java - 模运算符空指针异常

Question

这是我在 Android Studio 中的 java fragment 。

public static void makepack(Card[] pack, int[] a, int[] size) {

    for (int i = 1; i < 14; i++) {
        Random r = new Random();
        int Low = 1;
        int High = 53;
        int k = r.nextInt(High - Low) + Low;

        // 1st 13 are diamonds, then hearts, then clubs and then spades
        if (!inarray(a, k, size[0])) {
            pack[i - 1].num = (k % 13) + 1;
            pack[i - 1].type = (k / 13) + 1;
            a[size[0]] = k;
            size[0]++;
        } else {
            i--;
        }
    }

}

我收到以下错误:

"C:\Program Files\Java\jdk1.8.0_05\bin\java" -Didea.launcher.port=7534 "-Didea.launcher.bin.path=C:\Program Files\Android\Android Studio\bin" -Dfile.encoding=windows-1252 -classpath "C:\Users\nhar\AppData\Local\Android\sdk\platforms\android-21\android.jar;C:\Users\nhar\AppData\Local\Android\sdk\platforms\android-21\data\res;C:\Users\nhar\AndroidStudioProjects\MindicoatCardGame\app\build\intermediates\classes\debug;C:\Users\nhar\AppData\Local\Android\sdk\extras\android\m2repository\com\android\support\support-annotations\22.1.1\support-annotations-22.1.1.jar;C:\Users\nhar\AndroidStudioProjects\MindicoatCardGame\app\build\intermediates\exploded-aar\com.android.support\support-v4\22.1.1\jars\libs\internal_impl-22.1.1.jar;C:\Users\nhar\AndroidStudioProjects\MindicoatCardGame\app\build\intermediates\exploded-aar\com.android.support\support-v4\22.1.1\res;C:\Users\nhar\AndroidStudioProjects\MindicoatCardGame\app\build\intermediates\exploded-aar\com.android.support\support-v4\22.1.1\jars\classes.jar;C:\Users\nhar\AndroidStudioProjects\MindicoatCardGame\app\build\intermediates\exploded-aar\com.android.support\appcompat-v7\22.1.1\res;C:\Users\nhar\AndroidStudioProjects\MindicoatCardGame\app\build\intermediates\exploded-aar\com.android.support\appcompat-v7\22.1.1\jars\classes.jar;C:\Program Files\Android\Android Studio\lib\idea_rt.jar" com.intellij.rt.execution.application.AppMain com.example.nihar.mindicoatcardgame.MainFunction
Exception in thread "main" java.lang.NullPointerException
    at com.example.nihar.mindicoatcardgame.MainFunction.makepack(MainFunction.java:222)
    at com.example.nihar.mindicoatcardgame.MainFunction.main(MainFunction.java:244)
    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:483)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)

进程结束，退出代码为 1异常出现在 (%13) 行。知道我做错了什么吗？

Answer 1

唯一可能导致该行出现 NPE 的情况是 pack[i - 1] 是否为 null。

这就是您需要解决的问题。模数没问题。

关于您的评论，您可能想要这个...

if (!inarray(a, k, size[0])) {
    pack[i - 1] = new Card();
    pack[i - 1].num = (k % 13) + 1;
    pack[i - 1].type = (k / 13) + 1;
    a[size[0]] = k;
    size[0]++;
} else {
    i--;
}

更好的方法是为 Card 提供一个采用 num 和 type 的构造函数，然后您可以将三个 pack[i - 1] 行更改为一个。