Java Set集合去重的原理及实现

  Set<8大基本类型> set = new HashSet<8大基本类型>();
  set.add(8大基本类型);

public boolean add(E e) {
	return map.put(e, PRESENT)==null;
	}

public V put(K key, V value) {
    if (key == null)
      return putForNullKey(value);
    int hash = hash(key.hashCode());
    int i = indexFor(hash, table.length);
    for (Entry<K,V> e = table[i]; e != null; e = e.next) {//每添加一个，则循环判断是否与map中的元素相等
      Object k;
      // 先判断hashcode是否一致，然后再判断值是否相等
      if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
        V oldValue = e.value;
        e.value = value;
        e.recordAccess(this);
        return oldValue;
      }
    }
 
    modCount++;
    addEntry(hash, key, value, i);
    return null;
  }

public class User {
  //id
  protected Integer id;
  //username
  protected String username;

  //构造方法
  public User(int id,String username){
    this.id = id;
    this.username = username;
  }

  /**
   * 如果对象是USER，先比较hashcode，一致的场合在比价每个属性的值
   */
  @Override
  public boolean equals(Object obj) {
    if(obj == null)
      return false;
    if(this == obj)
      return true;
    if(obj instanceof User){
      User user = (User) obj;
      //if(user.id == this.id) return true; //只比较id
      //比较id和username 一致时才返回true，之后再去比较hashCode
      if(user.id == this.id && user.username.equals(this.username)){
        return true;
      }
    }
    return false;
  }

  /**
   *
   * 重写hashCode方法，返回的hashCode不一样 才认为是不一样的对象；
   */
  @Override
  public int hashCode() {
    //return id.hashCode(); 只比较id，id不一样就添加进集合；
    return id.hashCode() * username.hashCode();
  }
}

import java.util.HashSet;
import java.util.Set;

public class test {
  public static void main(String[] args){
    User user1 = new User(1,"xiaoqiang");
    User user2 = new User(2,"xiaoqiang");
    User user3 = new User(1,"xiaoqiang");
    User user4 = new User(1,"xiaoqiang");

    Set<User> set = new HashSet<User>();
    set.add(user1);
    set.add(user2);
    set.add(user3);
    set.add(user4);
    for(User u : set){
      System.out.println("id:" + u.id +" username:"+ u.username);
    }
  }
}