无法找到空指针异常

Question

我试图在按钮单击侦听器上滑动图像。我正在为此采取新的布局。每个布局由不同的全屏图像组成。点击按钮后应显示下一个布局。这是我的代码。

public class ImageSlider extends Activity { 

Button b, c, d; 
@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_image_slider); 

    b = (Button)findViewById(R.id.image); 
    b.setOnClickListener(new OnClickListener() { 

     @Override 
     public void onClick(View v) { 
      // TODO Auto-generated method stub 

      setContentView(R.layout.image1); 

     } 
    }); 

    c = (Button)findViewById(R.id.image1btn); 
     c.setOnClickListener(new OnClickListener() { 

      @Override 
      public void onClick(View v) { 
       // TODO Auto-generated method stub 

       setContentView(R.layout.image2); 
      } 
     }); 

     d = (Button)findViewById(R.id.image2btn); 
     d.setOnClickListener(new OnClickListener() { 

      @Override 
      public void onClick(View v) { 
       // TODO Auto-generated method stub 

       setContentView(R.layout.activity_image_slider); 
      } 
     }); 
} 

但我甚至正确intialising所有的按钮后，获得空指针异常。 这是错误日志。 请帮忙。对于主要活动

10-25 12:49:07.813: E/AndroidRuntime(13489): FATAL EXCEPTION: main 
10-25 12:49:07.813: E/AndroidRuntime(13489): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.fouractivities/com.example.fouractivities.ImageSlider}: java.lang.NullPointerException 
10-25 12:49:07.813: E/AndroidRuntime(13489): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1956) 
10-25 12:49:07.813: E/AndroidRuntime(13489):at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981) 
10-25 12:49:07.813: E/AndroidRuntime(13489):at android.app.ActivityThread.access$600(ActivityThread.java:123) 
10-25 12:49:07.813: E/AndroidRuntime(13489):at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147) 
10-25 12:49:07.813: E/AndroidRuntime(13489):at android.os.Handler.dispatchMessage(Handler.java:99) 
10-25 12:49:07.813: E/AndroidRuntime(13489):at android.os.Looper.loop(Looper.java:137) 
10-25 12:49:07.813: E/AndroidRuntime(13489):at android.app.ActivityThread.main(ActivityThread.java:4424) 
10-25 12:49:07.813: E/AndroidRuntime(13489):at java.lang.reflect.Method.invokeNative(Native Method) 
10-25 12:49:07.813: E/AndroidRuntime(13489):at java.lang.reflect.Method.invoke(Method.java:511) 
10-25 12:49:07.813: E/AndroidRuntime(13489):at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784) 
10-25 12:49:07.813: E/AndroidRuntime(13489):at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551) 
10-25 12:49:07.813: E/AndroidRuntime(13489):at dalvik.system.NativeStart.main(Native Method) 

XML文件

<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:paddingBottom="@dimen/activity_vertical_margin" 
android:paddingLeft="@dimen/activity_horizontal_margin" 
android:paddingRight="@dimen/activity_horizontal_margin" 
android:paddingTop="@dimen/activity_vertical_margin" 
tools:context=".ImageSlider" > 

<Button 
    android:id="@+id/image" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_alignParentTop="true" 
    android:layout_centerHorizontal="true" 
    android:layout_marginTop="185dp" 
    android:text="@string/image" /> 

XML的第一个图像

<?xml version="1.0" encoding="utf-8"?> 
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
android:layout_width="match_parent" 
android:layout_height="match_parent" 
android:orientation="vertical" > 

<ImageView 
    android:id="@+id/imageView1" 
    android:layout_width="150dp" 
    android:layout_height="300dp" 
    android:layout_alignParentBottom="true" 
    android:layout_alignParentLeft="true" 
    android:layout_alignParentRight="true" 
    android:layout_alignParentTop="true" 
    android:src="@drawable/a" /> 

<Button 
    android:id="@+id/image1btn" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_alignParentRight="true" 
    android:layout_centerVertical="true" 
    android:text="Next" /> 

Answer 1

这是错误的

你不能使用设置内容视图图像切换，

，当你的setContentView另一个布局，按钮将是空的

使用这个对所有...

<?xml version="1.0" encoding="utf-8"?> 
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
android:layout_width="match_parent" 
android:layout_height="match_parent"> 

<ImageView 
    android:id="@+id/imageView1" 
    android:layout_width="150dp" 
    android:layout_height="300dp" 
    android:layout_alignParentBottom="true" 
    android:layout_alignParentLeft="true" 
    android:layout_alignParentRight="true" 
    android:layout_alignParentTop="true" 
    android:src="@drawable/a" /> 
</RelativeLayout> 

可以使用像这样的MainActivity

public class MainActivity extends Activity { 


int[] pic_num = {R.drawable.img1, 
     R.drawable.img2, 
     R.drawable.img3}; 
int i = 1; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 

    final ImageView iv = (ImageView) findViewById(R.id.imageView1); 
    iv.setOnClickListener(new OnClickListener() { 

     @Override 
     public void onClick(View v) { 
      // TODO Auto-generated method stub 

      iv.setImageResource(pic_num[i+1]); 
      i ++; 
      if(i == 3){ 
       i = 1; 
      } 
     } 
    }); 
} 

} 

Answer 2

试试这个..

在activity_image_slider.xml我以前不具有这种Buttonimage1btn这ImageButtonimageView1这就是为什么NPE来到

使用ViewPager

https://github.com/chiuki/android-swipe-image-viewer

Answer 3

我想你已经非常不匹配的第一次布局。 设置setContentView（R.layout.main_activity）而不是setContentView（R.layout.activity_image_slider）;

super.onCreate(savedInstanceState); 
setContentView(R.layout.main_activity) //respective layout here 
b = (Button)findViewById(R.id.image); 

由于布局不匹配的ID缺少相应的布局和througs NPE