如何使用 ArrayList.contain() 检查 java 中的对象

Question

我正在练习 ArrayList with contains 方法。为了检查 ArrayList 中的对象，每次我需要使用 for each 循环遍历整个 ArrayList。因此，我在想是否有办法使用 contains 方法进行此类检查。

我提到过这个 post：How does a ArrayList's contains() method evaluate objects?

但我是java的新手，并不太了解该方法的含义。是不是要重写equals()方法来检查对象？非常感谢任何帮助。

这是我尝试检查用户是否存在于 playerList 中的方法。

public void showCertainPlayerInformation(ArrayList<NimPlayer> playerList, String inputName)
    {
        System.out.println("The request to show the name: "+ inputName);
        
        //trying to use the contains method, but it seems impossible
        //if (playerList.contains(NimPlayer.getUserName().equals(inputName)))

        for (NimPlayer player : playerList) 
        {
            String userCheck = player.getUserName();
            String familyName = player.getFamilyName();
            String givenName= player.getGivenName();
            int gamesWon = player.getGamesWon();
            int gamesPlayed = player.getGamesPlayed();
            
            if (userCheck.equals(inputName)) 
            {
                System.out.println(inputName + "," + givenName + "," 
                + familyName + "," + gamesPlayed + " games," + gamesWon + " wins");
                return;
            } 
                 
        }

        System.out.println("The player does not exist");
    }

Answer 1

是的。您需要像这样定义您的 NimPlayer class（如果您只想通过用户名比较两个 NimPlayer 对象）

package com.example.schooltimetable;

import java.util.Objects;

public class NimPlayer {
  private String userName;
  private String familyName;
  private String givenName;
  private int gamesWon;
  private int gamesPlayed;

  public String getUserName() {
    return userName;
  }

  public void setUserName(String userName) {
    this.userName = userName;
  }

  public String getFamilyName() {
    return familyName;
  }

  public void setFamilyName(String familyName) {
    this.familyName = familyName;
  }

  public String getGivenName() {
    return givenName;
  }

  public void setGivenName(String givenName) {
    this.givenName = givenName;
  }

  public int getGamesWon() {
    return gamesWon;
  }

  public void setGamesWon(int gamesWon) {
    this.gamesWon = gamesWon;
  }

  public int getGamesPlayed() {
    return gamesPlayed;
  }

  public void setGamesPlayed(int gamesPlayed) {
    this.gamesPlayed = gamesPlayed;
  }

  @Override
  public boolean equals(Object o) {
    if (this == o)
      return true;
    if (o == null || getClass() != o.getClass())
      return false;
    NimPlayer nimPlayer = (NimPlayer) o;
    return Objects.equals(userName, nimPlayer.userName);
  }

  @Override
  public int hashCode() {
    return Objects.hash(userName);
  }
}

现在检查数组列表是否包含：

public void showCertainPlayerInformation(ArrayList<NimPlayer> playerList, String inputName)
    {
      NimPlayer n = new NimPlayer();
      n.setUserName(inputName);
      if(playerList.contains(n)){
          ....
          return;
      }
      System.out.println("The player does not exist");
     }

Answer 2

您可以转到方法声明（CTRL + 单击方法名称）并检查其实现。 contains() 使用 equals() 检查传递的对象是否等于任何元素。并且 equals() 声明可以在 Object class:

中找到

数组列表Class:

public boolean contains(Object var1) {
    return this.indexOf(var1) >= 0;
}

public int indexOf(Object var1) {
    int var2;
    if (var1 == null) {
        for(var2 = 0; var2 < this.size; ++var2) {
            if (this.elementData[var2] == null) {
                return var2;
            }
        }
    } else {
        for(var2 = 0; var2 < this.size; ++var2) {
            if (var1.equals(this.elementData[var2])) {
                return var2;
            }
        }
    }

    return -1;
}

对象Class:

public boolean equals(Object var1) {
    return this == var1;
}

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