使用后退按钮创建 Activity 时 getActionBar() 不起作用

Question

我想在标题栏中创建一个带有后退按钮（后退箭头）的活动 我试过了：

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

但控制台显示下一个错误：

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) 

我应该怎么做我开始使用 Android 这是代码

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);

        // 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

您已将Toolbar设置为您的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);
    ....
}

并且添加这个方法：

@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);
    }

希望这会有所帮助。

Answer 2

尝试getSupportActionBar() 而不是getActionBar()。并添加setHomeAsUpEnabled(true)。

Answer 3

不要使用 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

使用getSupportActionBar().setDisplayHomeAsUpEnabled(true) 在ActionBar 上显示Back/Arrow 图标。

更新您的Result_Contract.java 如下：

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 5

1. AndroidManifest.xml 中的 setParentActivity（假设 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. 将 getActionBar() 替换为 getSupportActionBar()

Answer 6

你必须得到支持操作栏，使用这个

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