Java do-while Loop
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do {
// runs at least once
} while (condition);Basic do-while Loop
A do-while loop runs its body first and only checks the condition afterward, at the bottom -- the opposite order from a regular while loop, which tests the condition before ever entering the body.
Example: Basic do-while Loop
// Define the class Main
public class Main {
// Program entry point: the JVM starts running here
public static void main(String[] args) {
// Declare i and set it to 0
int i = 0;
// do-while: run the body once, then repeat while the condition is true
do {
// Print a line to the console
System.out.println(i);
i++;
} while (i < 3);
}
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Guaranteeing One Run
This ordering guarantees the loop body executes at least once even if the condition would have been false from the very start, which a standard while loop can never guarantee.
Example: Guaranteeing One Run
// Define the class Main
public class Main {
// Program entry point: the JVM starts running here
public static void main(String[] args) {
// Declare i and set it to 10
int i = 10;
// do-while: run the body once, then repeat while the condition is true
do {
// Print a line to the console
System.out.println("Runs once: " + i);
} while (i < 5);
}
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Incrementing Variables inside Loop
You must modify the variable checked in your loop condition somewhere inside the do-while block, or the loop keeps repeating forever since the condition never has a chance to become false.
Example: Incrementing Variables inside Loop
public class Main {
public static void main(String[] args) {
int i = 0;
do {
System.out.println(i);
i++; // without this, the loop never ends
} while (i < 3);
}
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Simulated Interactive Runs
Just like any loop, you must update the variable the condition depends on somewhere inside the do-while body -- if that variable never changes, the loop will run forever once it starts.
Example: Simulated Interactive Runs
// Define the class Main
public class Main {
// Program entry point: the JVM starts running here
public static void main(String[] args) {
// Declare attempts and set it to 0
int attempts = 0;
// do-while: run the body once, then repeat while the condition is true
do {
attempts++;
// Print a line to the console
System.out.println("Attempt " + attempts);
} while (attempts < 3);
}
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Nested do-while Loops
Nesting a do-while inside another do-while lets you build structures like a grid where each outer iteration always processes at least one full pass of the inner loop, guaranteed.
Example: Nested do-while Loops
// Define the class Main
public class Main {
// Program entry point: the JVM starts running here
public static void main(String[] args) {
// Declare row and set it to 1
int row = 1;
// do-while: run the body once, then repeat while the condition is true
do {
// Declare col and set it to 1
int col = 1;
// do-while: run the body once, then repeat while the condition is true
do {
// Print to the console without a newline
System.out.print(col + " ");
col++;
} while (col <= 3);
// Print a line to the console
System.out.println();
row++;
} while (row <= 2);
}
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- Forgetting the semicolon after
while (condition)in a do-while, which is a compile error. - Expecting the body to be skipped when the condition is false, when do-while always runs once.
- Declaring a variable inside the body and using it in the
whilecondition, where it is out of scope.
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