Switch

Format:

[sourcecode language=”java” toolbar=”false”]
switch (x){
case 1:
System. out .println(x);
break ;
case 2:
System. out .println(x);
break ;
default :
System. out .println(x);
break ;
}
[/sourcecode]

The switch argument:

• Must be a value that can become an int implicitly:
  • byte, short, char, int;
  • Byte, Short, Character, Integer;
  • an enum.
• Can be:
  • a variable reference,
  • a method call that returns a legal type,
  • an enum
• Cannot be:
  • boolean, long, float or double
  • Boolean, Long, Float or Double

The case constant:

• must be a compile time constant
  • a constant variable,
  • a final variable,
  • an enum.
• Must evaluate to the same type as the switch argument type.

Example with enums:

[sourcecode language=”java” toolbar=”false”]
public enum Day { SUNDAY, MONDAY , TUESDAY , WEDNESDAY}

Day d = Day.MONDAY;

switch (d){
case MONDAY :
System. out .println(x);
break ;
case TUESDAY :
System. out .println(x);
break ;
default :
System. out .println(x);
break ;
}
[/sourcecode]

Example: where the switch argument is an int and the case argument are capable of implicitly becoming and int.

[sourcecode language=”java” toolbar=”false”]
final char a = ‘a’;
final char b = 1;
final char c = ‘\u9874’;

int x = 1;

switch (x){
case a: System. out.println( "1" ); break ;
case 1: System. out.println( "1" ); break ;
case ‘\u9874’ : System. out.println( "1" ); break ;
}
[/sourcecode]

Example: where the switch arguement is generated by a for loop:

[sourcecode language=”java” toolbar=”false”]
public static void main(String args[])
{
for ( int i = 0 ; i < 3 ; i++)
{
switch (i)
{
case 0: System. out.println(i); break;
case 1: System. out.println(i); break;
case 2: System. out.println(i); break;
}
}
}
[/sourcecode]

Example of the use of a post increment in the switch clause.

[sourcecode language=”java” toolbar=”false”]
public static void main(String args[])
{
int i;
LOOP: for (i=0; i<5;i ++)
{
switch (i++)
{
case ‘0’ : System. out.println( "A" );
case 1: System. out.println( "B"); break LOOP;
case 2: System. out.println( "C"); break ;
case 3: System. out.println( "D"); break ;
case 4: System. out.println( "E");
case ‘E’ : System. out.println( "F" );
}
}
}
[/sourcecode]

1. the value of i used in the switch statement is 0,
2. there is no case 0,
3. the value of i is increased by 1 (i=1),
4. and the next loop of the for is executed, the for increases the loop by 1 (i=2)
5. the switch uses value of i = 2, and selects case 2.
6. the value of i is increased by 1 (i=3) and the value 3 is printed,
7. and the next loop of the for is executed, the for increases the loop by 1 (i=4)
8. the switch uses value of i = 4, and selects case 4.
9. the value of i is increased by 1 (i=5) and the value 5 is printed,
10. the loop finishes

The value of i is used to select the case before it is increased.

Illegal:

• Duplicate cases:
  • final char a = ‘a’;
  • and two cases
    • case a:
    • case 97:
  • These are duplicated cases, because their integer value is the same.
• Cases larger than the switch argument:
  • A switch argument of type byte and a case argument of 128,
  • 128 is larger than the maximum number allowed in a byte.

The default:

• only one allowed
• can be placed anywhere
• does not have an argument
• acts just like a case statement

Example of valid switch statements:

[sourcecode language=”java” toolbar=”false”]

switch (1) { default : break; }

[/sourcecode]

An empty switch block is a valid construct

NOTE: A variable defined within a switch statement has scope through out the block:

[sourcecode language=”java” toolbar=”false”]
switch(0)
{
case 0 :
boolean b = false; //1
break;

case 1 :
b = true; //2
break;
}
[/sourcecode]

b is defined at line 1 and used after at line 2, this is not out of scope because it is within the {}. However b MUST be defined before it is used -> normal rules.