Difference between while and do–while in Java - Visual Side-by-Side Comparison

Concept Flow - Difference between while and do–while

Start

↓

Check condition

|Yes↓

Execute loop body

↓

Repeat Check

No↓

Exit

↓

Start

↓

Execute loop body

↓

Check condition

|Yes↓

Repeat Execute

No↓

Exit

The while loop checks the condition before running the code inside, so it may skip the loop if false. The do-while loop runs the code once before checking the condition, so it always runs at least once.

Execution Sample

Java

int i = 0;
while (i < 2) {
  System.out.println(i);
  i++;
}

int j = 0;
do {
  System.out.println(j);
  j++;
} while (j < 2);

This code prints numbers 0 and 1 using both while and do-while loops, showing how they run.

Execution Table

Step	Loop Type	Variable	Condition	Condition Result	Action	Output
1	while	i=0	i < 2	true	Print i=0, i++	0
2	while	i=1	i < 2	true	Print i=1, i++	1
3	while	i=2	i < 2	false	Exit loop
4	do-while	j=0	N/A before first run	N/A	Print j=0, j++	0
5	do-while	j=1	j < 2	true	Print j=1, j++	1
6	do-while	j=2	j < 2	false	Exit loop

💡 while loop exits when i=2 because 2 < 2 is false; do-while loop exits after j=2 check fails

Variable Tracker

Variable	Start	After 1	After 2	Final
i	0	1	2	2
j	0	1	2	2

Key Moments - 2 Insights

Why does the do-while loop always run at least once?

Because the do-while loop runs the loop body first before checking the condition, as shown in execution_table rows 4 and 5 where output occurs before condition check.

Can the while loop skip running the loop body entirely?

Yes, if the condition is false at the start, the while loop does not run the body at all. This is shown in execution_table row 3 where the condition fails and loop exits.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of i when the while loop condition becomes false?

A 1

B 2

C 0

D 3

💡 Hint

Check execution_table row 3 where condition i < 2 is false.

At which step does the do-while loop print the first output?

A Step 6

B Step 5

C Step 4

D Step 3

💡 Hint

Look at execution_table row 4 where do-while prints j=0 before condition check.

If the initial value of i was 3, what would happen in the while loop?

A It would not print anything

B It would print 3 once

C It would print 0 and 1

D It would run forever

💡 Hint

Since while checks condition before running, if i=3 and condition is i<2, it fails immediately (see key_moments about while loop skipping).

Concept Snapshot

while loop:
- Checks condition first
- May skip loop if false
- Syntax: while(condition) { body }

do-while loop:
- Runs body once before checking
- Always runs at least once
- Syntax: do { body } while(condition);

Full Transcript

This lesson shows the difference between while and do-while loops in Java. The while loop checks the condition before running the loop body, so if the condition is false at the start, the loop body never runs. The do-while loop runs the loop body once before checking the condition, so it always runs at least once. The execution table traces variable values and outputs step-by-step for both loops. Key moments clarify why do-while always runs once and while can skip entirely. The quiz tests understanding of variable values and loop behavior from the execution steps.