java中的equals方法

equals()方法在包含继承时，需要注意写问题。equals的约定声明：如果p和q不是null，
p.equals(q)也该和q.equals(p)返回同样的值。下面的代码是种错误的写法

class BaseClass {
	private int x;
	
	public BaseClass(int i) {
		x = i;
	}
	
	public boolean equals(Object rhs) {
		if (!(rhs instanceof BaseClass))
			return false;
		return x == ((BaseClass) rhs).x;
	}
}

class DerivedClass extends BaseClass {
	private int y;
	
	public DerivedClass(int i, int j) {
		super(i);
		y = j;
	}
	
	public boolean equals(Object rhs) {
		if(!(rhs instanceof DerivedClass))
			return false;
		return super.equals(rhs) && y == ((DerivedClass) rhs).y;
	}
}

public class EqualWithInheritance {
	public static void main(String[] args) {
		BaseClass a = new BaseClass(5);
		DerivedClass b = new DerivedClass(5, 8);
		DerivedClass c = new DerivedClass(5, 8);
		
		System.out.println("b.equals(c)" + b.equals(c));
		System.out.println("a.equals(b)" + a.equals(b));
		System.out.println("b.equals(a)" + b.equals(a));
		
		System.out.println("b.instanceof BaseClass" + (b instanceof BaseClass));
	}
}

正确的实现应该是

class BaseClass {
	private int x;
	
	public BaseClass(int i) {
		x = i;
	}
	
	public boolean equals(Object rhs) {
		if (rhs == null || getClass() != rhs.getClass())
			return false;
		return x == ((BaseClass) rhs).x;
	}
}

class DerivedClass extends BaseClass {
	private int y;
	
	public DerivedClass(int i, int j) {
		super(i);
		y = j;
	}
	
	public boolean equals(Object rhs) {
		//Class test not needed; getClass() is done in superclass equals
		return super.equals(rhs) && y == ((DerivedClass) rhs).y;
	}
}

public class EqualWithInheritance {
	public static void main(String[] args) {
		BaseClass a = new BaseClass(5);
		DerivedClass b = new DerivedClass(5, 8);
		DerivedClass c = new DerivedClass(5, 8);
		
		System.out.println("b.equals(c)" + b.equals(c));
		System.out.println("a.equals(b)" + a.equals(b));
		System.out.println("b.equals(a)" + b.equals(a));
		
		System.out.println("b.instanceof BaseClass" + (b instanceof BaseClass));
	}
}