java - 为什么我的程序在我尝试添加 ArrayList 元素时说它存在，即使它是新元素？

Question

我正在向 ArrayList 添加元素，我将其设计为在用户尝试添加预先存在的元素时显示错误(与尝试删除不存在的元素时相同)。从它显示的内容来看，它添加了新元素，但仍然说该元素与 ArrayList 中已有的元素相匹配。我忽略了什么？ ArrayList 在 FacebookUser.java 类中创建和操作。谢谢(如果这是一个愚蠢的错误，我提前道歉)。

驱动类.java

public class DriverClass {

    public static void main(String[] args) {

        FacebookUser fu0 = new FacebookUser("Samuel", "password1");
        FacebookUser fu1 = new FacebookUser("Michael", "password2");
        FacebookUser fu2 = new FacebookUser("Amy", "password3");
        FacebookUser fu3 = new FacebookUser("Eugene", "password4");
        fu0.setPasswordHint("p1");
        fu3.setPasswordHint("p4");

        fu0.friend(fu1);
        fu0.friend(fu2);
        fu0.friend(fu3);
        fu0.friend(fu3);

        System.out.println(fu0.getFriends());

        fu0.defriend(fu1);
        fu0.defriend(fu1);

        System.out.println(fu0.getFriends());

        fu0.getPasswordHelp();
        fu3.getPasswordHelp();
    }

}

FacebookUser.java

import java.util.ArrayList;

public class FacebookUser extends UserAccount {

    private String passwordHint;
    private ArrayList<FacebookUser> friends = new ArrayList<FacebookUser>();

    public FacebookUser(String username, String password) {
        super(username, password);
        friends = new ArrayList<FacebookUser>();
    }

    @Override
    public void getPasswordHelp() {
        System.out.println("Password Hint: " + passwordHint);

    }

    void setPasswordHint(String hint) {
        passwordHint = hint;
    }

    void friend(FacebookUser newFriend) {
        System.out.println(friends.size());
        if (friends.size() == 0) {
            friends.add(newFriend);
        } else {
            for (int i = 0; i < friends.size(); i++) {
                if (friends.get(i).equals(newFriend)) {
                    System.out.println("That person is already in your friends list.");
                    break;
                } else if (!friends.get(i).equals(newFriend) && i == friends.size() - 1) {
                    friends.add(newFriend);
                }
            }
        }
    }

    void defriend(FacebookUser formerFriend) {
        if (friends.size() == 0) {
            System.out.println("That person is not in your friends list.");
        } else {
            for (int i = 0; i < friends.size(); i++) {
                if (friends.get(i).equals(formerFriend)) {
                    friends.remove(i);
                    break;
                } else if (!friends.get(i).equals(formerFriend) && i == friends.size() - 1) {
                    System.out.println("That person is not in your friends list.");
                }

            }
        }

    }

    ArrayList<FacebookUser> getFriends() {
        ArrayList<FacebookUser> friendsCopy = new ArrayList<FacebookUser>();
        for (int i = 0; i < friends.size(); i++) {
            friendsCopy.add(friends.get(i));
        }
        return friendsCopy;
    }

}

用户帐户.java

public abstract class UserAccount {

    private String username;
    private String password;
    private boolean active; 

    public UserAccount(String username, String password) {
        this.username = username;
        this.password = password;
        active = true;
    }

    public boolean checkPassword(String password) {
        if (password.equals(this.password)) {
            return true;
        } else {
            return false;
        }
    }

    public void deactivateAccount() {
        active = false;
    }

    public String toString() {
        return username;
    }

    public boolean checkActive() {
        if (active == true) {
            return true;
        } else {
            return false;
        }
    }

    public abstract void getPasswordHelp();

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        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;
        UserAccount other = (UserAccount) obj;
        if (username == null) {
            if (other.username != null)
                return false;
        } else if (!username.equals(other.username))
            return false;
        return true;
    }

}

执行:

0
1
That person is already in your friends list.
2
That person is already in your friends list.
3
That person is already in your friends list.
[Michael, Amy, Eugene]
That person is not in your friends list.
[Amy, Eugene]
Password Hint: p1
Password Hint: p4

Answer 1

您在 for 循环中与 size() 进行比较，即使您也可能将项目添加到循环中的列表中，所以您最终比较在最后一次迭代中针对自身的项目。

for (int i = 0; i < friends.size(); i++) { // result of size() will change
   if (friends.get(i).equals(newFriend)) {
      System.out.println("That person is already in your friends list.");
      break;
   } else if (!friends.get(i).equals(newFriend) && i == friends.size() - 1) {
      friends.add(newFriend);
   }
}

您可以只提取调用 size() 的结果，这样它就不会在您添加新项目时发生变化。或者您也可以在添加项目后退出循环。

保存大小:

int size = friends.size();
for (int i = 0; i < size; i++) {
   if (friends.get(i).equals(newFriend)) {
      System.out.println("That person is already in your friends list.");
      break;
   } else if (!friends.get(i).equals(newFriend) && i == friends.size() - 1) {
      friends.add(newFriend);
   }
}

或者，可能更好的是，在您决定应该添加该项目后使用 break:

for (int i = 0; i < friends.size(); i++) {
   if (friends.get(i).equals(newFriend)) {
      System.out.println("That person is already in your friends list.");
      break;
   } else if (!friends.get(i).equals(newFriend) && i == friends.size() - 1) {
      friends.add(newFriend);
      break;
   }
}

然后，一个更好的解决方案可能是避免循环并改用 contains:

void friend(FacebookUser newFriend) {
    System.out.println(friends.size());
    if (friends.contains(newFriend)) {
        System.out.println("That person is already in your friends list.");
        return;
    }

    friends.add(newFriend);
}