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Flow Control in Java - if, if-else-if, if-else

 Using Flow control in Java to control the flow of execution of the program based on certain conditions. It we can select a number of statements to be executed after a condition is met.

In Java we use the following to control the flow of the program:

  •     if
  •     if-else
  •     if-else-if
  •     Nested if etc.

1. if statement:
        It is used to decide whether a certain statement or block of code should be executed or not based on a condition. If the condition is true then the block of statement should be executed otherwise not.

Syntax:
    if(condition){
            //statements will execute only if condition is true
    }

For Example:
        int date = 1;
        int month = 1;
        if(date==1 && month ==1){
                System.out.println("Happy New Year");
        }

        Output: Happy New Year

2. if-else statement:
        It is used to decide whether a certain statement or block of code should be executed or not based on a condition. But what if the condition is false and we need to execute some statements in that case. We can use else part to execute a block of statements when the condition is false.

Syntax:
    if(condition){
            //statements will execute only if condition is true
    }else{
            //statements will execute only if codition is false
    }

For Example: To check whether a given number is even or odd
        int num = 13;
        if(num%2==0){
                System.out.println("Number is even");
        }
        else{
                System.out.println("Number is odd");
        }

        Output: Number is odd

3. if-else-if statement:
        When a user is given multiple conditions and based on those conditions they have to execute a certain block of code.
Here if statements will execute from top to bottom until a condition is met. Once a condition is met it will execute the statements and bypass rest of if -else blocks. 
       - In case if none of the condition is true then the final else block will be executed.

Syntax:
    if(condition-one){
            //statements will execute only if condition-one is true
    }else if(condition-two){
            //statements will execute only if codition-two is true
    }else{
            //statements will execute if none of the above conditions are true
    }

Note: There can be any number of else-if block

For Example:
        int percentage = 65;
        if(precentage>90 && percentage<=100){
                System.out.println("Grade A");
        }
        else if(precentage>80 && percentage<=90){
                System.out.println("Grade B");
        }
        else if(precentage>70 && percentage<=80){
                System.out.println("Grade C");
        }
        else if(precentage>60 && percentage<=70){
                System.out.println("Grade D");
        }else{
                System.out.println("Fail");
        }

        Output: Grade D

4. Nested if statement:
        When if statement is used inside another if statement then it is called nesting of if statements.

Syntax:
    if(condition-one){
            //statements will execute only if condition-one is true
             if(condition-two){
                //statements will execute only if condition is true
            }
    }

For Example: Check whether a given year is Leap Year or not
    int year = 1900;
    boolean leapYear = false;

    // if the year is divided by 4
    if (year % 4 == 0) {

      // if the year is century
      if (year % 100 == 0) {

        // if year is divided by 400
        // then it is a leap year
        if (year % 400 == 0)
          leapYear = true;
        else
          leapYear = false;
      }
      
      // if the year is not century
      else
        leapYear = true;
    }
    
    else
      leapYear = false;

    if (leapYear)
      System.out.println(year + " is a leap year.");
    else
      System.out.println(year + " is not a leap year.");
  }
}

Output: 1900 is not a leap year.




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