Difference between while and do–while in Java - Performance Comparison

Time Complexity: Difference between while and do-while

O(n)

Understanding Time Complexity

We want to see how the time it takes to run loops changes with input size.

Specifically, we compare two loop types: while and do-while.

Scenario Under Consideration

Analyze the time complexity of these two loops.

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

int j = 0;
do {
    System.out.println(j);
    j++;
} while (j < n);
    

Both loops print numbers from 0 up to n-1, but the order of checking the condition differs.

Identify Repeating Operations

Both loops repeat printing and incrementing.

• Primary operation: Printing and incrementing inside the loop.
• How many times: Exactly n times, assuming n > 0.

How Execution Grows With Input

As n grows, the number of print operations grows roughly the same for both loops.

Input Size (n)	Approx. Operations
10	About 10 prints
100	About 100 prints
1000	About 1000 prints

Pattern observation: The work grows linearly with n for both loops.

Final Time Complexity

Time Complexity: O(n)

This means the time to run grows directly in proportion to the input size.

Common Mistake

[X] Wrong: "do-while loops always run slower because they run the loop body before checking the condition."

[OK] Correct: Both loops run the body about n times, so their time grows the same way; the difference is only when the condition is checked.

Interview Connect

Understanding how loops grow with input helps you write efficient code and explain your choices clearly.

Self-Check

What if the loop body had a nested loop running n times inside? How would the time complexity change?