工作随笔

近日在开发中遇到了需要将一个list和另一个list重复的部分删除掉，嗯，本身是很简单的代码，只要用list自带的方法removeall就可以了，我的代码是这样的

public static void main(String[] args) {
		List<User> list1 = new ArrayList<>();
		List<User> list2 = new ArrayList<>();
		User user1 = new User("peipei", "123", "123.@163.com");
		User user2 = new User("peipei", "123", "123.@163.com");
		list1.add(user1);
		list2.add(user2);
		list1.removeAll(list2);
		System.out.println(list1);
	}
	
	static class User {
		String userName;
		String password;
		String email;
		
		public User(String userName, String password, String email) {
			super();
			this.userName = userName;
			this.password = password;
			this.email = email;
		}

		/**
		 * @return the userName
		 */
		public String getUserName() {
			return userName;
		}

		/**
		 * @param userName the userName to set
		 */
		public void setUserName(String userName) {
			this.userName = userName;
		}

		/**
		 * @return the password
		 */
		public String getPassword() {
			return password;
		}

		/**
		 * @param password the password to set
		 */
		public void setPassword(String password) {
			this.password = password;
		}

		/**
		 * @return the email
		 */
		public String getEmail() {
			return email;
		}

		/**
		 * @param email the email to set
		 */
		public void setEmail(String email) {
			this.email = email;
		}

		/* (non-Javadoc)
		 * @see java.lang.Object#toString()
		 */
		@Override
		public String toString() {
			return "User [userName=" + userName + ", password=" + password + ", email=" + email + "]";
		}
	}

输出结果为：[User [userName=peipei, password=123, email=123.@163.com]]Emmm…嗯？并没有移除掉list1和list2相同的部分，排除掉打错字的情况下,看了一下removeall方法的源代码发现是用equals来判断list中的元素是否相等的，ok，看到这里其实已经想明白了，我们经常用equals来判断字符串的内容是否相等，这是因为在String类中把Object类中的equals方法进行了重写，我们在用equals来判断对象是否相等是就等同于，对象1 == 对象2，比较的是两个对象的地址是否相等，把
User user2 = new User(“peipei”, “123”, “123.@163.com”);这行代码改为User user2 = user1；其他不变，打印结果为 [] ，那说回来我要怎样来判断两个对象的值是否相等呢，那就和String类一样来重写equals方法，修改后的User类代码如下：

public static void main(String[] args) {
		List<User> list1 = new ArrayList<>();
		List<User> list2 = new ArrayList<>();
		User user1 = new User("peipei", "123", "123.@163.com");
		User user2 = new User("peipei", "123", "123.@163.com");
		list1.add(user1);
		list2.add(user2);
		list1.removeAll(list2);
		System.out.println(list1);
	}

static class User {
		String userName;
		String password;
		String email;
		
		public User(String userName, String password, String email) {
			super();
			this.userName = userName;
			this.password = password;
			this.email = email;
		}

		/**
		 * @return the userName
		 */
		public String getUserName() {
			return userName;
		}

		/**
		 * @param userName the userName to set
		 */
		public void setUserName(String userName) {
			this.userName = userName;
		}

		/**
		 * @return the password
		 */
		public String getPassword() {
			return password;
		}

		/**
		 * @param password the password to set
		 */
		public void setPassword(String password) {
			this.password = password;
		}

		/**
		 * @return the email
		 */
		public String getEmail() {
			return email;
		}

		/**
		 * @param email the email to set
		 */
		public void setEmail(String email) {
			this.email = email;
		}
		
		@Override
		public String toString() {
			return "User [userName=" + userName + ", password=" + password + ", email=" + email + "]";
		}

		@Override
		public int hashCode() {
			final int prime = 31;
			int result = 1;
			result = prime * result + ((email == null) ? 0 : email.hashCode());
			result = prime * result + ((password == null) ? 0 : password.hashCode());
			result = prime * result + ((userName == null) ? 0 : userName.hashCode());
			return result;
		}
		
		@Override
		public boolean equals(Object obj) {
			if (this == obj)
				return true;
			if (obj == null)
				return false;
			if (getClass() != obj.getClass())
				return false;
			User other = (User) obj;
			if (email == null) {
				if (other.email != null)
					return false;
			} else if (!email.equals(other.email))
				return false;
			if (password == null) {
				if (other.password != null)
					return false;
			} else if (!password.equals(other.password))
				return false;
			if (userName == null) {
				if (other.userName != null)
					return false;
			} else if (!userName.equals(other.userName))
				return false;
			return true;
		}
	}

再次运行测试结果为 [] 搞定
注：在重写equals方法时，必须重写hashcode方法，一个是为了效率，一个是为了防止出现两个对象equals为ture但是hashcode不同这样违背定理的事情出现。