Do..While loop in java is similar to while loop except do-while loop evaluates its condition expression after the execution of loop code block. Do..While loop in java is used to execute a block of code multiple times.
Do while loop ensures that that code block inside do-while body will execute atleast once.
As the condition expression check is after do-while loop code block, it first execute it's code block and then decide whether to iterate again or not.
do {
// do while loop code block
statement(s);
} while(condition);
Control flow of do-while loop
condition expression is a boolean expression which gets evaluated every time after loop code block. Here is the description of do..while loop control flow.
- First of all, do-while loop code block gets executed.
- do-while loop evaluates condition expression inside the parenthesis.
- If condition expression evaluates to true, then do-while loop code block gets executed.
- condition expression is evaluated again.
- This execution process continues until the condition expression evaluates false.
- Whenever condition expression evaluates to false, do-while loop terminates and control goes to the next statement after the do-while loop.
Here is the flowchart of do-while loop execution
Java Do-While Loop Example Program
public class DoWhileLoop {
public static void main(String[] args) {
int count = 0;
do {
System.out.println(count);
count++;
} while(count < 5);
}
}
Output
0 1 2 3 4
In above program, do-while loop executes code block 5 times, till the time the value of count variable is less than 5. With each iteration the value of count variable is incremented. As soon as value of count becomes 5 do-while loop terminates.
Here are the execution steps for each iteration of do-while loop in above program.
| Iteration Count | Variable | Do-While Code Block | Condition(count < 5) |
|---|---|---|---|
| 1 | count = 0 | Print count(0). count is incremented to 1 |
true |
| 2 | count = 1 | Print count(1). count is incremented to 2 |
true |
| 3 | count = 2 | Print count(2). count is incremented to 3 |
true |
| 4 | count = 3 | Print count(3). count is incremented to 4 |
true |
| 5 | count = 4 | Print count(4). count is incremented to 5 |
false |
- Unlike for loop, there is no initialization and update statements.
- The condition in do-while loop is a boolean expression.
- We can also use infinite do while loops like while(1), which will never terminate. You should add terminating condition using break statement in order to terminate infinite do-while loop.
For Example:
do {
if(....){
break;
}
} while(1); - Opening and Closing braces are not required for single statement inside do while loop code block.
For Example:
do
statement;
while(condition);