如何为线性布局元素设置setLayoutParams

Question

如何为LinearLayout元素设置setLayoutParams()。在MainActivity.java。我写了下面的代码来设置布局参数，并在第50行显示nullPointerException，我最后粘贴Logcat文件。

layoutParams = new LinearLayout.LayoutParams(LayoutParams.WRAP_CONTENT, 50); 
imageview1.setLayoutParams(layoutParams); // line no 50 

layoutParams = new LinearLayout.LayoutParams(LayoutParams.WRAP_CONTENT, 50); 
textview1.setLayoutParams(layoutParams); 

layoutParams = new LinearLayout.LayoutParams(LayoutParams.FILL_PARENT,LayoutParams.WRAP_CONTENT); 
listview01.setLayoutParams(layoutParams); 

这是我的xml文件

<?xml version="1.0" encoding="utf-8"?> 
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
android:orientation="vertical" 
android:layout_width="wrap_content" 
android:layout_height="wrap_content" 
android:stackFromBottom="true" 
> 
<ImageView 
android:id="@+id/imageview1" 
android:layout_width="wrap_content" 
android:layout_height="50px" 
android:background="@drawable/applicationbar" 
android:layout_x="0px" 
android:layout_y="0px" 
> 
</ImageView> 
<TextView 
android:id="@+id/textview1" 
android:layout_width="wrap_content" 
android:layout_height="50px" 
android:text="TextView" 
android:layout_x="0px" 
android:layout_y="55px" 
> 
</TextView> 
<ListView android:id="@+id/ListView01" 
android:layout_width="fill_parent" 
android:layout_height="wrap_content" 
android:layout_marginLeft="10px" 
android:layout_marginRight="10px" 
android:layout_marginTop="35px" 
android:layout_marginBottom="40px" 
android:paddingLeft="0px" 
android:paddingRight="0px" /> 
</LinearLayout> 

登录猫：

04-19 16:47:22.221: ERROR/AndroidRuntime(3437): Uncaught handler: thread main exiting due to uncaught exception 
04-19 16:47:22.221: ERROR/AndroidRuntime(3437): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.android/com.android.Listview}: java.lang.NullPointerException 
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2496) 
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512) 
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):  at android.app.ActivityThread.access$2200(ActivityThread.java:119) 
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1863) 
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):  at android.os.Handler.dispatchMessage(Handler.java:99) 
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):  at android.os.Looper.loop(Looper.java:123) 
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):  at android.app.ActivityThread.main(ActivityThread.java:4363) 
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):  at java.lang.reflect.Method.invokeNative(Native Method) 
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):  at java.lang.reflect.Method.invoke(Method.java:521) 
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860) 
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618) 
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):  at dalvik.system.NativeStart.main(Native Method) 
04-19 16:47:22.221: ERROR/AndroidRuntime(3437): Caused by: java.lang.NullPointerException 
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):  at com.android.Listview.onCreate(Listview.java:40) 
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047) 
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2459) 

Answer 1

imageview1.setLayoutParams(new LayoutParams(LayoutParams.WRAP_CONTENT, 50)); 
textview1.setLayoutParams(new LayoutParams(LayoutParams.WRAP_CONTENT, 50)); 
listview01.setLayoutParams(new 
     LayoutParams(LayoutParams.MATCH_PARENT,LayoutParams.WRAP_CONTENT)); 

Answer 2

在你的活动开始尝试这种在onCreate（应虽然更换PIC）。不要忘记评论默认出现的setContentView：

// setContentView(R.layout.activity_main); 

TextView label = new TextView(this); 
label.setText("This is working, congrats!"); 
label.setTextSize(20); 
label.setGravity(Gravity.CENTER_HORIZONTAL); 

LinearLayout ll = new LinearLayout(this); 
ll.setOrientation(LinearLayout.VERTICAL); 
ll.setLayoutParams(new WindowManager.LayoutParams()); 

ll.setLayoutParams(new LinearLayout.LayoutParams(LinearLayout.LayoutParams.MATCH_PARENT, LinearLayout.LayoutParams.MATCH_PARENT)); 
ll.setGravity(Gravity.CENTER); 
ll.addView(label); 
setContentView(ll);