What is Loop Control?
There is a probability that we need to execute a block of code multiple times. Therefore, instead of writing the same piece of code multiple times, Java or any other programming language provide loop control statements. There are 3 types of loops in Java:-
- for Loop
- while Loop
- do-while loop
1) For Loop
For loop execute a block of code a number of times . In For loop, we first intialize the variable, check condition and increment/decrement value. It consists of 4 parts:-
Initialization: It is the initial condition which is executed once when the loop starts. Here, we can initialize the variable, or we can use an already initialized variable.
Condition: It is the second condition which is executed each time to test the condition of the loop. It continues execution until the condition is false. It must return boolean value either true or false.
Statement: The statement of the loop is executed each time until the second condition is false.
Increment/Decrement: It increments or decrements the variable value.
Syntax
for(Variable Initialization; Boolean_Expression Test; Increment/Decrement){
//Statements
}
for (int i = 0; i < 5; i++) {
System.out.println(i);
}
Example
public class For_loop {
public static void main(String[] args) {
for(int i=0;i<5;i++)
{
System.out.println("Value of i is: "+i);
}
System.out.println("-----------------------------------------------------");
for (int j=5;j>0;j--)
{
System.out.println("Value of j is: "+j);
}
}
}
Output
Value of i is: 0
Value of i is: 1
Value of i is: 2
Value of i is: 3
Value of i is: 4
--------------------------------------------------------
Value of j is: 5
Value of j is: 4
Value of j is: 3
Value of j is: 2
Value of j is: 1
- Statement 1 sets a variable before the loop starts (int i = 0).
- Statement 2 defines the condition for the loop to run (i must be less than 5). If the condition is true, the loop will start over again, if it is false, the loop will end.
- Statement 3 increases a value (i++) each time the code block in the loop has been executed.
2) While Loop
The Java while loop is used to iterate a part of the program several times. If the number of iteration is not fixed, it is recommended to use while loop.
Syntax
while(Boolean_Expression Test)
{
//Statements
}
Example
public class While_Loop {
public static void main(String[] args) {
int i=0;
while (i<15)
{
i=i+3;
System.out.println("Value of i is :"+i);
}
System.out.println("--------------------------------------------");
// In this case, value of j will be 0 and it will run infinite times.
int j=0;
while(j<15)
System.out.println("Value of j is :"+j);
{
j=j+3;
}
}
}
Output
Value of i is :3
Value of i is :6
Value of i is :9
Value of i is :12
Value of i is :15
--------------------------------------------
Value of j is :0
Value of j is :0
3) What is the difference between for and while loop?
While loop is used in situations where we do not know how many times loop needs to be executed beforehand.
For loop is used where we already know about the number of times loop needs to be executed. Typically for a index used in iteration.
4) Do While Loop
A do while loop is exactly similar to while loop and the only difference between two is that the do while loop executes the statement at least one time. As it start with do keyword and the boolean expression appears at the end of the loop.
Syntax
do{
//Statements
}while(Boolean_Expression Test);
Example
public class Do_While_Loop {
public static void main(String[] args) {
int i=0;
do
{
i=i+3
System.out.println("Value of i is :"+i);
}
while (i<15);
System.out.println("-------------------------------------------");
//Condition is already satisfied, still 1 round of execution
int j=10;
do
{
j=j+2;
System.out.println("Value of i is :"+j);
}
while (j<10);
}
}
Output
Value of i is :3
Value of i is :6
Value of i is :9
Value of i is :12
Value of i is :15
-----------------------------------------------------
Value of i is :12
5) What is the difference between While Loop or do- while loop?
1) while loop first check the condition then enter the body whereas do-while first enter the body and then check the condition.
2) while is a entry controlled loop whereas do-while is an exit controlled loop.
3) In while, condition comes before the body whereas in do-while, condition comes after the body.
6) Enhanced for loop in Java
It is used to traverse array or collection in java. It is easier to use than simple for loop because we don’t need to increment value and use subscript notation.
It works on elements basis, not index. It returns element one by one in the defined variable.
Syntax
for (data_type variable: array_name)
Example
public class Enhanced_Loop {
public static void main(String[] args) {
// Array of String storing days of the week
String days[] = { "Mon", "Tue", "Wed", "Thr", "Fri", "Sat", "Sun"};
// Enhanced for loop, this will automatically iterate on the array list
for (String dayName : days)
{
System.out.println("Days ==> "+ dayName);
}
System.out.println("");
// Normal for loop
for (int i=0; i < days.length; i++) {
System.out.println("Days ==> "+ days[i]);
}
}
}
Output
Days ==> Mon
Days ==> Tue
Days ==> Wed
Days ==> Thr
Days ==> Fri
Days ==> Sat
Days ==> Sun
Days ==> Mon
Days ==> Tue
Days ==> Wed
Days ==> Thr
Days ==> Fri
Days ==> Sat
Days ==> Sun
QA Automation Expert 
