尝试getText时出现空指针错误

Question

我是Android新手，请原谅我的无知。我试图搜索论坛，但一直没有找到任何可以使之发挥作用的东西。我试图使用.getText（）。toString（）（见下文），但得到一个java.lang.NullPointerException错误。我改变了我的setContentView，以便它可以查看fragment_main.xml，但仍然为我的不同组件返回null。这个目标非常简单，只是为了改变TextView上的文本，因为我刚刚开始。任何帮助，将不胜感激。

package com.example.app; 

import android.support.v7.app.ActionBarActivity; 
import android.support.v7.app.ActionBar; 
import android.support.v4.app.Fragment; 
import android.os.Bundle; 
import android.view.LayoutInflater; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
import android.view.ViewGroup; 
import android.os.Build; 
import android.widget.Button; 
import android.widget.TextView; 

public class MainActivity extends ActionBarActivity { 

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

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

     Button changeTxt = (Button) findViewById(R.id.btnChangeText); 
     final TextView textView = (TextView) findViewById(R.id.txtMessage); 

     changeTxt.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View view) { 
       String text = textView.getText().toString(); 

       if (text.contains("World")) { 
        textView.setText("Hello Android!"); 
       } 
       else { 
        textView.setText(("Hello World!")); 
       } 
      } 
     }); 
    } 


    public void onStart(){ 
     super.onStart(); 
    } 

    public void onResume(){ 
     super.onResume(); 
    } 

    public void onPause(){ 
     super.onPause(); 
    } 

    public void onStop(){ 
     super.onStop(); 
    } 

    public void onDestroy(){ 
     super.onDestroy(); 
    } 

    @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; 
     } 
    } 
} 

这里是XML：

<LinearLayout 
    android:orientation="vertical" 
    android:layout_width="fill_parent" 
    android:layout_height="fill_parent" 
    xmlns:android="http://schemas.android.com/apk/res/android"> 

    <TextView 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:textAppearance="?android:attr/textAppearanceLarge" 
     android:text="Large Text" 
     android:id="@+id/txtMessage" 
     android:layout_gravity="center_horizontal" /> 

    <Button 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:text="Change Text" 
     android:id="@+id/btnChangeText" 
     android:layout_gravity="center_horizontal" /> 
</LinearLayout> 

Fllo，更新的代码：

XML fragment_main.xml

​​

主要活动

package com.example.app; 

import android.support.v7.app.ActionBarActivity; 
import android.support.v7.app.ActionBar; 
import android.support.v4.app.Fragment; 
import android.os.Bundle; 
import android.view.LayoutInflater; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
import android.view.ViewGroup; 
import android.os.Build; 
import android.widget.Button; 
import android.widget.TextView; 

public class MainActivity extends ActionBarActivity { 

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

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


    } 


    public void onStart(){ 
     super.onStart(); 
    } 

    public void onResume(){ 
     super.onResume(); 
    } 

    public void onPause(){ 
     super.onPause(); 
    } 

    public void onStop(){ 
     super.onStop(); 
    } 

    public void onDestroy(){ 
     super.onDestroy(); 
    } 

    @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); 

      Button changeTxt = (Button) findViewById(R.id.btnChangeText); 
      final TextView textView = (TextView) findViewById(R.id.txtMessage); 

      changeTxt.setOnClickListener(new View.OnClickListener() { 
       @Override 
       public void onClick(View view) 
       { 
        String text = textView.getText().toString(); 

        if (text.contains("World")) 
        { 
         textView.setText("Hello Android!"); 
        } 
        else 
        { 
         textView.setText(("Hello World!")); 
        } 
       } 
      }); 

      return rootView; 
     } 
    } 

} 

Answer 1

的NullPointerException异常发生的原因，似乎你有你的Activity和你Fragment相同的布局：

// Activity 
setContentView(R.layout.fragment_main); 

// Fragment 
View rootView = inflater.inflate(R.layout.fragment_main, container, false); 
return rootView; 

你需要检查你的意见（TextView的&按钮）是在正确的布局，而不是进入aonther（我猜activity_main）。

两种解决方案：

• 复制/粘贴你的观点里fragment_main到activity_main和改变setContentView(R.layout.activity_main)。

• 或者让你的观点里fragment_main和使用下面的代码，以找到您Fragment里面你的看法，具体如下：

  @Override 
  public View onCreateView(LayoutInflater inflater, ViewGroup container, 
       Bundle savedInstanceState) { 
      View rootView = inflater.inflate(R.layout.fragment_main, container, false); 
      Button changeTxt = (Button) rootView.findViewById(R.id.btnChangeText); 
      final TextView textView = (TextView) rootView.findViewById(R.id.txtMessage); 
  
      changeTxt.setOnClickListener(new View.OnClickListener() { 
       @Override 
       public void onClick(View view) { 
        String text = textView.getText().toString(); 
  
        if (text.contains("World")) { 
         textView.setText("Hello Android!"); 
        } 
        else { 
         textView.setText(("Hello World!")); 
        } 
       } 
      }); 
      return rootView; 
  } 
  

希望这有助于。

---

UPDATE：

1。在fragment_main内删除您的FrameLayout。创建一个名为activity_main新的布局，并添加的FrameLayout进去（用ID）为：

<FrameLayout 
    xmlns:android="http://schemas.android.com/apk/res/android" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    android:id="@+id/container" /> 

2。设置此布局onCreate方法到您的活动像：

// onCreate method Activity 
setContentView(R.layout.activity_main); 

3。然后，删除static在片段声明，这应该是：

public class PlaceholderFragment ... { } 

4。最后，当你试图找到里面onCreateView方法ID的元素，你必须始终使用膨胀的观点为：

rootView.findViewById(R.id.some_id_example); 

瞧！这应该现在工作。