public class EqualsDemo {
    public static void main(String[] args) {

        String str0 = "Hello";
        String str1 = "Hello";

        System.out.println("str0==str1"+(str0==str1));
    }
}
运行结果：
str0==str1true

public static void main(String[] args) {

       String str0 = "Hello";

       String str1 = new String("Hello");

       System.out.println("str0==str1:"+(str0==str1));

    }
运行结果：
str0==str1:false

比较的是首地址，所以不相等。

public class EqualsDemo {
    public static void main(String[] args) {

        Dog dog0 = new Dog("zhangsan");
        Dog dog1 = new Dog("zhangsan");
        System.out.println(("dog0.equals(dog1):")+dog0.equals(dog1));

    }
}
运行结果：
dog0.equals(dog1):false

public class EqualsDemo {
    public static void main(String[] args) {

        Dog dog0 = new Dog("zhangsan");
        Dog dog1 = new Dog("zhangsan");
        System.out.println(("dog0.equals(dog1):")+dog0.getDogName().equals(dog1.getDogName()));

    }
}
运行结果：
dog0.equals(dog1):true

hashCode方法

未使用hashCode前

public class Dog{
    private int age;
    private String dogName;

    public Dog(int age, String dogName) {
        this.age = age;
        this.dogName = dogName;
    }
}

public class EqualsDemo {
    public static void main(String[] args) {

        Dog dog1 = new Dog(5,"xiaobai");
        Dog dog2 = new Dog(5,"xiaobai");
        Dog dog3 = new Dog(5,"xiaohei");

        System.out.println(dog1.hashCode());
        System.out.println(dog2.hashCode());
        System.out.println(dog3.hashCode());

        Set<Dog> dogSet = new HashSet<>();
        System.out.println(dog1);
        dogSet.add(dog1);
        System.out.println(dog2);
        dogSet.add(dog2);
        System.out.println(dog3);
        dogSet.add(dog3);

        System.out.println(dogSet.size());
        System.out.println(dog1==dog2);
        System.out.println(dog1.equals(dog2));

    }
}
运行结果：
1163157884
1956725890
356573597
com.filedemo.Dog@4554617c
com.filedemo.Dog@74a14482
com.filedemo.Dog@1540e19d
3
false
false

使用hashCode后

public class Dog{
    private int age;
    private String dogName;

    public Dog(int age, String dogName) {
        this.age = age;
        this.dogName = dogName;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        Dog dog = (Dog) o;

        if (age != dog.age) return false;
        return dogName != null ? dogName.equals(dog.dogName) : dog.dogName == null;
    }

    @Override
    public int hashCode() {
        int result = age;
        result = 31 * result + (dogName != null ? dogName.hashCode() : 0);
        return result;
    }
}

public class EqualsDemo {
    public static void main(String[] args) {

        Dog dog1 = new Dog(5,"xiaobai");
        Dog dog2 = new Dog(5,"xiaobai");
        Dog dog3 = new Dog(5,"xiaohei");

        System.out.println(dog1.hashCode());
        System.out.println(dog2.hashCode());
        System.out.println(dog3.hashCode());

        Set<Dog> dogSet = new HashSet<>();
        System.out.println(dog1);
        dogSet.add(dog1);
        System.out.println(dog2);
        dogSet.add(dog2);
        System.out.println(dog3);
        dogSet.add(dog3);

        System.out.println(dogSet.size());
        System.out.println(dog1==dog2);
        System.out.println(dog1.equals(dog2));

    }
}
运行结果：
-2069661338
-2069661338
-2069655448
com.filedemo.Dog@84a37966
com.filedemo.Dog@84a37966
com.filedemo.Dog@84a39068
2
false
true

image.png

image.png