Master Core Java Programming From Scratch

What is a Loop?

A loop is a control-flow construct that repeats a block of statements. Loops are useful when the same operation needs to be performed multiple times.

Start
  ↓
Check condition
  ↓
Execute statements
  ↓
Update
  ↓
Check condition again
  ↓
Stop when condition is false

Types of Loops

1. for Loop

The for loop combines initialization, a condition, and an update expression in one loop header.

for (initialization; condition; update) {

    // statements

}

for (int i = 1; i <= 5; i++) {

    System.out.println(i);

}

for Loop Output

1
2
3
4
5

How a for Loop Works

1. Initialization executes once.
2. The condition is evaluated.
3. If the condition is true, the loop body executes.
4. The update expression executes.
5. The condition is checked again.

2. while Loop

A while loop checks its condition before each iteration. If the condition is initially false, the body does not execute.

while (condition) {

    // statements

}

int i = 1;

while (i <= 5) {

    System.out.println(i);

    i++;

}

3. do-while Loop

A do-while loop executes its body first and checks the condition afterward. Therefore, the body executes at least once.

do {

    // statements

} while (condition);

int i = 1;

do {

    System.out.println(i);

    i++;

} while (i <= 5);

do-while Executes at Least Once

int number = 10;

do {

    System.out.println("Executed");

} while (number < 5);

Even though number < 5 is false, the message is printed once because the condition is checked after the body.

4. Enhanced for Loop

The enhanced for loop, also called the for-each loop, provides a simple way to iterate through arrays and other Iterable objects.

int[] numbers = {10, 20, 30, 40};

for (int number : numbers) {

    System.out.println(number);

}

Enhanced for with Strings

String[] names = {
    "Java",
    "Python",
    "C#"
};

for (String name : names) {

    System.out.println(name);

}

5. Nested Loops

A loop inside another loop is called a nested loop. Nested loops are commonly used for tables, matrices, and pattern-based problems.

for (int i = 1; i <= 3; i++) {

    for (int j = 1; j <= 3; j++) {

        System.out.println(i + " " + j);

    }

}

6. break Statement

The break statement immediately terminates the nearest enclosing loop or switch statement.

for (int i = 1; i <= 10; i++) {

    if (i == 5) {

        break;

    }

    System.out.println(i);

}

Output:

1
2
3
4

7. continue Statement

The continue statement skips the remaining statements in the current iteration and proceeds to the next iteration.

for (int i = 1; i <= 5; i++) {

    if (i == 3) {

        continue;

    }

    System.out.println(i);

}

Output:

1
2
4
5

Labeled break

Java supports labeled statements that can be used with break to terminate an enclosing labeled loop.

outer:
for (int i = 1; i <= 3; i++) {

    for (int j = 1; j <= 3; j++) {

        if (i == 2 && j == 2) {

            break outer;

        }

        System.out.println(i + " " + j);

    }

}

Infinite Loops

An infinite loop continues indefinitely because its condition never becomes false.

while (true) {

    System.out.println("Running...");

}

Loop with a Condition

int sum = 0;

for (int i = 1; i <= 10; i++) {

    sum += i;

}

System.out.println("Sum = " + sum);

Output:

Sum = 55

for vs while vs do-while

Practical Example: Multiplication Table

int number = 5;

for (int i = 1; i <= 10; i++) {

    System.out.println(
        number + " x " + i + " = " + (number * i)
    );

}

Output

5 x 1 = 5
5 x 2 = 10
5 x 3 = 15
5 x 4 = 20
5 x 5 = 25
5 x 6 = 30
5 x 7 = 35
5 x 8 = 40
5 x 9 = 45
5 x 10 = 50

Loop Best Practices

• Make sure the loop condition can eventually become false unless an infinite loop is intentional.
• Keep loop bodies small and readable.
• Avoid unnecessary nested loops when a simpler algorithm is available.
• Use enhanced for loops when you only need each element and not its index.
• Use break and continue only when they improve readability.
• Be careful when modifying a collection while iterating over it.