【java面试题】实现equals(Person person)方法的Person存入HashSet无法排重！

????先看下面的代码，想下会输出什么？

public class HashSetDemo {
    public static void main(String[] args) {
        Person p1 = new Person("a", 1);
        Person p2 = new Person("a", 1);
        HashSet<Person> hashSet = new HashSet<>();
        hashSet.add(p1);
        hashSet.add(p2);
        System.out.println(hashSet.size());
        System.out.println(p1.equals(p2));
    }
}

class Person {
    String name;
    int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public boolean equals(Person p) {
        return this.name.equals(p.name) && this.age == p.age;
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }
}

• 输出：

2
true

有哪位大佬知道是为什么？

????如果去掉Person的equals(Person p)方法，改成重写Object的equals(Object o)结果是什么？

import java.util.HashSet;
import java.util.Objects;

public class HashSetDemo {
    public static void main(String[] args) {
        Person p1 = new Person("a", 1);
        Person p2 = new Person("a", 1);
        HashSet<Person> hashSet = new HashSet<>();
        hashSet.add(p1);
        hashSet.add(p2);
        System.out.println(hashSet.size());
        System.out.println(p1.equals(p2));
    }
}

class Person {
    String name;
    int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public boolean equals(Object o) {
        System.out.println("equals(Object o)");
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Person person = (Person) o;
        return age == person.age &&
                Objects.equals(name, person.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }
}

输出：

1
true

以上的输出为什么？明明HashSet方法add时是指定了范型类型Person的，不应该是调用equals(Person p)方法吗？

• HashSet的add方法实现：

private transient HashMap<E,Object> map;
private static final Object PRESENT = new Object();
public HashSet() {
   map = new HashMap<>();
}
// 这里的E应该是Person类型的，但是考虑范型擦除，最终调用哪个方法还需要看看擦除后是何类型？
public boolean add(E e) {
   return map.put(e, PRESENT)==null;
}

• HashMap的put方法实现：

	public V put(K key, V value) {
    	return putVal(hash(key), key, value, false, true);
	}
    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            // 这里进行key是否相同的对比，比较引用和equals方法对比内容，但是这里key.equals(k)到底调用的是
            // key自定义类型的equals实现，还是覆写Object的equals方法
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }