空指針錯誤

Question

林有關於我的findViewById（一個空指針問題 其它只是一個模擬時鐘和按鈕加載到AnalogClock活動第一個非常簡單的應用程序去DigitalClock活動的第一項活動負載了罰款但是當點擊按鈕時，DigitalClock活動的onCreate中的viewById（）返回null，然後崩潰。

AnalogActivity.java 

package com.example.dualclocks2; 

import android.net.Uri; 
import android.os.Bundle; 
import android.app.Activity; 
import android.content.Intent; 
import android.view.Menu; 
import android.view.View; 
import android.view.View.OnClickListener; 
import android.widget.Button; 

public class AnalogActivity extends Activity { 
@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_analog); 

    Button go2DigitalButton = (Button) findViewById(R.id.btnGo2Digital); 
    go2DigitalButton.setOnClickListener(go2DigitalButtonListener); 

} 

public OnClickListener go2DigitalButtonListener = new OnClickListener() 
    { 

    @Override 
    public void onClick(View v) { 


     Intent intent = new Intent(AnalogActivity.this, DigitalActivity.class);    

     startActivity(intent); // execute the Intent 
    } 

    }; 

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

activity_analog.xml

<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" 
tools:context=".AnalogActivity" > 

<AnalogClock 
    android:id="@+id/analogClock1" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_alignParentBottom="true" 
    android:layout_centerHorizontal="true" 
    android:layout_marginBottom="150dp" /> 

<Button 
    android:id="@+id/btnGo2Digital" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_above="@+id/analogClock1" 
    android:layout_centerHorizontal="true" 
    android:layout_marginBottom="62dp" 
    android:text="@string/go2DigitalClock" /> 
</RelativeLayout> 

DigitalActivity.java

package com.example.dualclocks2; 

import android.os.Bundle; 
import android.app.Activity; 
import android.content.Intent; 
import android.view.Menu; 
import android.view.View; 
import android.view.View.OnClickListener; 
import android.widget.Button; 

public class DigitalActivity extends Activity { 
@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_digital); 

    Button go2AnalogButton = (Button) findViewById(R.id.btnGo2Digital);//null here 
    go2AnalogButton.setOnClickListener(go2AnalogButtonListener); 
} 

public OnClickListener go2AnalogButtonListener = new OnClickListener() 
    { 

    @Override 
    public void onClick(View v) { 


     Intent intent = new Intent(DigitalActivity.this, AnalogActivity.class);    

     startActivity(intent); // execute the Intent 
    } 

    }; 

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

}

activity_digital.xml

<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" 
tools:context=".DigitalActivity" > 

<Button 
    android:id="@+id/btnGo2Analog" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_alignParentTop="true" 
    android:layout_centerHorizontal="true" 
    android:layout_marginTop="69dp" 
    android:text="@string/go2Analog" /> 

<DigitalClock 
    android:id="@+id/digitalClock1" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_centerHorizontal="true" 
    android:layout_centerVertical="true" 
    android:gravity="center" 
    android:text="DigitalClock" 
    android:textSize="@dimen/digitalClockSize" /> 

</RelativeLayout> 

logcat的

01-18 17:05:35.835: E/AndroidRuntime(15800): FATAL EXCEPTION: main 
01-18 17:05:35.835: E/AndroidRuntime(15800): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.dualclocks2/com.example.dualclocks2.DigitalActivity}: java.lang.NullPointerException 
01-18 17:05:35.835: E/AndroidRuntime(15800): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1651) 
01-18 17:05:35.835: E/AndroidRuntime(15800): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1667) 
01-18 17:05:35.835: E/AndroidRuntime(15800): at android.app.ActivityThread.access$1500(ActivityThread.java:117) 
01-18 17:05:35.835: E/AndroidRuntime(15800): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:935) 
01-18 17:05:35.835: E/AndroidRuntime(15800): at android.os.Handler.dispatchMessage(Handler.java:99) 
01-18 17:05:35.835: E/AndroidRuntime(15800): at android.os.Looper.loop(Looper.java:130) 
01-18 17:05:35.835: E/AndroidRuntime(15800): at android.app.ActivityThread.main(ActivityThread.java:3688) 
01-18 17:05:35.835: E/AndroidRuntime(15800): at java.lang.reflect.Method.invokeNative(Native Method) 
01-18 17:05:35.835: E/AndroidRuntime(15800): at java.lang.reflect.Method.invoke(Method.java:507) 
01-18 17:05:35.835: E/AndroidRuntime(15800): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:842) 
01-18 17:05:35.835: E/AndroidRuntime(15800): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600) 
01-18 17:05:35.835: E/AndroidRuntime(15800): at dalvik.system.NativeStart.main(Native Method) 
01-18 17:05:35.835: E/AndroidRuntime(15800): Caused by: java.lang.NullPointerException 
01-18 17:05:35.835: E/AndroidRuntime(15800): at com.example.dualclocks2.DigitalActivity.onCreate(DigitalActivity.java:19) 
01-18 17:05:35.835: E/AndroidRuntime(15800): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047) 
01-18 17:05:35.835: E/AndroidRuntime(15800): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1615) 
01-18 17:05:35.835: E/AndroidRuntime(15800): ... 11 more 

Answer 1

R.layout.activity_digital 

不包含一個按鈕，其ID爲btnGo2Digital。

看來你想R.id.btnGo2Analog，而不是和的意思是寫

Button go2AnalogButton = (Button) findViewById(R.id.btnGo2Analog);//changed id