Enum equals() and == [duplicate]

Question

enum Drill{
        ATTENTION("Attention!"), AT_EASE("At Ease");    
        private String str; 
        private Drill(String str){
            this.str = str;
        }
        public String toString(){
            return str;
        }
    }
public class EnumExample {
    public static void main(String[] args) {
            Drill d1= Drill.valueOf("ATTENTION");
            Drill d2= Drill.valueOf("ATTENTION");
            **System.out.println(d1.equals(d2));//TRUE
            System.out.println(d1==d2);//TRUE**
            System.out.println(Drill.ATTENTION.equals(Drill.valueOf("ATTENTION")));
            System.out.println(Drill.ATTENTION.equals(Drill.AT_EASE));//FALSE 
            System.out.println(Drill.ATTENTION==Drill.valueOf("ATTENTION"));//TRUE
            System.out.println(Drill.ATTENTION==Drill.AT_EASE);//FALSE
}
}

Enum behaviour while using == and equals() seems to be same. According to my knowledge, == just check references. Therefore d1 == d2 should be FALSE. Can anyone explain this behavior why is is TRUE?

Answer 1

== should work fine with enums because there aren't multiple references of a given enum item; there's just one. The Java Language Specification section on enum types, 8.9 states that they are implicitly static and final and so can only be created once.

Answer 2

You are comparing enum constants. This means that for each enum constant, there is a single instance created.

This

enum Drill{
    ATTENTION("Attention!"), AT_EASE("At Ease"); 
    ...
}

is more or less equivalent to

final class Drill {
    public static final Drill ATTENTION = new Drill("Attention!") {};
    public static final Drill AT_EASE = new Drill("At Ease") {};
    ...
}

The valueOf method

/**
* Returns the enum constant of this type with the specified
* name.
* The string must match exactly an identifier used to declare
* an enum constant in this type.  (Extraneous whitespace 
* characters are not permitted.)
* 
* @return the enum constant with the specified name
* @throws IllegalArgumentException if this enum type has no
* constant with the specified name
*/
public static E valueOf(String name);

returns the value of instance referred to by the variable whose name equals the specified String value.

So

Drill.valueOf("ATTENTION") == Drill.ATTENTION

for every invocation of valueOf with that String value.