添加onClickListener Android时应用程序崩溃

我不知道为什么，但我似乎无法找出我的代码出了什么问题。我一添加onclick侦听器，应用程序就会崩溃。我真的需要帮助。我也看了其他线程，但我似乎无法找出我做错了什么。谢谢 代码如下：

package com.example.medrec;
import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.app.AlertDialog;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.view.Window;
import android.view.WindowManager;
import android.widget.Button;
import android.widget.Toast;
import android.os.Build;

public class MainActivity extends ActionBarActivity implements OnClickListener{

Button login;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, 
            WindowManager.LayoutParams.FLAG_FULLSCREEN);
    setContentView(R.layout.activity_main);

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }


    login = (Button)findViewById(R.id.w_login);
    //Crashes when I add this line.
    login.setOnClickListener(this);
}





@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;
    }
}

@Override
public void onClick(View v) {
    // TODO Auto-generated method stub

}

}

My activity_main.xml

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.example.medrec.MainActivity"
tools:ignore="MergeRootFrame" />

My fragment_main.xml

<RelativeLayout 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"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"

tools:context="com.example.medrec.MainActivity$PlaceholderFragment" >

<EditText
    android:id="@+id/w_number_edit_text"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/textView1"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="21dp"
    android:ems="10"
    android:inputType="number" >

    <requestFocus />
</EditText>

<TextView
    android:id="@+id/textView1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="118dp"
    android:text="@string/w_number" />

<Button
    android:id="@+id/w_login"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/w_number_edit_text"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="15dp"
    android:text="@string/w_login_text" />

---

你的应用程序崩溃，这是因为你的按钮w_登录属于你的fragment_main.xml，你必须为你的活动设置activity_main.xml布局

At代码

login = (Button) rootView.findViewById(R.id.w_login);
login.setOnClickListener(this);

---

您应该在片段onCreateView中引用此按钮

我认为它崩溃是因为您的登录名为null，而它为null是因为您的活动布局中不存在w_登录名。从片段中尝试此rootView

login = (Button) rootView.findViewById(R.id.w_login);
login.setOnClickListener(this);

请尝试以下代码：-

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);

    login = (Button)rootView.findViewById(R.id.w_login);
    //Crashes when I add this line.
    login.setOnClickListener(this);
        return rootView;
    }
}

您的应用程序在搜索activity_main.xml中的按钮时崩溃，显示NullPointerException，但在那里找不到。相反，您应该在占位符片段中添加按钮代码

从MainActivity中删除以下行

将以下内容添加到占位符片段中

---

类似的问题太多了。按钮属于片段布局，应该在片段中初始化，这是什么意思？你能详细说明一下吗？你应该在fragment中初始化你的视图，因为你在那里膨胀fragment_main.xml，视图属于fragment Layout最终得到了这个想法。谢谢你，我工作得很好。非常感谢你。不过，我仍然需要在片段方法上实现OnClickListener。

  assign on click listener in  onCreateView

   login = (Button) rootView.findViewById(R.id.w_login);
        //Crashes when I add this line.
      login.setOnClickListener(this);

login = (Button)findViewById(R.id.w_login);
    //Crashes when I add this line.
    login.setOnClickListener(this);

public static class PlaceholderFragment extends Fragment implements OnClickListener{

    public PlaceholderFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container, false);
        Button login = (Button)rootView.findViewById(R.id.w_login);
        login.setOnClickListener(this);
        return rootView;
    }
}