從工具欄管理工具欄中的按鈕從android中的片段

Question

我想在我的工具欄中找回按鈕，但我的應用崩潰了。 這是我MainActivity.java：

package com.penta.games.waslehrernichtdrfen; 

import android.os.Bundle; 
import android.support.v4.app.FragmentManager; 
import android.support.v4.app.FragmentTransaction; 
import android.support.v7.app.AppCompatActivity; 
import android.view.View; 
import android.widget.Button; 

public class MainActivity extends AppCompatActivity { 
    boolean status = false; 
    Button levelauswahl; 

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

     levelauswahl = (Button)findViewById(R.id.levelauswahl); 
     levelauswahl.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
       FragmentManager fragmentManager = getSupportFragmentManager(); 
       FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction(); 

        LevelAuswahl levelAuswahl = new LevelAuswahl(); 
        fragmentTransaction.add(R.id.fragment_container, levelAuswahl); 
        fragmentTransaction.addToBackStack(null); 
        fragmentTransaction.commit(); 


      } 
     }); 


    } 

} 

這是我的片段：

package com.penta.games.waslehrernichtdrfen; 


import android.os.Bundle; 
import android.support.v4.app.Fragment; 
import android.support.v7.widget.Toolbar; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 

public class LevelAuswahl extends Fragment { 
    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, 
          Bundle savedInstanceState) { 
     // Inflate the layout for this fragment 

     View rootView = inflater.inflate(R.layout.level_auswahl, container, false); 
     Toolbar toolbar = (Toolbar) getView().findViewById(R.id.toolbar); 
     toolbar.setTitle("Title"); 
     toolbar.setNavigationIcon(R.drawable.backarrow1); 
     toolbar.setNavigationOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
       getActivity().onBackPressed(); 
      } 
     }); 
     return rootView; 
    } 
} 

我想，如果我打開我可以回到在MainActivity與點擊後退箭頭的片段在我的工具欄中。 但是我的應用程序崩潰，如果我嘗試打開我的片段。

這裏是我的fragment.xml之：

<FrameLayout 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" 
    xmlns:app="http://schemas.android.com/apk/res-auto" 
    android:id="@+id/levelauswahl" 
    android:background="#ffffff" 
    tools:context="com.penta.games.waslehrernichtdrfen.LevelAuswahl"> 


    <android.support.v7.widget.Toolbar 
     xmlns:android="http://schemas.android.com/apk/res/android" 
     android:layout_width="match_parent" 
     android:layout_height="wrap_content" 
     android:background="#e70000" 
     android:id="@+id/toolbar" 
     android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar" 
     app:title="Was Lehrer nicht dürfen" /> 
    </RelativeLayout> 

工具欄應該是永遠存在的，但後退按鈕只有當我打開一個片段。 我正在爲java編程一週，所以請原諒我的小知識。

這是從我的logcat輸出：

08-15 22:44:47.799 2086-2086/? E/AndroidRuntime: FATAL EXCEPTION: main 
               Process: com.penta.games.waslehrernichtdrfen, PID: 2086 
               java.lang.NullPointerException 
                at com.penta.games.waslehrernichtdrfen.LevelAuswahl.onCreateView(LevelAuswahl.java:18) 
                at android.support.v4.app.Fragment.performCreateView(Fragment.java:2192) 
                at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1299) 
                at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1528) 
                at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1595) 
                at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:758) 
                at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2363) 
                at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2149) 
                at android.support.v4.app.FragmentManagerImpl.optimizeAndExecuteOps(FragmentManager.java:2103) 
                at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2013) 
                at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:710) 
                at android.os.Handler.handleCallback(Handler.java:733) 
                at android.os.Handler.dispatchMessage(Handler.java:95) 
                at android.os.Looper.loop(Looper.java:157) 
                at android.app.ActivityThread.main(ActivityThread.java:5356) 
                at java.lang.reflect.Method.invokeNative(Native Method) 
                at java.lang.reflect.Method.invoke(Method.java:515) 
                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265) 
                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081) 
                at dalvik.system.NativeStart.main(Native Method) 

Answer 1

我發現的錯誤。嘗試在你的片段的oncreateView方法

View rootView = inflater.inflate(R.layout.level_auswahl, container, false); 
    //the view here should be the one which inflates the layout which contains your toolbar. 
    //I assume it is rootview which u mentioned just above this line. 
    Toolbar toolbar = (Toolbar) rootView.findViewById(R.id.toolbar); 
    toolbar.setTitle("Title"); 
    toolbar.setNavigationIcon(R.drawable.backarrow1); 
    toolbar.setNavigationOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 

      //achieve your behaviour here 
     } 
    }); 
    return rootView;