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

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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;}@Overridepublic 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;}@Overridepublic 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);}@Overridepublic 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 1sttreeifyBin(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 keyV oldValue = e.value;if (!onlyIfAbsent || oldValue == null)e.value = value;afterNodeAccess(e);return oldValue;}}++modCount;if (++size > threshold)resize();afterNodeInsertion(evict);return null;}