Java基础--HashMap面试题

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * HashMap问题
 * @author 15735400536
 * 使用HashMap，如果key是自定义的类，就必须重写hashcode()和equals()
 * 使用ArrayList，如果元素是自定义的类，要做相等判断，就必须重写hashCode()和equals()
 */
public class HashMapTest {
	
	public static void main(String[] args) {
		Map<User,Doc> hashMap = new HashMap<User,Doc>();
		
		hashMap.put(new User("mxh","mxh"), new Doc("mxh","mxh"));
		hashMap.put(new User("root","root"), new Doc("root","root"));
		hashMap.put(new User("admin","admin"), new Doc("admin","admin"));
		
		//再不重写hashCode方法和equals方法的情况下get()方法拿到的结果为null
		//原因: new一个新的对象时，地址变了，不能保证hash值和equals结果还是一样。所以取不到对应的value。
		Object object = hashMap.get(new User("admin","admin"));
		System.out.println(object);
		
		List<Doc> list = new ArrayList<Doc>();
		list.add(new Doc("三国演义","罗贯中"));
		list.add(new Doc("红楼梦","曹雪芹"));
		list.add(new Doc("水浒传","施耐庵"));
		list.add(new Doc("西游记","吴承恩"));
		
		//Doc类未重写hashCode()和equals()方法前，结果为false
		boolean result = list.contains(new Doc("水浒传","施耐庵"));
		System.out.println("list集合是否存在该元素: " + result);
	}
}

/**
 * 用户
 * @author 15735400536
 *
 */
class User {
	private String username;
	private String password;
	public String getUsername() {
		return username;
	}
	public void setUsername(String username) {
		this.username = username;
	}
	public String getPassword() {
		return password;
	}
	public void setPassword(String password) {
		this.password = password;
	}
	public User(String username, String password) {
		super();
		this.username = username;
		this.password = password;
	}
	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		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 (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;
	}
	
}

/**
 * 文档
 * @author 15735400536
 *
 */
class Doc {
	private String title;
	private String content;
	public String getTitle() {
		return title;
	}
	public void setTitle(String title) {
		this.title = title;
	}
	public String getContent() {
		return content;
	}
	public void setContent(String content) {
		this.content = content;
	}
	public Doc(String title, String content) {
		super();
		this.title = title;
		this.content = content;
	}
	@Override
	public String toString() {
		return "Doc [title=" + title + ", content=" + content + "]";
	}
	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + ((content == null) ? 0 : content.hashCode());
		result = prime * result + ((title == null) ? 0 : title.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;
		Doc other = (Doc) obj;
		if (content == null) {
			if (other.content != null)
				return false;
		} else if (!content.equals(other.content))
			return false;
		if (title == null) {
			if (other.title != null)
				return false;
		} else if (!title.equals(other.title))
			return false;
		return true;
	}

}

　　Doc doc1 = new Doc("三国演义","罗贯中");
　　Doc doc2 = new Doc("三国演义","罗贯中");