尝试调用虚拟方法

Question

我正在尝试使用Timer和TimerTask类来制作一个简单的计时器。我不断收到以下错误：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference 

这里是我的代码：

package com.austinheitmann.stopwatch; 

import android.os.CountDownTimer; 
import android.os.SystemClock; 
import android.support.v7.app.ActionBarActivity; 
import android.os.Bundle; 
import android.util.Log; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
import android.widget.TextView; 

import java.util.Timer; 
import java.util.TimerTask; 


public class MainActivity extends ActionBarActivity { 

TextView timerf; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
timerf = (TextView) findViewById(R.id.timer); 
} 

int i=0; 




TimerTask time = new TimerTask() { 
    public void run() { 
     i++; 
     int k = i/60; 
     int j = i%60; 
     if (j>9) { 
      timerf.setText("seconds remaining: " + k + ":" + j); 
     }else { 
      timerf.setText("seconds remaining: " + k + ":0" + j); 
     } 
     Log.d("Uhh", "Sec:" + i); 
    } 
}; 

Timer timers = new Timer(); 

public void startB(View view) { 
timers.schedule(time, 100000000, 1000); 
} 

@Override 
public boolean onCreateOptionsMenu(Menu menu) { 
    // Inflate the menu; this adds items to the action bar if it is present. 
    getMenuInflater().inflate(R.menu.menu_main, menu); 
    return true; 

} 

@Override 
public boolean onOptionsItemSelected(MenuItem item) { 
    // Handle action bar item clicks here. The action bar will 
    // automatically handle clicks on the Home/Up button, so long 
    // as you specify a parent activity in AndroidManifest.xml. 
    int id = item.getItemId(); 

    //noinspection SimplifiableIfStatement 
    if (id == R.id.action_settings) { 
     return true; 
    } 

    return super.onOptionsItemSelected(item); 
} 
} 
} 

下面是XML文件（布局）：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
xmlns:tools="http://schemas.android.com/tools"   android:layout_width="match_parent" 
android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin" 
android:paddingRight="@dimen/activity_horizontal_margin" 
android:paddingTop="@dimen/activity_vertical_margin" 
android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity"> 

<TextView 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:textAppearance="?android:attr/textAppearanceLarge" 
    android:text="0:00" 
    android:id="@+id/timer" 
    android:layout_alignParentTop="true" 
    android:layout_centerHorizontal="true" 
    android:layout_marginTop="54dp" /> 

<Button 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:text="Start/Pause" 
    android:id="@+id/start" 
    android:layout_below="@+id/timer" 
    android:layout_alignParentLeft="true" 
    android:layout_alignParentStart="true" 
    android:layout_marginTop="119dp" 
    android:onClick="startB"/> 

<Button 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:text="Stop/Reset" 
    android:id="@+id/stop" 
    android:layout_alignBottom="@+id/start" 
    android:layout_alignParentRight="true" 
    android:layout_alignParentEnd="true" /> 
</RelativeLayout> 

