Why this .equals() code example returns “false”? [duplicate]

问题

class Dog{
    int height;
    int weight;
    String name;
}

public class DogTest {

    public static void main(String[] args) {
        Dog one = new Dog();
        one.height=4;
        one.name="fudo";
        one.weight =2;
        Dog two = new Dog();
        two.height=4;
        two.name="fudo";
        two.weight =2;
        if (one.equals(two)){
            System.out.println("True");
        }
        else{
            System.out.println("False");
        }
    }
}

Why this outputs "False"? If it is by default in Java that "all objects are not equal even if they have same values" then how can I "persuade" Java that these two objects actually are equal? Okay, even if two dogs have same name, height, weight one could be dalmatiner and the other one pit bull, and even if they are the same "race", in nature, they can always be different from one another.

PS: I understand that by saying if (one==two) {} we are comparing if they both refer to the same object on the heap, .equals on string's compares if they have same characters in the same order.

回答1:

The equals method by default says "Is this the same object in memory?" unless you override it.

You didn't override it.

The behavior didn't change.

You'll want to add a new method like this

public boolean equals(Object o) {
    if(o instanceof Dog) {
        Dog d = (Dog)(o);
        Dog t = this;
        return t.height == d.height && t.weight == d.weight && t.name.equals(d.name);
    }
    return false;
}

Stephan brings up a good point - never, ever, ever implment equals without hashCode. Always use the same fields in both.

public int hashCode() {
    int hash = name.hashCode();
    hash = hash * 31 + weight;
    hash = hash * 31 + height;
    return hash;
}

回答2:

You have to override your equals method in your Dog class. If not you are just comparing if those objects are the same instance in memory.

Here is an implementation of how to do this:

class Dog{
    int height;
    int weight;
    String name;

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Dog)) return false;

        Dog dog = (Dog) o;

        if (height != dog.height) return false;
        if (weight != dog.weight) return false;
        return name != null ? name.equals(dog.name) : dog.name == null;

    }
}

回答3:

Any Ide allow you to generate hashcode and equals in automatic way as getters and setters .

In your case without hashcode and equals you will get false , because the objects cannot being at the same memory location .

Your (working) example below :

class Dog{
    int height;
    int weight;
    String name;

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + height;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        result = prime * result + weight;
        return result;
    }
    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Dog other = (Dog) obj;
        if (height != other.height)
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        if (weight != other.weight)
            return false;
        return true;
    }

    public static void main(String[] args) {
        Dog one = new Dog();
        one.height=4;
        one.name="fudo";
        one.weight =2;
        Dog two = new Dog();
        two.height=4;
        two.name="fudo";
        two.weight =2;
        if (one.equals(two)){
            System.out.println("True");
        }
        else{
            System.out.println("False");
        }
    }
}

Result:

To generate hashcode and equals in automatic way in eclipse :

right click in your class and :

来源：https://stackoverflow.com/questions/46158500/why-this-equals-code-example-returns-false

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java

equals