1. Conditional Loops

2. Objectives:
Understand the semantics and learn the Java syntax pertaining to Conditional Loops
Be able to create a conditional loop based on problem circumstances

3. The Conditional Loop
A conditional loop repeats a block of code indefinitely until some condition is no longer met

4. condition is any boolean expression, like an if
The conditional Loop
while (test condition is true) {
statement1;
statement2;
...
statement; }
The body of the loop
condition is any boolean expression, like an if
The code inside will NEVER be executed unless the test condition is true.
This implies, it will never STOP executing as long as the test condition is true

5. The conditional Loop (cont’d)
Example: Continually ask the user for a number until it is in range
import java.util.Scanner;
static Scanner input = new Scanner(System.in);
public static void main(String [] args) {
int num = 0;
//Keep asking for input until the number is between 1 and 10
while (num < 1 || num > 10)
System.out.println(“Enter a number between 1 and 10, inclusive”);
num = Integer.parseInt(input.nextLine()); }
System.out.println(“w00t, you entered a valid number”);

6. private static void main(String[] args){
Rectangle[] obstacles = new Rectangle[3];
obstacles[0] = new Rectangle(0,0,50,100);
obstacles[1] = new Rectangle(10,90,50,80);
obstacles[2] = new Rectangle(200,10,100,30);
Rectangle player = new Rectangle(10,10,40,100);
int obsIndex = 0;
while(obsIndex < 3)
if (BoxBox(player,obstacles[obsIndex]) == true)
//obsIndex will hold the index of the
//obstacle that collided
break; }
obsIndex++;
//If obsIndex is 3, it fully looped without colliding
if (obsIndex < 3)
System.out.println(“Collision found at: “ + obsIndex);
private boolean BoxBox(Rectangle box1, Rectangle box2)
if (box1.x + box1.width < box2.x ||
box1.x > box2.x + box2.width ||
box1.y + box1.height < box2.y ||
box1.y > box2.y + box2.height)
return false;
return true;
Exiting a Loop Early
Sometimes it may be necessary to exit a loop early and go to the rest of the code
You can do this by using the keyword break
Example: A program that does box-to-box collision between a player and three obstacle rectangles. It stops as soon as it finds one, to not waste CPU