Java SlotMachine异常错误

Question

对于我的java类，我想创建一个SlotMachine类，它生成3个3个随机数的数组，并检查所有数字是否匹配，如果是，则将它们声明为胜利者。我还必须创建另一个程序来运行1000个老虎机并计算获胜者。问题是每次我运行SlotMachine类和main方法时，都会出现异常错误，我无法弄清楚如何修复。

异常错误：

Exception in thread "main" java.lang.NullPointerException
    at SlotMachine.isWinner(SlotMachine.java:41)
    at Play1000SlotMachines.main(Play1000SlotMachines.java:14)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at     sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)

SlotMachine类：

// import statements
import java.util.Random;

public class SlotMachine {


// fields
private int[] row1;
private int[] row2;
private int[] row3;

// methods
public SlotMachine(){
    playMachine();
}

public void playMachine(){

    Random random = new Random();

    int row1[] = new int[3];
    int row2[] = new int[3];
    int row3[] = new int[3];


    for (int i = 0; i < 3; i++) {
        row1[i] = random.nextInt(10);
    }

    for (int i = 0; i < 3; i++) {
        row2[i] = random.nextInt(10);
    }

    for (int i = 0; i < 3; i++) {
        row3[i] = random.nextInt(10);
    }
}

public boolean isWinner(){

    if (row1[0] == row1[1]){
        if (row1[0] == row1[2]){
            return true;
        }

    }

    if (row2[0] == row2[1]){
        if (row2[0] == row2[2]){
            return true;
        }
    }

    if (row3[0] == row3[1]){
        if (row3[0] == row3[2]){
            return true;
        }
    }
    return false;
}
}

主要方法：

public class Play1000SlotMachines{

public static void main(String[] args){

    // comment
    SlotMachine slotMachine = new SlotMachine();

    // comment
    int count = 0;

    // comment
    for (int i = 0; i < 1000; i++){
        slotMachine.playMachine();
        if (slotMachine.isWinner())
            count++;
    }

    // comment
    System.out.println("From 1000 slot machines, " + count + " were winners");
}
}