Core Java - Control Flow Statements

The execution of the program can be controlled by the programmer in a way that programmer can decide either which instruction has to be executed and ignored by using same statement are called as control flow statements.We are having two types of control flow statements such as

Decision making statements

Looping statements

1. Decision making statements:

We use decision making statement to decide which instruction need to be executed and ignored,We are having following decision making statements

if-else

else-if ladder

switch

nested if

break

returntype

a. if condition:
We go for if statement, if we are having only one option.If the Boolean expression evaluates to true, then the block of code inside the if statement will be executed. If not,the first set of code after the end of the if statement(after the closing curly brace) will be executed.
Syntax:

    if(Boolean){
        //Statements will execute if the Boolean expression is true
    }

Eg:

    public class Test{
        public static void main(String args[]){
        int x =10;
        if( x < 20){
        System.out.print("This is if statement condition...");
        }
        }
    }

Output:
This is if statement condition...

b. if-else Statement:
An if statement can be followed by an optional else statement, which executes when the Boolean expression is false.
Syntax:

    if(Boolean_expression){
        //Executes when the Boolean expression is true
        }else{
        //Executes when the Boolean expression is false
    }

Eg:

    public class Test{
        public static void main(String args[]){
        int x =30;
        if(x < 20 ){
        System.out.print("This is if statement condition...");
        }else{
        System.out.print("This is else statement condition...");
        }
        }
    }

Output:
This is else statement condition...

c. else-if ladder:
An condition we go for else-if ladder if we are having more than two option where execution of one option is mandatory. When using if, else if , else statements there are few points to keep in mind.

An if can have zero or one else's and it must come after any else if's.

An if can have zero to many else if's and they must come before the else.

Once an else if succeeds, none of the remaining else if's or else's will be tested.

Syntax:

    if(Boolean_expression1){
        //Executes when the Boolean expression 1 is true
        }elseif(Boolean_expression2){
        //Executes when the Boolean expression 2 is true
        }elseif(Boolean_expression3){
        //Executes when the Boolean expression 3 is true
        }else{
        //Executes when the none of the above condition is true.
    }

Eg:

    public class Test{
        public static void main(String args[]){
        int x =30;
        if( x ==10){
        System.out.print("Value of X : 10");
        }elseif( x ==20){
        System.out.print("Value of X : 20");
        }elseif( x ==30){
        System.out.print("Value of X : 30");
        }else{
        System.out.print("This is else statement");
        }
        }  
    }

Output:
Value of X : 30

d. switch statement:
A switch statement allows a variable to be tested for equality against a list of values. Each value is called a case, and the variable being switched on is checked for each case.
Syntax:

    switch(expression){
        case value :
        //Statements
        break;//optional
        case value :
        //Statements
        break;//optional
        //You can have any number of case statements.
        default://Optional
        //Statements
    }

The following rules apply to a switch statement:

The variable used in a switch statement can only be a byte, short, int, or char.

You can have any number of case statements within a switch. Each case is followed by the value to be compared to and a colon.

The value for a case must be the same data type as the variable in the switch and it must be a constant or a literal.

When the variable being switched on is equal to a case, the statements following that case will execute until a break statement is reached.

When a break statement is reached, the switch terminates, and the flow of control jumps to the next line following the switch statement.

Not every case needs to contain a break. If no break appears, the flow of control will fall throughto subsequent cases until a break is reached.

A switch statement can have an optional default case, which must appear at the end of the switch. The default case can be used for performing a task when none of the cases is true. No break is needed in the default case.

Eg:

    public class Test{
        public static void main(String args[]){
        char grade = args[0].charAt(0);
        switch(grade){
        case'A':
        System.out.println("Excellent!");
        break;
        case'B':
        case'C':
        System.out.println("Well done");
        break;
        case'D':
        System.out.println("You passed");
        case'F':
        System.out.println("Better try again");
        break;
        default:
        System.out.println("Invalid grade");
        }
        System.out.println("Your grade is "+ grade);
        }
    }

Output:
java Test a

Invalid grade java Test A

java Test C

e. Nested if statement:
It is always legal to nest if-else statements which means you can use one if or else if statement inside another if or else if statement.
Syntax:

    if(Boolean_expression1){
        //Executes when the Boolean expression 1 is true
        if(Boolean_expression2){
        //Executes when the Boolean expression 2 is true
        }
    }