每一个解决方案，我已经看到了这个错误与布局有关，但我无论如何都没有改变。我已经移动到startB类中的TimerTask并获得相同的错误。我该如何解决这个错误？

这里就是整个的错误，我从一个运行得到（其中在出现错误，我把**）清除logcat的后：

01-16 12:27:21.008 2455-2455/com.austinheitmann.stopwatch I/art﹕ Not late-enabling -Xcheck:jni (already on) 
01-16 12:27:21.119 2455-2455/com.austinheitmann.stopwatch D/AndroidRuntime﹕ Shutting down VM 
    --------- beginning of crash 
01-16 12:27:21.123 2455-2455/com.austinheitmann.stopwatch E/AndroidRuntime﹕ FATAL EXCEPTION: main 
    Process: com.austinheitmann.stopwatch, PID: 2455 
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.austinheitmann.stopwatch/com.austinheitmann.stopwatch.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2209) 
      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 'android.view.View android.view.Window.findViewById(int)' on a null object reference 
      at android.app.Activity.findViewById(Activity.java:2071) 
      at com.austinheitmann.stopwatch.MainActivity.<init>(MainActivity.java:25) 
      at java.lang.reflect.Constructor.newInstance(Native Method) 
      at java.lang.Class.newInstance(Class.java:1572) 
      at android.app.Instrumentation.newActivity(Instrumentation.java:1065) 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2199) 
            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) 
01-16 12:44:57.778 2582-2582/com.austinheitmann.stopwatch D/AndroidRuntime﹕ Shutting down VM 
01-16 12:44:57.778 2582-2582/com.austinheitmann.stopwatch E/AndroidRuntime﹕ FATAL EXCEPTION: main 
    Process: com.austinheitmann.stopwatch, PID: 2582 
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.austinheitmann.stopwatch/com.austinheitmann.stopwatch.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2209) 
      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 'android.view.View android.view.Window.findViewById(int)' on a null object reference 
      at android.app.Activity.findViewById(Activity.java:2071) 
      at com.austinheitmann.stopwatch.MainActivity.<init>(MainActivity.java:25) 
      at java.lang.reflect.Constructor.newInstance(Native Method) 
      at java.lang.Class.newInstance(Class.java:1572) 
      at android.app.Instrumentation.newActivity(Instrumentation.java:1065) 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2199) 
            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) 
01-16 12:46:25.059 2641-2641/com.austinheitmann.stopwatch D/AndroidRuntime﹕ Shutting down VM 
01-16 12:46:25.059 2641-2641/com.austinheitmann.stopwatch E/AndroidRuntime﹕ FATAL EXCEPTION: main 
    Process: com.austinheitmann.stopwatch, PID: 2641 
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.austinheitmann.stopwatch/com.austinheitmann.stopwatch.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2209) 
      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 'android.view.View android.view.Window.findViewById(int)' on a null object reference 
      at android.app.Activity.findViewById(Activity.java:2071) 
      at com.austinheitmann.stopwatch.MainActivity.<init>(MainActivity.java:25) 
      at java.lang.reflect.Constructor.newInstance(Native Method) 
      at java.lang.Class.newInstance(Class.java:1572) 
      at android.app.Instrumentation.newActivity(Instrumentation.java:1065) 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2199) 
            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) 
01-16 12:47:50.630 2702-2702/com.austinheitmann.stopwatch D/AndroidRuntime﹕ Shutting down VM 
01-16 12:47:50.630 2702-2702/com.austinheitmann.stopwatch E/AndroidRuntime﹕ FATAL EXCEPTION: main 
    Process: com.austinheitmann.stopwatch, PID: 2702 
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.austinheitmann.stopwatch/com.austinheitmann.stopwatch.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2209) 
      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 'android.view.View android.view.Window.findViewById(int)' on a null object reference** 
      at android.app.Activity.findViewById(Activity.java:2071) 
      at com.austinheitmann.stopwatch.MainActivity.<init>(MainActivity.java:25) 
      at java.lang.reflect.Constructor.newInstance(Native Method) 
      at java.lang.Class.newInstance(Class.java:1572) 
      at android.app.Instrumentation.newActivity(Instrumentation.java:1065) 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2199) 
            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) 

Answer 1

您试图findViewById活动的上下文之前已准备就绪。

如果你想在访问其他方法timerf太，创建一个类的成员，但使初始化onCreate后setContentView：

TextView timerf; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    timerf = (TextView) findViewById(R.id.timer); 
} 

更新：

我没注意到你正在实例化的其他类成员变量。它们引用的timerf为null。

您还需要在onCreate中初始化它们。下面是完整的代码应该工作：

package com.austinheitmann.stopwatch; 

import android.os.CountDownTimer; 
import android.os.SystemClock; 
import android.support.v7.app.ActionBarActivity; 
import android.os.Bundle; 
import android.util.Log; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
import android.widget.TextView; 

import java.util.Timer; 
import java.util.TimerTask; 


public class MainActivity extends ActionBarActivity { 

    Timer timers = new Timer(); 
    TextView timerf; 
    TimerTask time; 
    int i=0; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     timerf = (TextView) findViewById(R.id.timer); 

     time = new TimerTask() { 
      public void run() { 
       i++; 
       int k = i/60; 
       int j = i%60; 
       if (j>9) { 
        timerf.setText("seconds remaining: " + k + ":" + j); 
       }else { 
        timerf.setText("seconds remaining: " + k + ":0" + j); 
       } 
       Log.d("Uhh", "Sec:" + i); 
      } 
     }; 
    } 

    public void startB(View view) { 
     timers.schedule(time, 100000000, 1000); 
    } 

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.menu.menu_main, menu); 
     return true; 

    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     // Handle action bar item clicks here. The action bar will 
     // automatically handle clicks on the Home/Up button, so long 
     // as you specify a parent activity in AndroidManifest.xml. 
     int id = item.getItemId(); 

     //noinspection SimplifiableIfStatement 
     if (id == R.id.action_settings) { 
      return true; 
     } 

     return super.onOptionsItemSelected(item); 
    } 
} 

Answer 2

首先使用构造public MainActivity()和 Timer timer;和long startTime; long stopTime声明狼吞虎咽变量

public MainActivity() { 
    timer = new Timer(); 
    startTime = System.currentTimeMillis(); 
    stopTime = 0; 
}