Java 为什么Android中的Environment.getExternalStorageState（）案例为NullPointException

我的代码

原因：

private static String getSDCardPath() {
    boolean sdCardExist = Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState());
    if (sdCardExist) {
        return Environment.getExternalStorageDirectory().toString();
    }
    return null;
}

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我的代码是Environment.MEDIA\u MOUNTED.equalsEnvironment.getExternalStorageState和still case NPE

java.lang.NullPointerException
at android.os.Environment.getExternalStorageState(Environment.java:602)