Eg:

    public class Test{
        public static void main(String args[]){
        int x =30;
        int y =10;
        if( x ==30){
        if( y ==10){
        System.out.print("X = 30 and Y = 10");
        }
        }
    }

Output:
X =30 and Y =10

f. break statement:
It is a control transfer statement as well as keyword.It has to be used only inside switch (or) loop statement. The job of break is to take control of the main thread outside the switch block.
Syntax:

    break;

Eg:

    public class Test{
        public static void main(String args[]){
        int[] numbers ={10,20,30,40,50};
        for(int x : numbers){
        if(x ==30){
        break;
        }
        System.out.print( x );
        System.out.print("\n");
        }
        }
    }

Output :
10
20

g. return statement
In java, every method should return the data after performing the task.Returntype in a method will specify what type of data has to be returned back. In java, in place of returntype we can use primitive datatype, non-primitive datatype and void.

2. Looping statements

We use looping statement to execute the java instruction repeatedly (or) again and again. We are having following looping statements available such as

while

do-while

for loop

nested for loop

foreach loop

a.while loop :
A while loop is a control structure that allows you to repeat a task a certain number of times.
Syntax:

    while(Boolean_expression){
    //Statements
    }

When executing, if the boolean_expression result is true, then the actions inside the loop will be executed. This will continue as long as the expression result is true.
Here, key point of the while loop is that the loop might not ever run. When the expression is tested and the result is false, the loop body will be skipped and the first statement after the while loop will be executed.
Eg:

    public class Test{
        public static void main(String args[]){
        int x =10;
        while( x < 20){
        System.out.print("value of x : "+ x );
        x++;
        System.out.print("\n");
        }
        }
    }

Output:
value of x :10
value of x :11
value of x :12
value of x :13
value of x :14
value of x :15
value of x :16
value of x :17
value of x :18
value of x :19

b. do while loop:
A do...while loop is similar to a while loop, except that a do...while loop is guaranteed to execute at least one time.
Syntax:

    do{
    //Statements
    }while(Boolean_expression);

Eg:

    public class Test{
        public static void main(String args[]){
        int x =10;
        do{
        System.out.print("value of x : "+ x );
         x++;
        System.out.print("\n");
        }while( x < 20);
        }
    }

Output:
value of x :10
value of x :11
value of x :12
value of x :13

c. for loop statement:
A for loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times.A for loop is useful when you know how many times a task is to be repeated.
Syntax:

    for(initialization;Boolean_expression; update){
    //Statements
    }

Here is the flow of control in a for loop:

The initialization step is executed first, and only once. This step allows you to declare and initialize any loop control variables. You are not required to put a statement here, as long as a semicolon appears.

Next, the Boolean expression is evaluated. If it is true, the body of the loop is executed. If it is false, the body of the loop does not execute and flow of control jumps to the next statement past the for loop.

After the body of the for loop executes, the flow of control jumps back up to the update statement. This statement allows you to update any loop control variables. This statement can be left blank, as long as a semicolon appears after the Boolean expression.

The Boolean expression is now evaluated again. If it is true, the loop executes and the process repeats itself (body of loop, then update step,then Boolean expression). After the Boolean expression is false, the for loop terminates.

Eg:

    public class Test{
        public static void main(String args[]){
        for(int x =10; x < 20; x = x+1){
        System.out.print("value of x : "+ x );
        System.out.print("\n");
        }
        }
    }

Output:
value of x :10
value of x :11
value of x :12
value of x :13
value of x :14
value of x :15
value of x :16
value of x :17
value of x :18
value of x :19

d. for each loop statement(Enhanced for loop ):
As of Java 5, the enhanced for loop was introduced. This is mainly used for Arrays.
Syntax:

    for(declaration : expression){
    //Statements
    }

Declaration :

The newly declared block variable, which is of a type compatible with the elements of the array you are accessing. The variable will be available within the for block and its value would be the same as the current array element.

Expression :

This evaluates to the array you need to loop through. The expression can be an array variable or method call that returns an array.

Eg:

    public class Test{
        public static void main(String args[]){
        int[] numbers ={10,20,30,40,50};
        for(int x : numbers ){
        System.out.print(x);
        System.out.print(",");
        }
        System.out.print("\n");
        String[] names ={"James","Larry","Tom","Lacy"};
        for(String name : names ){
        System.out.print( name );
        System.out.print(",");
        }
        }
    }

Output:
10,20,30,40,50,
James,Larry,Tom,Lacy,

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