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From: Xue Feng <xf.lotus@yahoo.com>
Newsgroups: comp.lang.java.programmer
Subject: overriding with different return value type
Date: Thu, 16 Jul 2015 21:04:44 +0800
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Xref: csiph.com comp.lang.java.programmer:32042

According to the JDK manual, overriding requirs that
(1) the same signature
(2) the same return value type
The following example demonstrates it:

class Base{
	public int sum(int x, int y){
		System.out.println("Int");
		return x + y;
	}
}

class Ext extends Base{
	public double sum(int x, int y){
		System.out.println("Double");
		return (double)(x + y);
	}

}

The overriding sum in Ext returns double, while the overrided sum in 
Base returns int. Their return value type are different. So, we get 
error message when compiling.

But the other example seems work in different way. The Object class has 
a clone() method, and its header is like this:
     protected Object clone() throws CloneNotSupportedException
The clone() in Object class returns an Object's instance. Suppose that 
there is a class implements cloneable interface:

class Foobar implements Cloneable{
	public Foobar clone() throws CloneNotSupportedException{
		System.out.println("Foobar");
		return (Foobar)super.clone();
	}
}

The clone() in Foobar class returns an Foobar's instance. It's obvious 
that the two clone() methods have different return value type, but no 
error message comes while compiling. I wonder why the clone() method 
works in different way.