java - 声明的数组给我 "Exception in thread "main"java.lang.NullPointerException"错误

Question

线程“main”中出现异常 java.lang.NullPointerException 在 Game.main(Game.java:12)

我尝试过搜索，这显然意味着我试图在“机器[0].setPayoutRate(35);”

当我尝试调试程序时，对象数组不会显示为三个对象。只是一个值为“SlotMachine[3]”的值，这对我来说没有多大意义。有什么想法吗？

import java.util.Scanner;

public class Game {
public static void main(String[] args) {

    Scanner userInput = new Scanner(System.in);
    SlotMachine[] machines;
    machines = new SlotMachine[3];

    int cash = 0, totalPlays = 0;

    machines[0].setPayoutRate(35);
    machines[1].setPayoutRate(100);
    machines[2].setPayoutRate(10);

    machines[0].setPayoutAmount(30);
    machines[1].setPayoutAmount(60);
    machines[2].setPayoutAmount(11);

    System.out.println("How many quarters do you have?");
    cash = userInput.nextInt();

    System.out.println("how many times has the first machine been played?");
    machines[0].settimesPlayed(userInput.nextInt());

    System.out
            .println("how many times has the second machine been played?");
    machines[1].settimesPlayed(userInput.nextInt());

    System.out.println("how many times has the third machine been played?");
    machines[2].settimesPlayed(userInput.nextInt());

    while (cash >= 1) {
        if (cash >= 1) {
            cash = machines[0].play();
            totalPlays++;
        }
        if (cash >= 1) {
            cash = machines[0].play();
            totalPlays++;
        }
        if (cash >= 1) {
            cash = machines[0].play();
            totalPlays++;
        }
    }
 System.out.println("You played: " + totalPlays + " times.");
  }
}

主类在上面^对象类如下

public class SlotMachine {

private int timesPlayed;
private int payoutRate;
private int payoutAmount;

public SlotMachine(int timesPlayed, int payoutRate, int payoutAmount) {
    this.timesPlayed = timesPlayed;
    this.payoutRate = payoutRate;
    this.payoutAmount = payoutAmount;
}

public int getTimesPlayed() {
    return this.timesPlayed;
}

public void settimesPlayed(int timesPlayed) {
    this.timesPlayed = timesPlayed;
}

public int getPayoutRate() {
    return this.payoutRate;
}

public void setPayoutRate(int payoutRate) {
    this.payoutRate = payoutRate;
}

public int getPayoutAmount() {
    return this.payoutAmount;
}

public void setPayoutAmount(int payoutAmount) {
    this.payoutAmount = payoutAmount;
}

public int play() {
    int moneyExchange = 0;
    timesPlayed++;

    if (timesPlayed != payoutRate) {
        moneyExchange = -1;
    } else if (timesPlayed == payoutRate) {
        moneyExchange = payoutAmount;
        timesPlayed = 0;
    }

    return moneyExchange;
}

}

Answer 1

您声明了长度为 3 的数组，但未为其分配任何新的 SlotMachine，因此所有元素仍为 null，导致 NullPointerException。初始化您的数组元素。

machines = new SlotMachine[3];
machines[0] = new SlotMachine(0, 35, 30);
machines[1] = new SlotMachine(0, 100, 60);
machines[2] = new SlotMachine(0, 10, 11);

然后你可以通过数组访问它们。