getActionBar（）创建与后退按钮的活动

Question

我想新建与在标题栏中 后退按钮（后退箭头）的活动我已经试过不工作时：

ActionBar actionBar = getActionBar(); 
actionBar.setDisplayHomeAsUpEnabled(true); 

但控制台正显示出我的下一个错误：

E/AndroidRuntime: FATAL EXCEPTION: main 
     Process: com.example.ernesto.apptranscargo, PID: 23286 
     java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.ernesto.apptranscargo/com.example.ernesto.apptranscargo.Result_contract}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416) 
      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
      at android.app.ActivityThread.-wrap11(ActivityThread.java) 
      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
      at android.os.Handler.dispatchMessage(Handler.java:102) 
      at android.os.Looper.loop(Looper.java:148) 
      at android.app.ActivityThread.main(ActivityThread.java:5417) 
      at java.lang.reflect.Method.invoke(Native Method) 
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference 
      at com.example.ernesto.apptranscargo.Result_contract.onCreate(Result_contract.java:23) 
      at android.app.Activity.performCreate(Activity.java:6237) 
      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107) 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369) 
      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)  
      at android.app.ActivityThread.-wrap11(ActivityThread.java)  
      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)  
      at android.os.Handler.dispatchMessage(Handler.java:102)  
      at android.os.Looper.loop(Looper.java:148)  
      at android.app.ActivityThread.main(ActivityThread.java:5417)  
      at java.lang.reflect.Method.invoke(Native Method)  
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)  
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)  

我应该怎么做我正在beggining的Android 下面是代码

RESU lt_contract.java

public class Result_contract extends AppCompatActivity { 

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

     Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
     setSupportActionBar(toolbar); 

     // I put these lines here and there is an error 
     //ActionBar actionBar = getActionBar(); 
     //actionBar.setDisplayHomeAsUpEnabled(true); 

     FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab); 
     fab.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View view) { 
       Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG) 
         .setAction("Action", null).show(); 
      } 
     }); 
    } 

} 

activity_result_contract.xml

<?xml version="1.0" encoding="utf-8"?> 
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:app="http://schemas.android.com/apk/res-auto" 
    xmlns:tools="http://schemas.android.com/tools" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    android:fitsSystemWindows="true" 
    tools:context="com.example.ernesto.apptranscargo.Result_contract"> 

    <android.support.design.widget.AppBarLayout 
     android:layout_width="match_parent" 
     android:layout_height="wrap_content" 
     android:theme="@style/AppTheme.AppBarOverlay"> 

     <android.support.v7.widget.Toolbar 
      android:id="@+id/toolbar" 
      android:layout_width="match_parent" 
      android:layout_height="?attr/actionBarSize" 
      android:background="?attr/colorPrimary" 
      app:popupTheme="@style/AppTheme.PopupOverlay" /> 

    </android.support.design.widget.AppBarLayout> 

    <include layout="@layout/content_result_contract" /> 

    <android.support.design.widget.FloatingActionButton 
     android:id="@+id/fab" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_gravity="bottom|end" 
     android:layout_margin="@dimen/fab_margin" 
     android:src="@android:drawable/ic_dialog_email" /> 

</android.support.design.widget.CoordinatorLayout> 

Answer 1

您已经设置为Toolbar为您的supportActionBar。因此将setDisplayHomeAsUpEnabled设置为ActionBar将导致NullPointerExeption。试试下面的代码：

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

    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
    setSupportActionBar(toolbar); 
    getSupportActionBar().setDisplayHomeAsUpEnabled(true); 
    getSupportActionBar().setHomeButtonEnabled(true); 
    .... 
} 

而且添加这种方法：

@Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     int id = item.getItemId(); 
     switch (id) { 
      // Respond to the action bar's Up/Home button 
      case android.R.id.home: 
       onBackPressed(); 
       return true; 
     } 
     return super.onOptionsItemSelected(item); 
    } 

希望这有助于。

Answer 2

尝试getSupportActionBar()代替getActionBar()。并添加setHomeAsUpEnabled(true)。

Answer 3

请勿使用getActionBar。使用getSupportActionBar（）

Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
setSupportActionBar(toolbar); 

    //This line helps to display back arrow 
getSupportActionBar().setDisplayHomeAsUpEnabled(true); 
getSupportActionBar().setDisplayShowHomeEnabled(true); 

Answer 4

你必须得到您的支持行动起来吧，用这个

Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
setSupportActionBar(toolbar); 
getSupportActionBar().setTitle(" ");//Set title 
getSupportActionBar().setSubtitle(" ");//Set Subtitle 
getSupportActionBar().setDisplayHomeAsUpEnabled(true);//Enable Back button 

Answer 5

使用getSupportActionBar().setDisplayHomeAsUpEnabled(true)上显示ActionBarBack/Arrow图标。

更新您如下Result_Contract.java：

public class Result_contract extends AppCompatActivity { 

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

     Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
     setSupportActionBar(toolbar); 

     // Required to show ARROW icon 
     getSupportActionBar().setDisplayHomeAsUpEnabled(true); 

     FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab); 
     fab.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View view) { 
       Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG) 
         .setAction("Action", null).show(); 
      } 
     }); 
    } 

} 

Answer 6

setParentActivity在AndroidManifest.xml（比方说ParentActivity是MainActivity）

<activity 
    android:name=".DisplayMessageActivity" 
    android:parentActivityName=".MainActivity" > 
    <!-- Parent activity meta-data to support 4.0 and lower --> 
    <meta-data 
     android:name="android.support.PARENT_ACTIVITY" 
     android:value=".MainActivity" /> 
</activity> 

2.更换getActionBar（）与getSupportActionBar（）