的NullPointerException添加TabListener

Question

，當我在logcat中收到此錯誤信息：

06-25 15：17：34.495：E/AndroidRuntime（5891）： 了java.lang.RuntimeException：無法啓動活動 ComponentInfo {com。示例/ com.example.MainActivity}： 顯示java.lang.NullPointerException 06-25 16：07：43.589：E/AndroidRuntime（6587）：由：顯示java.lang.NullPointerException 06-25 16：07：43.589：E/AndroidRuntime（6587）：at com.example.MyTabsListener.onTabSelected（MyTabListener.java:28）

這是我的主要活動：

package com.example; 

import android.content.Context; 
import android.os.Bundle; 
import android.support.v4.app.Fragment; 

import com.actionbarsherlock.app.ActionBar; 
import com.actionbarsherlock.app.SherlockActivity; 

public class MainActivity extends SherlockActivity {  
    /** Called when the activity is first created. */ 
    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.main); 

     ActionBar actionbar = getSupportActionBar(); 
     actionbar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS); 

     ActionBar.Tab tab1 = actionbar.newTab().setText("Fragment A"); 
     SherlockFragment fragment1 = new AFragment(); 

     tab1.setTabListener(new MyTabsListener(fragment1, this 
      .getApplicationContext())); 
     actionbar.addTab(tab1); 
    } 

} 

而且我簡單TabListener：

package com.example;  

import android.content.Context; 
import android.support.v4.app.FragmentTransaction; 
import android.widget.Toast; 

import com.actionbarsherlock.app.ActionBar; 
import com.actionbarsherlock.app.ActionBar.Tab; 
import com.actionbarsherlock.app.SherlockFragment; 

class MyTabsListener implements ActionBar.TabListener { 
    public SherlockFragment fragment; 
    private final Context appContext; 

    public MyTabsListener(SherlockFragment fragment, Context ctx) { 
        this.fragment = fragment; 
        this.appContext = ctx; 
    } 

    @Override 
    public void onTabReselected(Tab tab, FragmentTransaction ft) { 
        Toast.makeText(this.appContext, "Reselected!", Toast.LENGTH_LONG) 
                .show(); 
    } 

    @Override 
    public void onTabSelected(Tab tab, FragmentTransaction ft) { 
        ft.replace(R.id.fragment_container, fragment); 
    } 

    @Override 
    public void onTabUnselected(Tab tab, FragmentTransaction ft) { 
        ft.remove(fragment); 
    } 
} 

最後我班片段A：

package com.example; 

import android.os.Bundle; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 

import com.actionbarsherlock.app.SherlockFragment; 

public class AFragment extends SherlockFragment { 

    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, 
      Bundle savedInstanceState) { 
     // Inflate the layout for this fragment 
     return inflater.inflate(R.layout.weekdays, container, false); 
    } 

} 

所以這功能導致錯誤的TabListener：

@Override 
public void onTabSelected(Tab tab, FragmentTransaction ft) { 
    ft.replace(R.id.fragment_container, fragment); 
} 

Answer 1

1. MainActivity必須延長SherlockFragmentActivity（信貸給JosipPranjic！）。

2. 的NullPointerException異常可以固定從本SO問題下面的代碼：Null FragmentTransaction being passed to TabListener.onTabSelected()

   @Override 
   public void onTabSelected(Tab tab, FragmentTransaction ignoredFt) { 
       FragmentManager fragMgr = ((FragmentActivity) mActivity) 
        .getSupportFragmentManager(); 
       FragmentTransaction ft = fragMgr.beginTransaction(); 
   
       // Check if the fragment is already initialized 
       if (mFragment == null) { 
        // If not, instantiate and add it to the activity 
        mFragment = Fragment.instantiate(mActivity, mClass.getName()); 
   
        ft.add(android.R.id.content, mFragment, mTag); 
       } else { 
        // If it exists, simply attach it in order to show it 
        ft.attach(mFragment); 
       } 
   
       ft.commit(); 
   } 
   
   @Override 
   public void onTabUnselected(Tab tab, FragmentTransaction ignoredFt) { 
       FragmentManager fragMgr = ((FragmentActivity) mActivity) 
         .getSupportFragmentManager(); 
       FragmentTransaction ft = fragMgr.beginTransaction(); 
   
       // Check if the fragment is already initialized 
       if (mFragment == null) { 
        // If not, instantiate and add it to the activity 
        mFragment = Fragment.instantiate(mActivity, mClass.getName()); 
   
        ft.add(android.R.id.content, mFragment, mTag); 
       } else { 
        // If it exists, simply attach it in order to show it 
        ft.detach(mFragment); 
       } 
   
       ft.commit(); 
   } 
   

Answer 2

看起來appContext在代碼中沒有初始化......請初始化...

用於MyTabsListener

Toast.makeText(MainActivity.appContext, "Reselected!", Toast.LENGTH_LONG).show(); 

但如果可能的話將它傳遞給MyTabsListener的構造函數，而不是使其成爲靜態

new MyTabsListener(fragment1,this) 

Answer 3

appContext未初始化。 注意該類Activity擴展Context類，等等，表現出Toast，這將足以

Toast.makeText(MainActivity, "Reselected!", Toast.LENGTH_LONG).show(); 

另一個問題：如果你想使用的實際應用背景下，嘗試getApplicationContext()。這將返回單例應用程序上下文。更多的信息：link

Answer 4

我剛剛遇到了同樣的問題，我想我解決了它。

基本上，你的MainActivity應該延伸SherlockFragmentActivity和NOT SherlockActivity。