Decision Making

There are three types of decision making statements They are:

• If statement.
• If _Else statement
• Switch Statement 

1.The if Statement:

An if statement consists of a Boolean expression followed by one or more statements.  

Syntax:

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

 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. 

Example:

public class Test 

 {

   public static void main(String args[]){
      int x = 10;

      if( x < 20 ){
         System.out.print("This is if statement");
      }
   }
}

This would produce the following result:

This is if statement

2.The 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
}

Example:

 public class Test {

   public static void main(String args[]){
      int x = 30;

      if( x < 20 ){
         System.out.print("This is if statement");
      }else{
         System.out.print("This is else statement");
      }
   }
}

This would produce the following result:

This is else statement

3.The 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 through to 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.
  
  
  

  Example:

  public class Test {
  
     public static void main(String args[]){
        //char grade = args[0].charAt(0);
        char grade = 'C';
  
        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);
     }
  }

  Compile and run above program using various command line arguments. 
  This would produce the following result:
  
  

  $ java Test
  Well done
  Your grade is a C
  $

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