Java Language Enum constant specific body


Example

In an enum it is possible to define a specific behavior for a particular constant of the enum which overrides the default behavior of the enum, this technique is known as constant specific body.

Suppose three piano students - John, Ben and Luke - are defined in an enum named PianoClass, as follows:

    enum PianoClass {
    JOHN, BEN, LUKE;
        public String getSex() {
            return "Male";
        }
        public String getLevel() {
            return "Beginner";
        }
    }

And one day two other students arrive - Rita and Tom - with a sex (Female) and level (Intermediate) that do not match the previous ones:

    enum PianoClass2 {
    JOHN, BEN, LUKE, RITA, TOM;
        public String getSex() {
            return "Male"; // issue, Rita is a female
        }
        public String getLevel() {
            return "Beginner"; // issue, Tom is an intermediate student
        }
    }

so that simply adding the new students to the constant declaration, as follows, is not correct:

PianoClass2 tom = PianoClass2.TOM;
PianoClass2 rita = PianoClass2.RITA;
System.out.println(tom.getLevel()); // prints Beginner -> wrong Tom's not a beginner
System.out.println(rita.getSex()); // prints Male -> wrong Rita's not a male

It's possible to define a specific behavior for each of the constant, Rita and Tom, which overrides the PianoClass2 default behavior as follows:

enum PianoClass3 {
    JOHN, BEN, LUKE,
    RITA {
        @Override
        public String getSex() {
            return "Female";
        }
    },
    TOM {
        @Override
        public String getLevel() {
            return "Intermediate";
        }
    };
    public String getSex() {
        return "Male";
    }
    public String getLevel() {
        return "Beginner";
    }
}

and now Tom's level and Rita's sex are as they should be:

PianoClass3 tom = PianoClass3.TOM;
PianoClass3 rita = PianoClass3.RITA;
System.out.println(tom.getLevel()); // prints Intermediate
System.out.println(rita.getSex()); // prints Female

Another way to define content specific body is by using constructor, for instance:

enum Friend {
    MAT("Male"),
    JOHN("Male"),
    JANE("Female");
    
    private String gender;

    Friend(String gender) {
        this.gender = gender;
    }

    public String getGender() {
        return this.gender;
    }
}

and usage:

Friend mat = Friend.MAT;
Friend john = Friend.JOHN;
Friend jane = Friend.JANE;
System.out.println(mat.getGender());     // Male
System.out.println(john.getGender());    // Male
System.out.println(jane.getGender());    // Female