Java 11 Developer Certification - Java Loop Structures

October 25, 2020

What we are covering in this lesson

What is a loop
Types of loop in java
Loop Syntax
Code examples

What is a loop

A loop causes a set of instructions to execute repeatedly, until a certain condition is met, or the element list being looped through is exhausted. Loops are very fundamental to programming.

Types of loop in java

Java supports 3 types of loops statements

for loop
while loop
do while loop

while and do while loops are quite similar in nature, the only difference being that do while will always execute atleast once before checking the condition in while. while loop keeps executing until the conditions are met, and the iteration counters are done within the code block. for loop is mostly used when the number of iterations needed are known beforehand.

Loop Syntax

while loop Syntax

while (expression) {
    statement1;
    statement2;
    statementn;
}

do while loop syntax

do {
    statement1;
    statement2;
    statementn;
} while (expression);

for loop syntax

//traditional for loop
for (initialization; termination condition; increment/decrement) {
    statement1;
    statement2;
    statementn;
}

OR

//advanced for loop
for (formalParameter : expression) {
    statement1;
    statement2;
    statementn;
}

Code examples

Lets have a look at a simple code snippet to understand these loops and the basic difference between them.

package maxCode.online.Loops;

public class WhileDoWhileLoops {
	public static void main(String[] args) {
		// set up a boolean that while be the condition for our while loop
        boolean isTrue = true;
        int iterations = 0;

        while (isTrue) {
            //if (++iterations > 2) break;
            //if (iterations == 2) continue;
        	if (++iterations > 2) {
        		isTrue = false;
        	}
            System.out.println("Iteration: " + iterations + ", isTrue = " + isTrue);
            
            //Nested do while loop
            do {
                System.out.println("Do while start," + " iteration = " + iterations);
                System.out.println("Do while end");
            } while (iterations < 0);
        }  // end while
	}
}

Output

Iteration: 1, isTrue = true
Do while start, iteration = 1
Do while end
Iteration: 2, isTrue = true
Do while start, iteration = 2
Do while end
Iteration: 3, isTrue = false
Do while start, iteration = 3
Do while end

The above code clearly indicates that the while loop keeps on executing until the condition isTrue remains true. As soon as it is false, the next loop does not execute. Also, keep in mind that the line where this condition is made false. The lines below that will also get executed and it only in the next loop condition when the condition will be evaluated false. The condition variable isTrue needs to be manipulated within the loop only, such that the statements execute for as much time as we want.

The do while loop always executes only once because the condition we have specified will never stand true. Unlike while, it will always execute once regardless of the condition.

In case of for loop, we have 2 types of loop. One is the basic traditional for loop and the other one is a advanced loop. The tradional one has a initialization expression, termination condition and increment/decrement and all these are separated by semi-colon.

The init expression initializes the for loop, only executed once when the loop begins, prior to the termination condition being evaluated for the first time. It is optional.

The termination condition specifies the condition till when the loop must run. As soon as this condition is false, the loop terminates. This expression is also optional and its default value is true. It is evaluated prior to each loop iteration.

The increment/decrement value is invoked after each iteration of the loop. This is also optional.

As you can see, all the 3 parameters are optional, so if we have a for (;;), it will create an infinite loop, and anything within it will execute forever!

The advanced for loop is used to loop through an iterable set of data and so some operation. The type of expression must be iterable or an array type. If we want an iterator index in advanced loop, we have to create and maintain it ourselves.

package maxCode.online.Loops;

public class ForLoop {
	public static void main(String[] args) {
		System.out.println("-----------  TRADITIONAL FOR LOOP -----------");
		String[] strArray = {"1", "2", "3"};
		for (int i = 0; i < strArray.length; i++) {
			System.out.println("Argument " + (i + 1) + " = " + strArray[i]);
		}

		System.out.println("-----------  ENHANCED FOR LOOP -----------");
		int i = 0;	//iterator index, you have to maintain it yourself
		for (String arg: strArray) {
			System.out.println("Argument " + (++i) + " = " + arg);
		}
	}
}