When overriding equals in Java, why does it not work to use a parameter other than Object?

Question

I ran into an interesting behavior recently. It seems that if I override .equals() to take a parameter other than Object, it doesn\'t get called. Can anyone explain to me

Answer 1

Notice that the method you are calling is defined in the javadoc for ArrayList> as

boolean contains(Object o)
    Returns true if this list contains the specified element. 

instead of

boolean contains(E o)
    Returns true if this list contains the specified element. 

Implementation of ArrayList.java:

private transient Object elementData[];

public boolean contains(Object elem) {
    return indexOf(elem) >= 0;
}

public int indexOf(Object elem) {
    if (elem == null) {
        for (int i = 0; i < size; i++)
            if (elementData[i]==null)
                return i;
    } else {
        for (int i = 0; i < size; i++)
            if (elem.equals(elementData[i]))
                return i;
    }
    return -1;
}

It uses the equals method defined in the Object superclass since the equals method is not overridden in ArrayList>'s implementation.

When overriding Object equals in java, you should override the Object hashCode method as well.

Anyway you might want to try the following code:

class A{    
    public int content;    
    A(){
        this(0);
    }    
    A(int value){
        content = value;
    }   
    public boolean equals(Object obj){
        System.out.println("overriding equals method");
        return this.content == ((A) obj).content;
    }    
    public boolean equals(A a){
        System.out.println("overloading equals method");
        return this.content == a.content;
    }    
    public static void main(String[] args){
        A x = new A(1);
        A y = new A(2);
        Object z  = new A(1);
        System.out.println(x.equals(y));
        System.out.println(x.equals(x));
        System.out.println(x.equals(z));
        //override as z is declared as Object at compile time
        //so it will use methods in class Object instead of class A
        System.out.println(x.equals((Object) y));
        System.out.println(x.equals((Object) x));        
    }   
}
//rant: they didn't teach me these in javaschool and I had to learn it the hard way.