Skip to content

8. Looping Constructs in Java

Table of Contents

Java provides several looping constructs that allow repeated execution of a block of code as long as a condition holds.

Loops are essential for iteration, traversal of data structures, repeated computations, and implementing algorithms.

8.1 The while Loop

The while loop evaluates its boolean condition before each iteration.
If the condition is false from the beginning, the body is never executed.

Syntax 

while (condition) {
    // loop body
}

  • The condition must evaluate to a boolean.
  • The loop may execute zero or more times.

  • Common pitfalls include forgetting to update the loop variable, causing an infinite loop.

  • Example: 

    int i = 0;
while (i < 3) {
    System.out.println(i);
    i++;
}

Output: 

0
1
2

8.2 The do-while Loop

The do-while loop evaluates its condition after executing the body, ensuring the body runs at least once.

Syntax 

do {
    // loop body
} while (condition);

Tip

do-while requires a semicolon after the closing parenthesis.

  • Example: 
    int x = 5;
do {
    System.out.println(x);
    x--;
} while (x > 5); // body runs once even though condition is false

Output: 

5

8.3 The for Loop

The traditional for loop is best suited for loops with a counter variable. It consists of three parts: initialization, condition, update.

Syntax 

for (initialization; condition; update) {
    // loop body
}

  • Initialization runs once before the loop starts.
  • Condition is evaluated before each iteration.
  • Update runs after each iteration.
  • Initialization and update may contain multiple statements separated by commas.
  • Variables in initialization must all be of the same type.

  • Any component may be omitted, but semicolons remain.

  • Example: 

    for (int i = 0; i < 3; i++) {
    System.out.println(i);
}

Omitting parts: 

int j = 0;
for (; j < 3;) {  // valid
    j++;
}

Multiple statements: 

int x = 0;
for (long i = 0, c = 3; x < 3 && i < 12; x++, i++) {
    System.out.println(i);
}

8.4 The Enhanced for-each Loop

The enhanced for simplifies iteration over arrays and collections.

Syntax 

for (ElementType var : arrayOrCollection) {
    // loop body
}

  • Loop variable is read-only relative to the underlying collection.
  • Works with any Iterable or array.

  • Cannot remove elements without an iterator.

  • Example: 

    String[] names = {"A", "B", "C"};
for (String n : names) {
    System.out.println(n);
}

Output: 

A
B
C

8.5 Nested Loops

Loops may be nested; each maintains its own variables and conditions.

for (int i = 1; i <= 2; i++) {
    for (int j = 1; j <= 3; j++) {
        System.out.println(i + "," + j);
    }
}

Output: 

1,1
1,2
1,3
2,1
2,2
2,3

8.6 Infinite Loops

A loop is infinite when its condition always evaluates to true or is omitted.

while (true) { ... }

for (;;) { ... }

Tip

Infinite loops must contain break, return, or external control.

8.7 break and continue

break Exits the innermost loop immediately. 

for (int i = 0; i < 5; i++) {
    if (i == 2) break;
    System.out.println(i);
}

continue Skips the rest of the loop body and continues to next iteration. 

for (int i = 0; i < 5; i++) {
    if (i % 2 == 0) continue;
    System.out.println(i);
}

Note

break and continue apply to the nearest loop unless labels are used.

8.8 Labeled Loops

A label (identifier + colon) may be applied to a loop to allow break/continue to affect outer loops.

labelName:
for (...) {
    for (...) {
        break labelName;
    }
}
  • Example: 
    outer:
for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        if (j == 2) break outer;
        System.out.println(i + "," + j);
    }
}

8.9 Loop Variable Scope

  • Variables declared in the loop header are scoped to that loop.
  • Variables declared inside the body exist only inside that block.
for (int i = 0; i < 3; i++) {
    int x = i * 2;
}
// i and x are not accessible here

8.10 Unreachable code after break, continue, and return

Any statement placed after break, continue, or return in the same block is considered unreachable and will not compile.

8.10.1 Unreachable Code After break

for (int i = 0; i < 3; i++) {
    break;
    System.out.println("Unreachable"); // ❌ Compile-time error
}

8.10.2 Unreachable Code After continue

for (int i = 0; i < 3; i++) {
    continue;
    System.out.println("Unreachable"); // ❌ Compile-time error
}

Note

continue jumps to the next iteration, so following code is never executed.

8.10.3 Unreachable Code After return

int test() {
    return 5;
    System.out.println("Unreachable"); // ❌ Compile-time error
}

Note

return exits the method immediately; no statements can follow.