contains() method in List not working as expected

问题:

the api of contains() method says

"Returns true if this list contains the specified element. More formally, returns true if and only if this list contains at least one element e such that (o==null ? e==null : o.equals(e)). "

I overrode the equals() method in my class but contains() still returns me false when i check

my code

class Animal implements Comparable<Animal>{
    int legs;
    Animal(int legs){this.legs=legs;}
    public int compareTo(Animal otherAnimal){
        return this.legs-otherAnimal.legs;
    }
    public String toString(){return this.getClass().getName();}

    public boolean equals(Animal otherAnimal){
        return (this.legs==otherAnimal.legs) && 
                (this.getClass().getName().equals(otherAnimal.getClass().getName()));
    }

    public int hashCode(){
        byte[] byteVal = this.getClass().getName().getBytes();
        int sum=0;
        for(int i=0, n=byteVal.length; i<n ; i++)
            sum+=byteVal[i];
        sum+=this.legs;
        return sum;
    }

}
class Spider extends Animal{
    Spider(int legs){super(legs);}
}
class Dog extends Animal{
    Dog(int legs){super(legs);}
}
class Man extends Animal{
    Man(int legs){super(legs);}
}

pardon the bad concept behind classes but i was just testing understanding of my concepts.

now when I try this, it prints false even though equals is overriden

List<Animal> li=new ArrayList<Animal>();
Animal a1=new Dog(4);
li.add(a1);
li.add(new Man(2));
li.add(new Spider(6));

List<Animal> li2=new ArrayList<Animal>();
Collections.addAll(li2,new Dog(4),new Man(2),new Spider(6));
System.out.println(li2.size());
System.out.println(li.contains(li2.get(0))); //should return true but returns false

回答1:

You overloaded equals instead of overriding it. To override Object's equals method, you must use the same signature, which means the argument must be of Object type.

Change to:

@Override
public boolean equals(Object other){
    if (!(other instanceof Animal))
        return false;
    Animal otherAnimal = (Animal) other;
    return (this.legs==otherAnimal.legs) && 
           (this.getClass().getName().equals(otherAnimal.getClass().getName()));
}

回答2:

As JLS-8.4.8.1 specify

An instance method m1, declared in class C, overrides another instance method m2, declared in class A , if all of the following are true:

C is a subclass of A.

The signature of m1 is a subsignature  of the signature of m2.

Either:

m2 is public, protected, or declared with default access in the same package as C, or

m1 overrides a method m3 (m3 distinct from m1, m3 distinct from m2), such that m3 overrides m2.

Signature Must be same to override which in your case is ignored !!!