我只想开始在Android上制作aps，我对C ++有点了解，但是我用最简单的东西战斗了几天 - 我不能让我的按钮工作。
我尝试了每个教程方法，但我不能让听众按钮 - 每次我这样做，我的程序就会崩溃。
代码很简单（我不会在我的方法之下改变任何东西），我也开始了全新的Android项目，只是在所有教程中进行了那些按钮和TextView编辑：
package com.example.buttonwork;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v7.app.ActionBarActivity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;
public class MainActivity extends ActionBarActivity {
Button but1;
TextView tv1;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment())
.commit();
}
tv1 = (TextView) findViewById(R.id.tv1);
but1 = (Button) findViewById(R.id.b1);
but1.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
tv1.setText("Button pressed");
}
});
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
/**
* A placeholder fragment containing a simple view.
*/
public static class PlaceholderFragment extends Fragment {
public PlaceholderFragment() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container, false);
return rootView;
}
}
}
The LOGCAT:
04-26 15:51:39.435: W/dalvikvm(7185): threadid=1: thread exiting with uncaught exception (group=0x418762a0)
04-26 15:51:39.450: E/AndroidRuntime(7185): FATAL EXCEPTION: main
04-26 15:51:39.450: E/AndroidRuntime(7185): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.buttonwork/com.example.buttonwork.MainActivity}: java.lang.NullPointerException
04-26 15:51:39.450: E/AndroidRuntime(7185): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2110)
04-26 15:51:39.450: E/AndroidRuntime(7185): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2135)
04-26 15:51:39.450: E/AndroidRuntime(7185): at android.app.ActivityThread.access$700(ActivityThread.java:140)
04-26 15:51:39.450: E/AndroidRuntime(7185): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1237)
04-26 15:51:39.450: E/AndroidRuntime(7185): at android.os.Handler.dispatchMessage(Handler.java:99)
04-26 15:51:39.450: E/AndroidRuntime(7185): at android.os.Looper.loop(Looper.java:137)
04-26 15:51:39.450: E/AndroidRuntime(7185): at android.app.ActivityThread.main(ActivityThread.java:4921)
04-26 15:51:39.450: E/AndroidRuntime(7185): at java.lang.reflect.Method.invokeNative(Native Method)
04-26 15:51:39.450: E/AndroidRuntime(7185): at java.lang.reflect.Method.invoke(Method.java:511)
04-26 15:51:39.450: E/AndroidRuntime(7185): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1027)
04-26 15:51:39.450: E/AndroidRuntime(7185): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:794)
04-26 15:51:39.450: E/AndroidRuntime(7185): at dalvik.system.NativeStart.main(Native Method)
04-26 15:51:39.450: E/AndroidRuntime(7185): Caused by: java.lang.NullPointerException
04-26 15:51:39.450: E/AndroidRuntime(7185): at com.example.buttonwork.MainActivity.onCreate(MainActivity.java:29)
04-26 15:51:39.450: E/AndroidRuntime(7185): at android.app.Activity.performCreate(Activity.java:5206)
04-26 15:51:39.450: E/AndroidRuntime(7185): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1094)
04-26 15:51:39.450: E/AndroidRuntime(7185): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2074)
04-26 15:51:39.450: E/AndroidRuntime(7185): ... 11 more
程序每次都崩溃了。我已导入所有内容，ID和名称也很好。请帮助我，因为现在我很绝望。我只是想通过这些简单的事情来开始编写真正有效且有输出的东西，但我陷入了这么简单的一步......
答案 0 :(得分：1)
在这里你 ..这就是你犯错误的地方。您试图获取活动中的引用而不是您定义的片段。
从 onCreate（）和占位符片段中删除按钮，添加这些组件。这就是你得到的原因空指针。
注意：使用 getActivity（）来引用片段中的组件。
在你片段中，你必须做出这样的改变：
public static class PlaceholderFragment extends Fragment {
public PlaceholderFragment (){}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container, false);
return rootView;
}
@Override
public void onActivityCreated(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onActivityCreated(savedInstanceState);
Button but1 = (Button)getActivity(). findViewById(R.id.b1);
TextView tv1 = (TextView)getActivity(). findViewById(R.id.tv1);
but1.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
tv1.setText("Button pressed");
}
});
}
}
希望这会有所帮助.. :)
答案 1 :(得分：0)
使用此功能并从tb1方法中删除bt1和onCreate(Bundle savedInstanceState)初始化代码
/**
* A placeholder fragment containing a simple view.
*/
public static class PlaceholderFragment extends Fragment {
public PlaceholderFragment() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container, false);
tv1 = (TextView) (rootView.findViewById(R.id.tv1));
but1 = (Button) (rootView.findViewById(R.id.b1));
but1.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
tv1.setText("Button pressed");
}
});
return rootView;
}
}