Question

我在已经定义的上下文变量上得到android.content.Context android.content.Context.getApplicationContext()' on a null object reference。我正在关注Android开发的Big Nerd Ranch指南，但是代码给了我这个看似毫无根据的错误。

这是我声明我的Context变量mContext：

的地方

public class CrimeLab {
    private static CrimeLab sCrimeLab;

    private List<Crime> mCrimes = new ArrayList<>();
    private Context mContext;
    private SQLiteDatabase mDatabase;
...
}

但是这行代码抛出了一个空对象引用：

public CrimeLab(Context context) {
        mContext = context.getApplicationContext(); <- This one
        mDatabase = new CrimeBaseHelper(mContext)
                .getWritableDatabase();
        mCrimes = new ArrayList<>();
    }

这是堆栈跟踪。我尝试了不同的方式来声明上下文，但它都给出了同样的错误。

Process: myapp.onur.criminalintent, PID: 5095
 java.lang.RuntimeException: Unable to start activity ComponentInfo{myapp.onur.criminalintent/myapp.onur.criminalintent.CrimeListActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2726)
     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2787)
     at android.app.ActivityThread.-wrap12(ActivityThread.java)
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1504)
     at android.os.Handler.dispatchMessage(Handler.java:102)
     at android.os.Looper.loop(Looper.java:154)
     at android.app.ActivityThread.main(ActivityThread.java:6247)
     at java.lang.reflect.Method.invoke(Native Method)
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:872)
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:762)
  Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
     at myapp.onur.criminalintent.CrimeLab.<init>(CrimeLab.java:26)

编辑这是我的整个CrimeLab.java类

public class CrimeLab {
    private static CrimeLab sCrimeLab;

    private List<Crime> mCrimes = new ArrayList<>();
    private Context mContext;
    private SQLiteDatabase mDatabase;

    public void addCrime(Crime c){
        mCrimes.add(c);
    }

    public CrimeLab(Context context) {
        mContext = context.getApplicationContext();
        mDatabase = new CrimeBaseHelper(mContext)
                .getWritableDatabase();
        mCrimes = new ArrayList<>();
    }

    public static CrimeLab get(Context context) {
        if (sCrimeLab == null) {
            sCrimeLab = new CrimeLab(context);
        }return sCrimeLab;
    }

    public List<Crime> getCrimes() {
        return mCrimes;
    }

    public Crime getCrime(UUID id){
        for (Crime crime : mCrimes){
            if(crime.getID().equals(id)){
                return crime;
            }
        }
        return null;
    }
}

Answer 1

某处，您正在调用new CrimeLab(null)或正在将null传递给CrimeLab构造函数。

在您现在更新的问题中，您要在：

中创建新的CrimeLab

public static CrimeLab get(Context context) {
    if (sCrimeLab == null) {
        sCrimeLab = new CrimeLab(context);
    }return sCrimeLab;
}

因此，传递给Context的{{1}}为get()。因此，找到您呼叫null的位置并修复参数，使其不是CrimeLab.get()。

Answer 2

我解决了。问题出在另一个班级。它没有明确表示null，但它传递了一个错误的变量。我所要做的就是将其删除。

为什么在已声明的Context上有NullPointerException？

2 个答案: