Question

我在LogCat中收到此错误消息：

06-25 15：17：34.495：E / AndroidRuntime（5891）： java.lang.RuntimeException：无法启动活动 ComponentInfo {com。示例/ com.example.MainActivity}：的显示java.lang.NullPointerException 06-25 16：07：43.589：E / AndroidRuntime（6587）：引起：java.lang.NullPointerException 06-25 16：07：43.589：E / AndroidRuntime（6587）： at com.example.MyTabsListener.onTabSelected（MyTabListener.java:28）

这是我的主要活动：

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);
    }

}

我简单的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);
    }
}

最后我的班级片段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);
    }

}

所以这个函数导致了TabListener中的错误：

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

Answer 1

我遇到了同样的问题，我想我已经解决了。

基本上，您的 MainActivity 应该扩展 SherlockFragmentActivity 而不是 SherlockActivity 。

Answer 2

MainActivity必须延长SherlockFragmentActivity（积分给JosipPranjic！）。

可以使用此SO问题中的以下代码修复 NullPointerException ：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 3

看起来appContext未在代码中初始化......请初始化...

在MyTabsListener

中使用

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

但是如果可能的话，在MyTabsListener的构造函数中传递它而不是使那个静态

new MyTabsListener(fragment1,this)

Answer 4

appContext未初始化。请注意，类Activity扩展了类Context，因此，要显示Toast，就足够了

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

另一个问题：如果您想使用真实的应用程序上下文，请尝试getApplicationContext()。这将返回单例应用程序上下文。有关此问题的更多信息：link

添加TabListener时出现NullPointerException

4 个答案: