Loop controls in Java

Loop control is one of java's language feature like any other programming language feature, which allows to execute written statements in loop body to execute sequentially those many time until loop condition not get satisfied.

Java programming language provides four types of loop controls or constructs.

1. while loop
2. do...while loop
3. for loop
4. for-each loop

we will see all those constructs with example one by one.

while loop

• while loop execute the statements or group of statements multiple times unless given loop condition not fail
• while loop checks for condition first then execute statements in loop body 
• repeat above step till condition satiesfied

see, below code snippet explains while loop:

	public static void main(String[] args) {
	int index = 1;
	while (index <= 5) { //check for condition first
	System.out.println("Index:" + index);
	index = index + 1; //increase index otherwise loop will never end
	}
	}

view raw LoopTest.java hosted with ❤ by GitHub

Output:

	Index:1
	Index:2
	Index:3
	Index:4
	Index:5
	Process finished with exit code 0

view raw gistfile1.txt hosted with ❤ by GitHub

do...while loop

• do...while loop checks for condition later executing body of loop
• unlike while loop it does not check condition first, this is one of major difference between do--while and while loop
• like while loop, it executes loop body until condition not failed
• in failure condition scenario, do...while loop executes at-least once.

see, below code snippet:

	public static void main(String[] args) {
	int index = 1;
	do {
	System.out.println("Index:" + index);
	index = index + 1; //increase index otherwise loop will never end
	} while (index <= 5);//check for condition later
	}

view raw LoopTest.java hosted with ❤ by GitHub

Output:

	Index:1
	Index:2
	Index:3
	Index:4
	Index:5
	Process finished with exit code 0

view raw gistfile1.txt hosted with ❤ by GitHub

for loop

• for loop is useful when you know how many times the loop body to be executed.
• this loop construct allows you to compactly define or initialise loop condition then define how long condition will stay true and condition update statement
• unlike do...while loop for loop checks for condition after condition initialisation, then execute body and then execute condition update statement
• repeat above step until condition stays true.

see, below code snippet:

	public static void main(String[] args) {
	for(int i = 1 ; i <= 5 ; i++){ //condition <initialisation> ; <condition validation> ; <condition update>
	System.out.println("Index:"+i); //execute body
	}
	}

view raw LoopTest.java hosted with ❤ by GitHub

Output:

	Index:1
	Index:2
	Index:3
	Index:4
	Index:5
	Process finished with exit code 0

view raw gistfile1.txt hosted with ❤ by GitHub

for each loop

• for each loop is advance loop in java.
• it introduced to improve code readability and beautify code.
• this loop iterates over array and Iterable type objects
• one can nest for each loop inside for each loop
• for each is not useful for filtering list or array.
• it travers list sequentially without index to access elements

see, below code snippet:

	public static void main(String[] args) {
	int[] idArray = {1, 2, 3, 4, 5}; //traverse over array
	for (int id : idArray) { //read it as: for each id in idArray
	System.out.println("Id:" + id);// print id
	}
	//traverse over list of String objects
	List<String> dictonary = Arrays.asList("John", "Ana", "George", "Sara");
	for (String name : dictonary) { //read it as: for each name in dictonary
	System.out.println("Name:" + name); // print name
	}
	}

view raw LoopTest.java hosted with ❤ by GitHub

Output:

	Id:1
	Id:2
	Id:3
	Id:4
	Id:5
	Name:John
	Name:Ana
	Name:George
	Name:Sara
	Process finished with exit code 0

view raw gistfile1.txt hosted with ❤ by GitHub