尝试调用空对象引用的方法

我正在编写一个android应用程序，并且出现此错误。我评论的我的代码几乎每一个部分，并且仍然未发表意见的只有部分有以下几种：尝试调用空对象引用的方法

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; 
}

，这是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)

我不明白是什么原因造成的问题。

来源

2016-01-19 Neshat

你在哪里分配pointCalculator？ – ajantha

您还没有分配的内存给pointCalculator对象：

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

来源

2016-01-19 08:34:44

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(); 

}

来源

2016-01-19 08:34:50

您还没有初始化您的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(); 

}

来源

2016-01-19 08:36:45

您正在使用对象PointCalculator没有初始化，所以这是你的问题。尝试在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(); 

}

来源

2016-01-19 08:45:33 Unii

尝试调用空对象引用的方法

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