc.nextInt();

        if (age < 0) {
            System.out.println("Invalid age");
        } else if (age <= 12) {
            System.out.println("Child");
        } else if (age <= 19) {
            System.out.println("Teenager");
        } else if (age <= 59) {
            System.out.println("Adult");
        } else {
            System.out.println("Senior");
        }

        sc.close();
    }
}

75. Practical Program — Discount Calculator#

Suppose:

Output
Purchase >= 5000 → 20%
Purchase >= 2000 → 10%
Otherwise         → 0%

Program:

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter purchase amount: ");
        double amount = sc.nextDouble();

        double discountRate;

        if (amount >= 5000) {
            discountRate = 0.20;
        } else if (amount >= 2000) {
            discountRate = 0.10;
        } else {
            discountRate = 0.0;
        }

        double discount = amount * discountRate;
        double finalAmount = amount - discount;

        System.out.printf("Discount = %.2f%n", discount);
        System.out.printf("Final amount = %.2f%n", finalAmount);

        sc.close();
    }
}

76. Practical Program — Simple ATM Decision#

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        double balance = 10000;

        System.out.print("Enter withdrawal amount: ");
        double amount = sc.nextDouble();

        if (amount <= 0) {
            System.out.println("Invalid amount");
        } else if (amount > balance) {
            System.out.println("Insufficient balance");
        } else {
            balance -= amount;
            System.out.printf("Withdrawal successful%n");
            System.out.printf("Remaining balance = %.2f%n", balance);
        }

        sc.close();
    }
}

This is still a simple educational example, not a real banking system.

77. Common Mistake — Assignment Instead of Comparison#

This is wrong for a condition:

Java
if (age = 18) {
}

= means assignment.

Comparison uses:

Java
==

Correct:

Java
if (age == 18) {
}

Remember:

Output
=   → assign
==  → compare

78. Common Mistake — Semicolon After if#

Do not write:

Java
if (age >= 18);
{
    System.out.println("Adult");
}

The semicolon terminates the if statement.

The block afterward is no longer controlled by the condition.

Correct:

Java
if (age >= 18) {
    System.out.println("Adult");
}

79. Common Mistake — Missing Braces#

Java technically allows:

Java
if (age >= 18)
    System.out.println("Adult");

But when there are multiple statements, braces are important.

Prefer:

Java
if (age >= 18) {
    System.out.println("Adult");
    System.out.println("Eligible");
}

Braces make the scope obvious and reduce mistakes.

80. The Dangling else Problem#

Consider:

Java
if (a > 0)
    if (b > 0)
        System.out.println("Both positive");
    else
        System.out.println("b is not positive");

Which if does the else belong to?

Java associates an else with the nearest unmatched if.

To make code clear, use braces:

Java
if (a > 0) {
    if (b > 0) {
        System.out.println("Both positive");
    } else {
        System.out.println("b is not positive");
    }
}

This is much easier to understand.

81. Common Mistake — Wrong else-if Order#

Bad:

Java
if (marks >= 40) {
    System.out.println("Pass");
} else if (marks >= 90) {
    System.out.println("A");
}

The >= 90 branch is unreachable in practice within this chain because every mark of 90 or more already satisfies marks >= 40.

Better:

Java
if (marks >= 90) {
    System.out.println("A");
} else if (marks >= 40) {
    System.out.println("Pass");
}

82. Common Mistake — Using == for String Content#

Avoid:

Java
if (name == "Aman") {
}

Use:

Java
if ("Aman".equals(name)) {
}

or:

Java
if (name != null && name.equals("Aman")) {
}

The first version is often simpler and null-safe.

83. Common Mistake — Integer Division in Conditions#

Consider:

Java
int a = 1;
int b = 2;

if (a / b == 0.5) {
    System.out.println("Equal");
}

This does not behave as a floating-point division because both operands are integers.

1 / 2
↓
0

Use:

Java
if ((double) a / b == 0.5) {
    ...
}

when floating-point division is intended.

84. Common Mistake — Floating-Point Exact Comparison#

Avoid relying on:

Java
if (0.1 + 0.2 == 0.3)

for exact equality.

