java - 我无法让我的按钮工作

Question

我只是想开始在 Android 上制作 aps，我懂一点 C++，但我为最简单的事情奋斗了几天 - 我无法让我的按钮工作。

我尝试了所有教程方法，但我无法让听众按下按钮 - 每次我这样做时，我的程序都会崩溃。

代码很简单(我没有更改我的方法下面的任何内容)，我还启动了全新的 Android 项目，只是像所有教程中那样进行了这些按钮和 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

而且程序每次都会崩溃。我已经导入了所有内容，ID 和名称也很好。请帮助我，因为现在我很绝望。我只是想完成这些简单的事情并开始编写一些真正有效、具有和输出的东西，但我陷入了如此简单的步骤......

Answer 1

给你..这就是你犯错误的地方。您试图获取 Activity 中的引用而不是您定义的 fragment 。

从 onCreate() 中删除按钮，并在占位符 fragment 中添加这些组件。这就是您得到空指针。

注意:使用getActivity()来引用 fragment 中的组件。

在您的 fragment 中，您必须进行如下更改:

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");
                
            }
        });
    }
}

希望这有帮助..:)