Enum Question

[peter kosmas]

Hello ...
This piece of code is from the source question Bank ! the Sun Microsystems scjp training they have with mock exams
i would like to know why c.equals("BLUE") does not work and Other.Colors.RED.equals(c) works fine
what's the catch .(The reference did not give me a good clue just one line saying Lines 18 and 20 are correct
here is the code

  package scjp;

 class Other{
   enum Colors{RED, GREEN, BLUE, YELLOW};
 }

 class UseEnums{
   public static void main(String [] args) {
    for( Other.Colors c : Other.Colors.values()) {
    
        if (Other.Colors.RED.equals(c))
         System.out.print("red ");
       if (c == Other.Colors.GREEN)
         System.out.print("green ");
      if (c.equals("BLUE"))
         System.out.print("blue ");
 } } }
  

and here is what it displays
red green

[Sebastian Janisch]

It is because you are comparing a String with an instance of your enum, which can never be the same!

Note that every element of an enum has it's own instance.

like this:

enum Colors {
    RED, GREEN, BLUE
}

public static void main(String... args) {
    Colors red = Colors.RED;
    Colors green = Colors.GREEN;
    Colors blue = Colors.BLUE;  
  
    Colors red2 = Colors.RED;
}

whenever you assign an enum to somewhere, you are invoking it's constructor, hence get a fresh instance.

[pete stein]

Sebastian Janisch wrote:whenever you assign an enum to somewhere, you are invoking it's constructor, hence get a fresh instance.

Can you show a source for this statement? I'm not sure that I agree with it.

[Sebastian Janisch]

pete stein wrote:
Can you show a source for this statement? I'm not sure that I agree with it.

The fact that you can pass arguments to an enum supports this statement. Check this code...

public enum EnginePowers {
    KW(1d), PS(.135d), HP(1.34d);

    private double multiplicator;

    private EnginePowers (double multiplicator) {
        this.multiplicator = multiplicator;
    }

    public double getMultiplicator() {
        return multiplicator;
    }
}

I have to correct myself though. A call to the same Enum element will always return the same instance. My bad

[Nitish Bangera]

if (c.equals("BLUE"))
System.out.print("blue ");

How can this be possible??? as far as i think "BLUE" will be a String object in the String pool whereas the c is an enum reference variable which will have RED,GREEN,BLUE and YELLOW enum constants. The two cannot be meaningful equivalents at all because enum constants are not a string.

  class Other{  
   enum Colors{
RED("RED"), GREEN("GREEN"), BLUE("BLUE"), YELLOW("YELLOW");
Colours(String name){this.name = name;}
private String name;
public String getName(){return name;}
};  
 }  

then you can compare

c.getName().equals("BLUE");

 class Other{
   enum Colors{
   RED("RED"), GREEN("GREEN"), BLUE("BLUE"), YELLOW("YELLOW");
   Colors(String name){this.name = name;}
   private String name;
   public String getName(){return name;}
   };

 }

 public class EnumD{
   public static void main(String [] args) {
    for( Other.Colors c : Other.Colors.values()) {

        if (Other.Colors.RED.equals(c))
         System.out.print("red ");
       if (c == Other.Colors.GREEN)
         System.out.print("green ");
      if (c.getName().equals("BLUE"))
         System.out.print("blue ");
 } } }   

Well one more method is convert your enum constant to a String and then compare. The original code that you had given. change c.equals("Blue"); to c.toString().equals("BLUE");

[pete stein]

Sebastian Janisch wrote:I have to correct myself though. A call to the same Enum element will always return the same instance. My bad

Thanks for the clarification. This is what I remember reading when learning about enums and is why == will work with enums.