Floating-point representation can make the result slightly different.

Use an appropriate tolerance for numerical applications.

85. if Conditions Must Be Boolean#

Java does not allow C-style implicit integer-to-boolean conditions.

This is invalid:

Java
int x = 10;

if (x) {
}

Java requires:

Java
if (x > 0) {
}

or:

Java
boolean positive = true;

if (positive) {
}

This is different from languages where non-zero numbers can be treated as true.

86. No Implicit Boolean Conversion#

These are not valid:

Java
if (1) {
}

or:

Java
if ("hello") {
}

Java conditions must have type:

Java
boolean

or evaluate to a boolean expression.

87. Complex Condition Example#

Consider:

Java
int age = 25;
boolean citizen = true;
boolean specialPermission = false;

if ((age >= 18 && citizen) || specialPermission) {
    System.out.println("Eligible");
}

Break it down:

age >= 18
   ↓
true

citizen
   ↓
true

true && true
   ↓
true

specialPermission
   ↓
false

true || false
   ↓
true

Therefore:

Output
Eligible

Learning to break complex conditions into small pieces is an important programming skill.

88. Boolean Variables Can Simplify Conditions#

Instead of:

Java
if (isLoggedIn == true) {
}

write:

Java
if (isLoggedIn) {
}

Similarly:

Instead of:

Java
if (isLoggedIn == false) {
}

write:

Java
if (!isLoggedIn) {
}

Example:

Java
boolean isLoggedIn = true;

if (isLoggedIn) {
    System.out.println("Welcome");
}

89. Avoid Unnecessary Boolean Comparisons#

Prefer:

Java
if (hasPermission) {
}

instead of:

Java
if (hasPermission == true) {
}

Prefer:

Java
if (!hasPermission) {
}

instead of:

Java
if (hasPermission == false) {
}

The shorter versions are easier to read.

90. Combining Conditions with Methods#

Conditions can use method calls.

Example:

Java
String name = "Aman";

if (!name.isEmpty()) {
    System.out.println("Name provided");
}

Another:

Java
String email = "user@example.com";

if (email.contains("@")) {
    System.out.println("Looks like an email");
}

These are simple examples; real validation should be more rigorous.

91. Conditions and Null#

Reference variables can contain:

Java
null

You can check:

Java
if (name == null) {
    System.out.println("No name");
}

Or:

Java
if (name != null) {
    System.out.println(name);
}

Remember:

Output
null means no object reference

You will study references and null much more deeply in the OOP chapters.

92. instanceof in Conditions#

Java provides:

Java
instanceof

to test whether an object is compatible with a type.

Example:

Java
Object value = "Java";

if (value instanceof String) {
    System.out.println("It is a String");
}

Output:

Output
It is a String

This becomes especially important when studying inheritance and polymorphism.

93. Pattern Matching with instanceof#

Modern Java can combine the type test and variable binding.

Example:

Java
Object value = "Java";

if (value instanceof String text) {
    System.out.println(text.length());
}

If the object is a String, Java makes the matched variable:

Java
text

available in the appropriate scope.

You will study this more deeply in the modern Java chapter.

94. Conditions in Loops#

Conditions are not limited to if.

Loops also use conditions.

For example:

Java
int i = 1;

while (i <= 5) {
    System.out.println(i);
    i++;
}

The condition:

Java
i <= 5

determines whether the loop continues.

You will study loops in Chapter 7.

95. Conditions in Methods#

Methods can return boolean values.

Example:

Java
public static boolean isEven(int number) {
    return number % 2 == 0;
}

Then:

Java
if (isEven(10)) {
    System.out.println("Even");
}

Output:

Output
Even

This is an important connection between conditions and methods.

96. Good Condition Design#

A good condition should be:

Output
clear
correct
easy to read
easy to test

Instead of:

Java
if (!(age < 18) && !(!citizen || banned)) {
}

sometimes it is better to introduce meaningful boolean variables:

Java
boolean adult = age >= 18;
boolean allowed = citizen && !banned;

if (adult && allowed) {
    ...
}

Readable logic is easier to maintain.

97. Practical Example — Password Strength#

A very basic educational check:

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter password: ");
        String password = sc.nextLine();

        if (password.length() >= 8) {
            System.out.println("Password has at least 8 characters.");
        } else {
            System.out.println("Password is too short.");
        }

        sc.close();
    }
}

This is not a complete password-security check. It simply demonstrates a condition.

98. Practical Example — Shipping Eligibility#

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter order amount: ");
        double amount = sc.nextDouble();

        System.out.print("Enter distance in km: ");
        double distance = sc.nextDouble();

        if (amount >= 1000 && distance <= 20) {
            System.out.println("Free shipping");
        } else {
            System.out.println("Shipping charges apply");
        }

        sc.close();
    }
}

99. Practical Example — Login with Account Status#

Java
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Username: ");
        String username = sc.nextLine();

        System.out.print("Password: ");
        String password = sc.nextLine();

        boolean active = true;

        if ("admin".equals(username) && "1234".equals(password)) {
            if (active) {
                System.out.println("Login successful");
            } else {
                System.out.println("Account is inactive");
            }
        } else {
            System.out.println("Invalid credentials");
        }

        sc.close();
    }
}

This example demonstrates nested decisions.

100. Decision-Making Cheat Sheet#

Output
if
→ execute something only when a condition is true

if-else
→ choose between two paths

else-if
→ choose among multiple conditions

nested if
→ condition inside another condition

switch
→ choose among discrete values

switch expression
→ choose a value based on a case

ternary
→ compact two-way value selection

101. if Decision Tree#

Diagram
                 Condition
                    │
              ┌─────┴─────┐
             true        false
              │             │
              ▼             ▼
          execute         skip
           block           block

102. if-else Decision Tree#

Diagram
                 Condition
                    │
              ┌─────┴─────┐
             true        false
              │             │
              ▼             ▼
           if block      else block
              │             │
              └──────┬──────┘
                     ▼
                   End

103. else-if Decision Tree#

Diagram
             Condition 1
              /       \
           true       false
            │           │
            ▼           ▼
         Block 1    Condition 2
                      /     \
                   true     false
                    │         │
                    ▼         ▼
                 Block 2   Condition 3

Continue until one condition matches or else is reached.

104. switch Decision Tree#

Diagram
                 value
                   │
        ┌──────────┼──────────┐
        │          │          │
      case 1     case 2     case 3
        │          │          │
        ▼          ▼          ▼
      Action     Action      Action
        │          │          │
        └──────────┴──────────┘
                   │
                 default

105. Practice Questions — Basic#

Write programs for:

  1. Check whether a number is positive.
  2. Check whether a number is negative.
  3. Check whether a number is zero.
  4. Check whether a number is even or odd.
  5. Check whether a person is an adult.
  6. Check whether marks are passing.
  7. Find the larger of two numbers.
  8. Check whether two numbers are equal.
  9. Check whether a character is uppercase.
  10. Check whether a character is a digit.

106. Practice Questions — if-else-if#

Write programs to:

  1. Find the grade from marks.
  2. Categorize age.
  3. Find the largest of three numbers.
  4. Find the smallest of three numbers.
  5. Determine whether a year is a leap year.
  6. Calculate a discount based on purchase amount.
  7. Calculate an electricity bill using slabs.
  8. Classify a number as positive, negative, or zero.
  9. Check whether a triangle angle value is valid.
  10. Determine a student's result category.

107. Practice Questions — Logical Operators#

Write programs that check:

  1. Age and citizenship eligibility.
  2. Username and password.
  3. Whether a number lies within a range.
  4. Whether a number is divisible by both 3 and 5.
  5. Whether a number is divisible by either 3 or 5.
  6. Whether a person is eligible based on marks and attendance.
  7. Whether a product gets free shipping.
  8. Whether a user is logged in and has permission.
  9. Whether an account is active and verified.
  10. Whether a number is outside a specified range.

