java - 类(class)测试代码中出现 NullPointerException？

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 3 年前。

所以测试我的练习的代码给了我失败测试:

"nullpointerException，在调用 addPlayer(Player: Atro, 0) 时。导致失败的代码 Team j = new Team("HIFK"); Player p = new Player("Atro"); j.addPlayer(p) ;

我已经在我的主程序中运行了这段代码，它运行得很好，所以我不明白为什么测试代码有问题。

public class Main {
public static void main(String[] args) {
    Team barcelona = new Team("FC Barcelona");
    barcelona.setMaxSize(16);
    Player brian = new Player("Brian");
    Player pekka = new Player("Pekka", 39);
    barcelona.addPlayer(brian);
    barcelona.addPlayer(pekka);
    barcelona.addPlayer(new Player("Mikael", 1));

    System.out.println("Total goals: " + barcelona.goals());
}

}

import java.util.ArrayList;

public class Team {
private String name;
private ArrayList<Player> playersOnTeam = new ArrayList<>();
private int maxSize;


public Team(String teamName) {
    name = teamName;
}

public String getName() {
    return name;
}

public void addPlayer(Player playerToAdd) {
    if (playersOnTeam.size() == maxSize) {
        return;
    }
    playersOnTeam.add(playerToAdd);
}

public void printPlayers() {
    String playerList = "";
    for (Player i : this.playersOnTeam) {
        System.out.println(i.getName() + ", " + "goals " + i.goals());
    }
}

public void setMaxSize(int setSize) {
    maxSize = setSize;
}

public int size() {
    return playersOnTeam.size();
}

public int goals() {
    int allGoals = 0;
    for (Player i : playersOnTeam) {
        allGoals = allGoals + i.goals();
    }
    return allGoals;
}

}

public class Player {
private String playerName;
private int goals;

public Player(String name) {
    this(name,0);
}

public Player(String name, int score) {
    playerName = name;
    goals = score;
}

public String getName() {
    return this.playerName;
}

public int goals() {
    return goals;
}

public String toString() {
    return "Player: " + playerName + ", " + "Goals " + goals;
}

}

Answer 1

您需要初始化playersOnTeam

private ArrayList<Player> playersOnTeam = new ArrayList<>();

如果没有初始化，playersOnTeam将为null(并且无法使用.add()访问