java - 将按钮添加到 LinearLayout 不起作用

Question

我试图以编程方式向 LinearLayout 添加按钮，但应用程序崩溃:

activity_main.xml:

<LinearLayout 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=".MainActivity"
    android:id="@+id/main_view" >

    <Button
        android:id="@+id/button1"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_alignRight="@+id/textView1"
        android:layout_below="@+id/textView1"
        android:text="Button"
        android:onClick="onClickBtn" />

</LinearLayout>

这里是代码:

package com.chovanec.runo;

import android.os.Bundle; // and more

public class MainActivity extends Activity {

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

    @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_main, menu);  
        return true;
    }

    public void onClickBtn(View v)
    {
        Button btn = new Button(getBaseContext());
        btn.setText("New Button");

        LinearLayout l = (LinearLayout) findViewById(R.layout.activity_main);
        l.addView(btn);

    }

}

LogCat:

04-14 03:09:00.150: D/AndroidRuntime(7375): Shutting down VM
04-14 03:09:00.150: W/dalvikvm(7375): threadid=1: thread exiting with uncaught exception (group=0x415102a0)
04-14 03:09:00.155: E/AndroidRuntime(7375): FATAL EXCEPTION: main
04-14 03:09:00.155: E/AndroidRuntime(7375): java.lang.IllegalStateException: Could not execute method of the activity
04-14 03:09:00.155: E/AndroidRuntime(7375):     at android.view.View$1.onClick(View.java:3691)
04-14 03:09:00.155: E/AndroidRuntime(7375):     at android.view.View.performClick(View.java:4211)
04-14 03:09:00.155: E/AndroidRuntime(7375):     at android.view.View$PerformClick.run(View.java:17267)
04-14 03:09:00.155: E/AndroidRuntime(7375):     at android.os.Handler.handleCallback(Handler.java:615)
04-14 03:09:00.155: E/AndroidRuntime(7375):     at android.os.Handler.dispatchMessage(Handler.java:92)
04-14 03:09:00.155: E/AndroidRuntime(7375):     at android.os.Looper.loop(Looper.java:137)
04-14 03:09:00.155: E/AndroidRuntime(7375):     at android.app.ActivityThread.main(ActivityThread.java:4898)
04-14 03:09:00.155: E/AndroidRuntime(7375):     at java.lang.reflect.Method.invokeNative(Native Method)
04-14 03:09:00.155: E/AndroidRuntime(7375):     at java.lang.reflect.Method.invoke(Method.java:511)
04-14 03:09:00.155: E/AndroidRuntime(7375):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1006)
04-14 03:09:00.155: E/AndroidRuntime(7375):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:773)
04-14 03:09:00.155: E/AndroidRuntime(7375):     at dalvik.system.NativeStart.main(Native Method)
04-14 03:09:00.155: E/AndroidRuntime(7375): Caused by: java.lang.reflect.InvocationTargetException
04-14 03:09:00.155: E/AndroidRuntime(7375):     at java.lang.reflect.Method.invokeNative(Native Method)
04-14 03:09:00.155: E/AndroidRuntime(7375):     at java.lang.reflect.Method.invoke(Method.java:511)
04-14 03:09:00.155: E/AndroidRuntime(7375):     at android.view.View$1.onClick(View.java:3686)
04-14 03:09:00.155: E/AndroidRuntime(7375):     ... 11 more
04-14 03:09:00.155: E/AndroidRuntime(7375): Caused by: java.lang.NullPointerException
04-14 03:09:00.155: E/AndroidRuntime(7375):     at com.chovanec.runo.MainActivity.onClickBtn(MainActivity.java:37)
04-14 03:09:00.155: E/AndroidRuntime(7375):     ... 14 more

Answer 1

应该是LinearLayout l = (LinearLayout) findViewById(R.id.main_view);

您还想念android:orientation="vertical"在您的线性布局中，这就是新按钮没有出现的原因。除非您希望按钮并排显示，否则您必须更改 android:layout_width="fill_parent"至android:layout_width="wrap_content"在你的按钮 xml 中

并且你不能使用

android:layout_alignRight="@+id/textView1"
android:layout_below="@+id/textView1"  

除非是相对布局。