A. Sundararajan's Weblog

Multimethods in Groovy

Let us look at a simple class called "Person".

class Person {
    private String name;
    public Person(String name) {
        this.name = name;
    }
    public Person() {
        this.name = "";
    }
    
    public boolean equals(Object o) {
        System.out.println("Huh! not comparable!");
        return false;
    }

    // This does not override java.lang.Object.equals(Object) method
    // See also: Java(TM) Puzzlers: Traps, Pitfalls, and Corner Cases 
    // Puzzle 58 (It is easy to overload when you intend to override!)
    public boolean equals(Person emp) {        
        return name.equals(emp.name);
    }

    public static void main(String[] args) {
        Object e1 = new Person("Sundar");
        Object e2 = new Person("Rajan");
        System.out.println(e1.equals(e2));
        System.out.println(e1.equals(e1));
        System.out.println(e1.equals("Sundar"));   
    }
}

Huh! not comparable!
false
Huh! not comparable!
false
Huh! not comparable!
false

    public static void main(String[] args) {
        Person e1 = new Person("Sundar");
        Person e2 = new Person("Rajan");
        System.out.println(e1.equals(e2));
        System.out.println(e1.equals(e1));
        System.out.println(e1.equals("Sundar"));   
    }

false
true
Huh! not comparable!
false

This is because javac's overload resolution mechanism has selected the "right" method (at the compile time) for us!

Let us check the output of the unmodified (i.e., main not changed as above) with Groovy. I'll use

• Groovy JSR-223 engine from http://scripting.dev.java.net project
• Groovy version 1.0-JSR-06 from http://groovy.codehaus.org/download

jrunscript -cp ../build/groovy-engine.jar;../lib/groovy-all-1.0-JSR-06.jar  -l groovy Person.java

false
true
Huh! not comparable!
false

Groovy invokes the "correct" method in all cases. This is because Groovy supports multimethods. The method actully invoked depends not only on the "receiver" - but, also on the dynamic type of the arguments. For each equals call, the correct equals method is chosen (regardless of compiled time type declared in the program).

• Languages like CLOS support symmetric multimethods, where every argument of the multimethod is treated in the same way. The multimethods cannot belong to particular classes.
• Encapsulated or asymmetric multimethods are an alternative to symmetric multimethods: these methods belong to a class and hence are encapsulated within a single class - the Groovy.

If you want to know more about "binary methods" (single argument methods where argument type is same as that of the "receiver" - like the equals method), then you may want to refer to On Binary Methods

( Aug 08 2006, 06:48:01 PM IST ) Permalink Comments [4] | | | | |