Decisions

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In this section we shall study how we can implement control based on certain conditions.

if

File: if1.java

import java.util.* ;

public class if1
{
    public static void main( String args[] )
    {
        int x1 = 5 ;
        int y1 = 5 ;

        System.out.println( "Enter a value between 1 and 10" ) ;
        Scanner scanner = new Scanner(System.in);

       x1  =  scanner.nextInt(); ;
       if ( x1 <= 5 )
        System.out.println( "Value was less than or equal to 5." ) ;

    //Common error the ; is the empty statement associated
    //if . The  statement following it,  is
    //executed every time.

    if ( x1 > 5 );
      System.out.println( "Value was greater than 5." ) ;
    //Correct way
    if ( x1 > 5 )
      System.out.println( "Value was greater than 5." ) ;

    if ( x1 <= 5 )
     { //Block associate multiple statement with if
       System.out.println( "Inside block 1st line:Value was less than or equal to 5." ) ;
       System.out.println( "Inside block 2nd line: Value was less than or equal to 5." ) ;
     }

    if ( x1 <= 5 )
      System.out.println( "Inside if else: Value was less than or equal to 5." ) ;
    else
      System.out.println( "Inside if else: Value was greater than 5." ) ;

    //Nested if We can have an if statement inside another if
    if ( x1 <= 5 )
     {
       System.out.println( "Inside nested if: Value was less than or equal to 5." ) ;
       if ( x1 <= 3 )
         System.out.println( "Inside nested if: Value was less than or equal to 3." ) ;
       else
         System.out.println( "Inside nested if: Value was between 3 and 5." ) ;
     }
    else
      System.out.println( "Inside if else: Value was greater than 5." ) ;

    //Multi way if else if
    if ( x1 <= 3 )
      System.out.println( "Inside multi-way if: Value was less than or equal to 3." ) ;
    else if ( x1 <= 5 )
      System.out.println( "Inside multi-way if: Value was between 4 and 5." ) ;
    else if ( x1 <= 7 )
      System.out.println( "Inside multi-way if: Value was between 6 and 7." ) ;
    else
      System.out.println( "Inside multi-way if: Value was between 8 and 10." ) ;



    }


} //class
C:\WebSite\Learn\2\Java\decisions>java if1
Enter a value between 1 and 10
3
Value was less than or equal to 5.
Value was greater than 5.
Inside block 1st line:Value was less than or equal to 5.
Inside block 2nd line: Value was less than or equal to 5.
Inside if else: Value was less than or equal to 5.
Inside nested if: Value was less than or equal to 5.
Inside nested if: Value was less than or equal to 3.
Inside multi-way if: Value was less than or equal to 3.

Ternary

The ternary expression is a shorthand form for the "if else" statement.
File: Ternary.java

import java.util.* ;

public class Ternary
{
    public static void main( String args[] )
    {
        int age = 5 ;
        int y1 = 5 ;

        System.out.println( "Enter an age: " ) ;
        Scanner scanner = new Scanner(System.in);
        age  =  scanner.nextInt() ;
        String text = (age < 18) ? "Minor" : "Adult";
        System.out.println( "Person is a " + text ) ;

     //Not necessary to assign to something
     System.out.println(   (age < 18) ?  "Minor"  :
         "Adult"  ) ;

 }


} //class
C:\WebSite\Learn\2\Java\decisions>java Ternary
Enter an age:
12
Person is a Minor
Minor

Exercise 1)

File: Ex1.java
Convert the below "if/else" statement into an equivalent ternary statement.

import java.util.* ;

public class Ex1
{
    public static void main( String args[] )
    {
        int x1 ;


        System.out.println( "Enter a number: " ) ;
        Scanner scanner = new Scanner(System.in);
        x1  =  scanner.nextInt() ;
        if ( x1 < 5 )
          System.out.println("The number is less than 5" ) ;
        else
          System.out.println("The number is greater than 5" ) ;



 }


} //class

Switch

The switch is like a multi-way if else statement. It is more readable and concise and is useful if the statements are small.
File: Switch.java

import java.util.* ;
import java.time.* ;

public class Switch
{
    public static void main( String args[] )
    {
        int x1 = 5 ;
        int y1 = 5 ;

        //System.out.println( "Enter a value between 1 and 10" ) ;
        //Scanner scanner = new Scanner(System.in);

        // Get today's date
        LocalDate today = LocalDate.now();

        // Get the DayOfWeek enum instance
        int dayNo = today.getDayOfWeek().getValue() ;
        System.out.println( "dayNo: " + dayNo ) ;
        String day = "" ;
        switch ( dayNo )
        {
          case 1:
            day = "Monday";
            break;
          case 2:
             day = "Tuesday";
            break;
          case 3:
            day = "Wednesday";
            break;
          case 4:
            day = "Thursday";
            break;
          case 5:
            day = "Friday";
            break;
          case 6:
            day = "Saturday";
          case 7:
            day = "Sunday";
            break;

          }
        System.out.println(  "Today's day is :" +  day ) ;

        //System.out.println( "----" )
        //Without break the control continues
        x1 = 0 ;
        switch ( x1 )
        {
          case 0:
            System.out.println( "Sunday" ) ;
          case 1:
            System.out.println( "Monday" ) ;
          case 2:
            System.out.println( "Tuesday" ) ;
            break;
        }

        System.out.println( "----" ) ;
        //multiple case statements
        //share same code block
        switch ( x1 )
        {
          case 0:
          case 1:
            System.out.println( "x1 is 0" ) ;
            System.out.println( "x1 is 1" ) ;
            break ;
          case 2:
            System.out.println( "x1 is 2" ) ;
            break;
        }


        System.out.println( "----" ) ;
        //default case statement is executed
        //if there is no match.
        x1 = 5 ;
        switch ( x1 )
        {
          case 0:
              System.out.println( "x1 is 0" ) ;
              break ;
          case 1:
            System.out.println( "x1 is 1" ) ;
            break ;
          case 2:
            System.out.println( "x1 is 2" ) ;
            break;
          default:
            System.out.println( "x1 is something else." ) ;
        }

    }


} //class
C:\WebSite\Learn\2\Java\decisions>java Switch
dayNo: 1
Today's day is :Monday
Sunday
Monday
Tuesday
----
x1 is 0
x1 is 1
----
x1 is something else.