嘗試調用空對象引用的方法

Question

我正在編寫一個android應用程序，並且出現此錯誤。我評論的我的代碼幾乎每一個部分，並且仍然未發表意見的只有部分有以下幾種：

public class MainActivity extends AppCompatActivity { 

int digi=0; 
public PointCalculator pointCalculator; 
@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    digi = pointCalculator.digicalculator(); 
    Toast.makeText(getApplicationContext(), digi, Toast.LENGTH_LONG).show(); 

} 

和PointCalculator：

public int digicalculator() 
{ 
    int digipoints=0; 
    digipoints = digipoints + 30; 
    return digipoints; 
} 

，這是logcat的：

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.neshat.test/com.example.neshat.test.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'int com.example.neshat.test.PointCalculator.digicalculator()' on a null object reference 
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298) 
     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) 
     at android.app.ActivityThread.access$800(ActivityThread.java:144) 
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) 
     at android.os.Handler.dispatchMessage(Handler.java:102) 
     at android.os.Looper.loop(Looper.java:135) 
     at android.app.ActivityThread.main(ActivityThread.java:5221) 
     at java.lang.reflect.Method.invoke(Native Method) 
     at java.lang.reflect.Method.invoke(Method.java:372) 
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) 
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) 
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int com.example.neshat.test.PointCalculator.digicalculator()' on a null object reference 
     at com.example.neshat.test.MainActivity.onCreate(MainActivity.java:18) 
     at android.app.Activity.performCreate(Activity.java:5933) 
     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105) 
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251) 
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) 
            at android.app.ActivityThread.access$800(ActivityThread.java:144) 
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) 
            at android.os.Handler.dispatchMessage(Handler.java:102) 
            at android.os.Looper.loop(Looper.java:135) 
            at android.app.ActivityThread.main(ActivityThread.java:5221) 
            at java.lang.reflect.Method.invoke(Native Method) 
            at java.lang.reflect.Method.invoke(Method.java:372) 
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) 
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) 

我不明白是什麼原因造成的問題。

Answer 1

您還沒有分配的內存給pointCalculator對象：

pointCalculator = new PointCalculator(); 
digi = pointCalculator.digicalculator(); 

Answer 2

需要初始化你PointCalculator第一。

public class MainActivity extends AppCompatActivity { 

int digi=0; 
public PointCalculator pointCalculator = new PointCalculator(); //<--- Initialized 
@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    digi = pointCalculator.digicalculator(); 
    Toast.makeText(getApplicationContext(), digi, Toast.LENGTH_LONG).show(); 

} 

Answer 3

您還沒有初始化您的PointCalculator。

public PointCalculator pointCalculator; 
@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    pointCalculator = new PointCalculator(); // Here it's initialised 
    digi = pointCalculator.digicalculator(); 
    Toast.makeText(getApplicationContext(), digi, Toast.LENGTH_LONG).show(); 

} 

Answer 4

您正在使用對象PointCalculator沒有初始化，所以這是你的問題。嘗試在onCreate中初始化它：

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    pointCalculator = new PointCalculator(); //init 
    digi = pointCalculator.digicalculator(); 
    Toast.makeText(getApplicationContext(), digi, Toast.LENGTH_LONG).show(); 

}