108. Practice Questions — switch#

Write programs for:

  1. Day of week.
  2. Month number to month name.
  3. Simple calculator.
  4. Menu selection.
  5. Grade description.
  6. Traffic signal.
  7. Basic unit converter.
  8. Number to word for values 1–5.
  9. Season selection.
  10. Character-based menu.

109. Challenge — ATM Menu#

Create:

Output
===== ATM =====
1. Check Balance
2. Deposit
3. Withdraw
4. Exit

Take the user's choice and perform the appropriate action.

Use:

Java
switch

and conditions where needed.

110. Challenge — Student Result System#

Take:

Output
student name
marks of 5 subjects

Calculate:

Output
total
percentage
grade
pass/fail

Validation:

Output
Each mark must be between 0 and 100.

Suggested logic:

invalid marks
    ↓
show error

otherwise
    ↓
calculate percentage
    ↓
determine grade
    ↓
display result

111. Challenge — Login System#

Create a simple program with:

Output
username
password
account status

Rules:

Output
wrong username/password
    → invalid credentials

correct credentials + inactive account
    → account inactive

correct credentials + active account
    → login successful

Use:

Java
if
else-if
&&

112. Challenge — Shopping Discount#

Rules:

Output
amount >= 10000 → 30%
amount >= 5000  → 20%
amount >= 2000  → 10%
otherwise       → 0%

Print:

Output
Original amount
Discount percentage
Discount amount
Final amount

Use printf() for money formatting.

113. Challenge — Number Analyzer#

Take an integer and determine:

Output
positive/negative/zero
even/odd
divisible by 3?
divisible by 5?

For example:

Output
Input: 30

Positive
Even
Divisible by 3
Divisible by 5

Notice that multiple independent if statements are useful here because multiple properties can be true at the same time.

114. Interview Questions#

Q1. What is an if statement?#

An if statement conditionally executes a block of code when a boolean condition is true.

Q2. What is if-else?#

It provides two alternative execution paths:

Output
true  → if
false → else

Q3. What is else-if?#

It allows multiple conditions to be checked in sequence.

The first true condition's block executes.

Q4. Can multiple if blocks execute?#

Yes.

With separate if statements, every condition is checked independently.

Q5. Can multiple else-if branches execute?#

No.

In a single if-else-if-else chain, at most one branch executes.

Q6. What is nested if?#

An if statement placed inside another conditional block.

Q7. What is switch used for?#

switch is useful when selecting among discrete values or cases.

Q8. What is break used for in traditional switch?#

It exits the switch statement and prevents execution from falling through to later cases.

Q9. What is fall-through?#

In traditional switch syntax, if a matching case does not end with a control transfer such as break, execution can continue into subsequent cases.

Q10. What is default?#

It is the fallback branch of a traditional switch when no case matches.

Q11. What is a switch expression?#

A modern switch form that produces a value.

Example:

Java
String result = switch (day) {
    case 1 -> "Monday";
    default -> "Unknown";
};

Q12. What is the ternary operator?#

A conditional expression with three parts:

Java
condition ? trueValue : falseValue

Q13. Can Java use an integer directly as an if condition?#

No.

Java requires a boolean expression.

This is invalid:

Java
if (1) {
}

Q14. Difference between = and ==?#

Output
=  → assignment
== → equality comparison

Q15. How do you compare Strings?#

Use .equals() for content equality.

Example:

Java
"Java".equals(name)

Q16. Why is "Java".equals(name) useful?#

It avoids calling a method on name when name is null.

Q17. What is short-circuit evaluation?#

With && and ||, Java may skip evaluation of the right-hand operand when the result is already determined by the left-hand operand.

Q18. Why is this safe?#

Java
if (name != null && name.length() > 0)

Because if name != null is false, && does not evaluate name.length().

Q19. What is the difference between if and switch?#

if is more flexible for comparisons, ranges, and complex boolean expressions.

switch is convenient for choosing among discrete values.

Q20. What is a nested condition?#

A condition placed inside another condition.

115. Interview Output Questions#

Question 1#

What is the output?

Java
int x = 10;

if (x > 5) {
    System.out.println("A");
}

System.out.println("B");

Answer:

Output
A
B

Question 2#

Java
int x = 10;

if (x > 20) {
    System.out.println("A");
} else {
    System.out.println("B");
}

Output:

Output
B

Question 3#

Java
int x = 10;

if (x > 0) {
    System.out.println("Positive");
} else if (x % 2 == 0) {
    System.out.println("Even");
}

Output:

Output
Positive

The second condition is not reached.

Question 4#

Java
int x = 10;

if (x > 0) {
    System.out.println("Positive");
}

if (x % 2 == 0) {
    System.out.println("Even");
}

Output:

Output
Positive
Even

Question 5#

Java
int x = 5;

if (x > 0 && x < 10) {
    System.out.println("A");
}

Output:

Output
A

Question 6#

Java
int x = 10;

if (x < 5 || x == 10) {
    System.out.println("Yes");
}

Output:

Output
Yes

116. Important Mental Model#

When reading a condition, ask:

What is being checked?
        ↓
What boolean value does it produce?
        ↓
Which branch executes?

For example:

Java
if (marks >= 40) {
    System.out.println("Pass");
} else {
    System.out.println("Fail");
}

Think:

Diagram
marks >= 40?
      │
   ┌──┴──┐
 true   false
   │       │
 Pass     Fail

This mental model makes conditional code much easier to understand.

117. Conditions in Real Programs#

Almost every serious application uses conditions.

Examples:

Login
  ↓
Is username correct?
  ↓
Is password correct?
  ↓
Is account active?

Shopping:

Cart
  ↓
Is coupon valid?
  ↓
Is minimum amount reached?
  ↓
Apply discount

Banking:

Withdrawal request
  ↓
Is amount valid?
  ↓
Is balance sufficient?
  ↓
Process withdrawal

Games:

Player action
  ↓
Is move valid?
  ↓
Is player alive?
  ↓
Update game

Web applications:

Request
  ↓
Is user authenticated?
  ↓
Is user authorized?
  ↓
Perform operation

Conditions are therefore not just beginner syntax. They are a fundamental part of software logic.

118. Connection to Previous Chapters#

You now have:

Diagram
Chapter 3
Variables & Data Types
        ↓
Chapter 4
Operators
        ↓
Chapter 5
Input & Output
        ↓
Chapter 6
Conditions

Together, these allow you to build programs like:

User Input
    ↓
Store in Variables
    ↓
Perform Calculations
    ↓
Check Conditions
    ↓
Choose Action
    ↓
Display Output

This is the basic structure of many programs.

119. Connection to the Next Chapter#

The next major step is repetition.

Conditions answer:

Output
Should I do this?

Loops answer:

Output
Should I keep doing this?

For example:

Java
if (age >= 18) {
    System.out.println("Adult");
}

checks something once.

A loop can repeatedly check a condition:

Java
while (age < 18) {
    // repeat
}

You will learn this in:

Chapter 7 — Loops#

Topics will include:

Output
while
do-while
for
nested loops
break
continue
enhanced for loop
loop patterns
common loop mistakes
practical problems

120. Final Revision#

Remember these structures.

Basic if:

Java
if (condition) {
    // code
}

if-else:

Java
if (condition) {
    // true
} else {
    // false
}

else-if:

Java
if (condition1) {
    // ...
} else if (condition2) {
    // ...
} else {
    // ...
}

Nested if:

Java
if (condition1) {
    if (condition2) {
        // ...
    }
}

Traditional switch:

Java
switch (value) {
    case 1:
        // ...
        break;

    case 2:
        // ...
        break;

    default:
        // ...
}

Modern switch:

Java
switch (value) {
    case 1 -> "One";
    case 2 -> "Two";
    default -> "Other";
}

Ternary:

Java
condition ? value1 : value2

Logical operators:

Output
&& → AND
|| → OR
!  → NOT

Comparison operators:

Output
>   <   >=   <=   ==   !=

Most importantly:

if
    ↓
decision

else
    ↓
alternative

else-if
    ↓
multiple alternatives

switch
    ↓
discrete choices

Once conditions become comfortable, you are ready to move from simple one-time decisions to repeated logic with loops.