Java ==,equals()与hashcode()的使用

public class Main {
    public static void main(String[] args) {
        int a = 5;
        int b = 5;
        int c = 6;
        System.out.println(a==b); // true
        System.out.println(a==c); // false
        Person p1 = new Person("hhh", 15);
        Person p2 = new Person("hhh", 15);
        // 属性相同，但引用不同。按照常理应该被认作是相同的。实际却是不同的
        System.out.println(p1 == p2); // false
    }
}
class Person{
    String name;
    Integer age;
    Person(String name, Integer age){
        this.age = age;
        this.name = name;
    }
}

public class Point {
    private int x;
    private int y;
    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
    // getters and setters
    @Override
    public boolean equals(Object obj) {
        return (this == obj);
    }
}

public class Main {
    public static void main(String[] args) {
        List<String> list = List.of("A", "B", "C");
        // 下面的例子中即使对象的地址不同，但是属性值相同，因此在列表中能查到
        System.out.println(list.contains(new String("C"))); // true
        System.out.println(list.indexOf(new String("C"))); // 2
    }
}

public class Main {
    public static void main(String[] args) {
        HashMap<String, String> map = new HashMap<>();
        map.put(new String("sss"),"hhh");
        map.put(new String("sss"), "aaa");
        // map里只有一个数据，因为String重写了equals方法，所以key属性值一样的会被覆盖
        System.out.println(map.size());
        HashMap<Person, String> map1 = new HashMap<>();
        map1.put(new Person("xiao",15), "ok");
        map1.put(new Person("xiao",15), "hao");
        // map1里有两个数据，明明key是一样的，却没有覆盖
        System.out.println(map1.size());
    }
}
class Person{
    String name;
    int age;
    Person(String name, int age){
        this.name = name;
        this.age = age;
    }
}

public class Main {
    public static void main(String[] args) {
        HashMap<String, String> map = new HashMap<>();
        map.put(new String("sss"),"hhh");
        map.put(new String("sss"), "aaa");
        // map里只有一个数据，因为String重写了equals方法和hashcode方法，所以key属性值一样的会被覆盖
        System.out.println(map.size()); // 1
        HashMap<Person, String> map1 = new HashMap<>();
        Person p1 = new Person("xiao", 15);
        map1.put(p1, "ok");
        Person p2 = new Person("xiao", 15);
        map1.put(p2, "hao");
        // map现在也只有一个数据了，因为两个相同属性的对象被视作同一个
        System.out.println(map1.size()); // 1
    }
}
class Person{
    String name;
    int age;
    Person(String name, int age){
        this.name = name;
        this.age = age;
    }
    // 覆写equals方法
    @Override
    public boolean equals(Object o) {
        if (o instanceof Person p) {
            return this.name.equals(p.name) && this.age == p.age;
        }
        return false;
    }
    // 覆写hashcode方法
    @Override
    public int hashCode() {
        int h = 0;
        h = 31 * h + name.hashCode();
        h = 31 * h + age;
        return h;
    }
}