Java设计一个类（已有一个User类），实现接口，查询是否存在某个用户，怎么写？

Question

这两个（设计一个类，实现接口，查询是否存在某个用户）,（设计一个类，实现接口，修改某个用户密码中的字母全部为小写）具体（代码）要怎么写啊？

不太懂这题的两个（设计类实现接口）怎么写。是写个接口，写个类实现接口吗？还是User类实现接口？

可以的话帮我做完这道题，不尽感激！

Java程序设计题

User 有属性：用户名、密码、地址；

有一个接口如下： interface OperationService{ public String action(LinkedList client，String key); }

1.设计一个 User 类（按简单 Java 类的要求设计）

2.设计一个类，实现接口，查询是否存在某个用户 。

3.设计一个类，实现接口，修改某个用户密码中的字母全部为小写。

4.设计主类，执行程序，输出查询、修改后的结果。

3月28日21点前截止

Answer 1

整上 

// OperationService.java文件
import java.util.LinkedList;

public interface OperationService {
    public String action(LinkedList client, String key);
}

// User.java文件
public class User{
    private String username;
    private String password;
    private String address;
    public User(){
        this.username = "";
        this.password = "";
        this.address = "";
    }
    public User(String username, String password, String address){
        this.username = username;
        this.password = password;
        this.address = address;
    }
    public String getUsername(){
        return this.username;
    }
    public String getPassword(){
        return this.password;
    }
    public String getAddress(){
        return this.address;
    }
    public void setPassword(String password){
        this.password = password;
    }
}

// SearchUser.java文件
import java.util.LinkedList;

public class searchUser {
    public searchUser(){
        ;
    }
    public String action(LinkedList client, String key){
        if(client!=null && client.size()>0 && key!=null){
            for(Object eachUser: client){
                User thisUser = (User) eachUser;
                if(thisUser.getUsername().equals(key)){
                    return thisUser.getUsername()+":"+thisUser.getAddress();
                }
            }
            return "The user "+key+" is not in the list";
        }
        return "Find failed";
    }
}

// setPasswordLowercase.java文件
import java.util.LinkedList;

public class setPasswordLowercase {
    public setPasswordLowercase(){
        ;
    }
    public String action(LinkedList client, String key){
        if(client!=null && client.size()>0 && key!=null){
            for(int i = 0; i < client.size(); i++){
                User eachUser = (User) client.get(i);
                if(eachUser.getUsername().equals(key)){
                    System.out.println("Before set password, " + key+ "'s password is :" + eachUser.getPassword());
                    eachUser.setPassword(eachUser.getPassword().toLowerCase());
                    client.set(i, eachUser);
                    System.out.println("After set password, " + key+ "'s password is :" + eachUser.getPassword());
                    return "Set " + key + "'s password to its lowercased format. ";
                }
            }
            return "Set failed. Not find " + key;
        }
        return "Set failed, for the input arguments invalid.";
    }
}

// mainClass.java文件
import java.util.LinkedList;

public class mainClass {
    public static void main(String[] args){
        LinkedList<User> client = new LinkedList<User>();
        client.add(new User("Xiaoming", "xiaomingPassWord", "xiaomingDeAddress"));
        client.add(new User("Zhangsan", "zhangSanPassWord", "zhangSanDeAddress"));
        client.add(new User("Lisi", "LisiPassWord", "LisiDeAddress"));
        client.add(new User("WangWu", "WangwuPassWord", "wangWuDeAddress"));
        System.out.println(new searchUser().action(client,"Xiaoming"));
        System.out.println(new searchUser().action(client,"xiaoming"));
        System.out.println(new searchUser().action(client,"Wangwu"));
        System.out.println(new searchUser().action(client,"ProfSnail"));
        System.out.println(new setPasswordLowercase().action(client,"Wangwu"));
        System.out.println(new setPasswordLowercase().action(client,"Wangwu"));
        System.out.println(new setPasswordLowercase().action(client,"Xiaoming"));
        System.out.println(new setPasswordLowercase().action(client,"ProfSnail"));
    }
}

在我这里已经测试运行通过了。题主可以私信找我要一个打包好的程序包。