java - 在android中更改背景图像

Question

每次单击按钮时，我都试图更改我的背景图像。问题是，我总是遇到 javanullexception。

这是我的代码:

    public class Help extends Activity {

    int pageCtr = 0;
    RelativeLayout hLayout;
    ImageView nextbtn, prevtbtn;

     @Override
     public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            this.requestWindowFeature(Window.FEATURE_NO_TITLE);
            setContentView(R.layout.activity_help);

            hLayout = (RelativeLayout)findViewById(R.id.helpLayout);
            hLayout.setBackgroundResource(R.drawable.helpbg1);

     }

     public void showNextPage(View v)
     {
         pageCtr++;
         switchPage(pageCtr);
     }

     public void showPrevPage(View v)
     {
         pageCtr--;
         switchPage(pageCtr);
     }

     public void switchPage(int ctr)
     {
         hLayout = (RelativeLayout)findViewById(R.id.helpLayout);
         nextbtn = (ImageView)findViewById(R.id.nextBtn);
         prevtbtn = (ImageView)findViewById(R.id.backbtn);

         switch (ctr)
         {
         case 0:
             prevtbtn.setVisibility(View.INVISIBLE);
             hLayout.setBackgroundResource(R.drawable.helpbg1);
             break;
         case 1:
             prevtbtn.setVisibility(View.VISIBLE);
             hLayout.setBackgroundResource(R.drawable.helpbg2);
             break;
         case 2:
             hLayout.setBackgroundResource(R.drawable.helpbg3);
             break;
         case 3:
             nextbtn.setVisibility(View.VISIBLE);
             hLayout.setBackgroundResource(R.drawable.helpbg4);
             break;
         case 4:
             nextbtn.setVisibility(View.INVISIBLE);
             hLayout.setBackgroundResource(R.drawable.helpbg5);
             break;
         }
     }

}

我的 XML:

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

<ImageView
    android:id="@+id/backBtn"
    android:layout_width="36dp"
    android:layout_height="36dp"
    android:src="@drawable/backbtn" 
    android:gravity="left"
    android:layout_marginTop="150dp"
    android:onClick="showPrevPage"
    android:visibility="gone"/>


<ImageView
    android:id="@+id/nextBtn"
    android:layout_width="36dp"
    android:layout_height="36dp"
    android:src="@drawable/nextbtn" 
    android:layout_alignParentRight="true"
    android:layout_marginTop="150dp"
    android:onClick="showNextPage"/>

</RelativeLayout> 

堆栈跟踪/日志:

    01-11 23:36:28.476: E/AndroidRuntime(1351): FATAL EXCEPTION: main
    01-11 23:36:28.476: E/AndroidRuntime(1351): java.lang.IllegalStateException: Could not execute method of the activity
    01-11 23:36:28.476: E/AndroidRuntime(1351):     at android.view.View$1.onClick(View.java:3591)
    01-11 23:36:28.476: E/AndroidRuntime(1351):     at android.view.View.performClick(View.java:4084)
    01-11 23:36:28.476: E/AndroidRuntime(1351):     at android.view.View$PerformClick.run(View.java:16966)
    01-11 23:36:28.476: E/AndroidRuntime(1351):     at android.os.Handler.handleCallback(Handler.java:615)
    01-11 23:36:28.476: E/AndroidRuntime(1351):     at  android.os.Handler.dispatchMessage(Handler.java:92)
    01-11 23:36:28.476: E/AndroidRuntime(1351):     at android.os.Looper.loop(Looper.java:137)
    01-11 23:36:28.476: E/AndroidRuntime(1351):     at android.app.ActivityThread.main(ActivityThread.java:4745)
    01-11 23:36:28.476: E/AndroidRuntime(1351):     at  java.lang.reflect.Method.invokeNative(Native Method)
    01-11 23:36:28.476: E/AndroidRuntime(1351):     at java.lang.reflect.Method.invoke(Method.java:511)
    01-11 23:36:28.476: E/AndroidRuntime(1351):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
    01-11 23:36:28.476: E/AndroidRuntime(1351):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
01-11 23:36:28.476: E/AndroidRuntime(1351):     at dalvik.system.NativeStart.main(Native Method)

        01-11 23:36:28.476: E/AndroidRuntime(1351): Caused by: java.lang.reflect.InvocationTargetException
        01-11 23:36:28.476: E/AndroidRuntime(1351):     at java.lang.reflect.Method.invokeNative(Native Method)
       01-11 23:36:28.476: E/AndroidRuntime(1351):  at java.lang.reflect.Method.invoke(Method.java:511)
        01-11 23:36:28.476: E/AndroidRuntime(1351):     at  android.view.View$1.onClick(View.java:3586)
        01-11 23:36:28.476: E/AndroidRuntime(1351):     ... 11 more
        01-11 23:36:28.476: E/AndroidRuntime(1351): Caused by: java.lang.NullPointerException
        01-11 23:36:28.476: E/AndroidRuntime(1351):     at com.example.gems.Help.switchPage(Help.java:53)
        01-11 23:36:28.476: E/AndroidRuntime(1351):     at com.example.gems.Help.showNextPage(Help.java:31)
    01-11 23:36:28.476: E/AndroidRuntime(1351):     ... 14 more

很简单的代码，但是错误指向了我的pageCtr。它似乎没有看到我的 pageCtr 的任何值。试图将全局变量传递给希望它能工作的方法，但没有成功。错误是当我尝试增加或减少 pageCtr 时。

Answer 1

backBtn 的名称在您的 java 代码和 xml 声明中不匹配。

在 XML 中，android:id="@+id/backBtn"

在 Java 中，prevtbtn = (ImageView)findViewById(R.id.backbtn);

你应该使用:

public void switchPage(int ctr)
     {
         hLayout = (RelativeLayout)findViewById(R.id.helpLayout);
         nextbtn = (ImageView)findViewById(R.id.nextBtn);
         prevtbtn = (ImageView)findViewById(R.id.backBtn);
...
}