JavaでnullPointerExceptionが発生するのはなぜですか？

Question

startGameボタンをクリックするまで例外が表示されません。私はエラーがException in thread "JavaFX Application Thread" java.lang.NullPointerException at helloworld.HelloWorld$3.handle(HelloWorld.java:123)と言っています私はそのエラーが何を意味するか分かりません。エラーが発生しましたplayers.add(new Player(0, "Amrit"));私は間違っていますか？ここで

public String gameStatus = "Pending"; 
public ArrayList<Player> players; 

    startGame.addEventHandler(MouseEvent.MOUSE_CLICKED, 
     new EventHandler<MouseEvent>() { 
      @Override public void handle(MouseEvent e) { 

       gameStatus = "Started"; 
       players.add(new Player(0, "Amrit")); 
       players.add(new Player(1, "Tyler")); 
       players.add(new Player(2, "Scott")); 
       players.add(new Player(3, "Ryker")); 

       while(gameStatus == "Started"){ 

        //Select first two settlements with one road extending from each 
        for(int s = 0; s < 1; s++){ 

         for(int p = 0; p < players.size(); p++){ 

          Alert alert = new Alert(Alert.AlertType.CONFIRMATION); 
          alert.setTitle(players.get(p).name); 
          alert.setContentText(players.get(p).name + ", please select your first settlement and place a road next to it."); 

         } 
        } 

        gameStatus = "Ended"; 
       } 

      } 
    }); 

} 

はPlayer.javaです：あなたはそれがnullで使用しようとすると

package helloworld; 

import java.awt.Point; 
import java.util.ArrayList; 

public class Player { 

    public int ID; 
    public String name; 
    public ArrayList<String> hand = new ArrayList<String>(); 
    public int points; 
    public Point[] citiesBuilt; 
    public Point[] settlementsBuilt; 
    public int citiesAvail; 
    public int settlementsAvail; 
    public Point[] roadsBuilt; 
    public int roadsAvail; 

    Player(int id, String playerName){ 

     ID = id; 
     name = playerName; 

    } 
} 

Answer 1

playersは、初期化されていません。そのコードを次の場所に変更してください：

public ArrayList<Player> players = new ArrayList<>();