添加后退按钮活动

Question

我正在使用android 2.1，希望在Mapsavity上添加一个后退按钮。我在这一页上尝试了什么[在操作栏上显示后退按钮，但应用程序崩溃。我已经按照另一个论坛的建议将extends FragmentActivity替换为AppCompatActivity，但应用程序仍然崩溃。我知道这与操作栏有关，因为如果我删除它，应用程序就会工作。操作栏似乎为空。我被卡住了，已经启动了应用程序数次。下面是错误：

 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.pc.canda.theplacestosee/com.pc.canda.theplacestosee.MapsActivity}: 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:2298)
                                                                                        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
                                                                                        at android.app.ActivityThread.access$800(ActivityThread.java:144)
                                                                                        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
                                                                                        at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                                        at android.os.Looper.loop(Looper.java:135)
                                                                                        at android.app.ActivityThread.main(ActivityThread.java:5221)
                                                                                        at java.lang.reflect.Method.invoke(Native Method)
                                                                                        at java.lang.reflect.Method.invoke(Method.java:372)
                                                                                        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
                                                                                        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
                                                                                     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference 
                                                                                        at com.pc.canda.theplacestosee.MapsActivity.onCreate(MapsActivity.java:31)
                                                                                        at android.app.Activity.performCreate(Activity.java:5937)
                                                                                        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
                                                                                        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
                                                                                        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
                                                                                        at android.app.ActivityThread.access$800(ActivityThread.java:144)
                                                                                        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
                                                                                        at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                                        at android.os.Looper.loop(Looper.java:135)
                                                                                        at android.app.ActivityThread.main(ActivityThread.java:5221)
                                                                                        at java.lang.reflect.Method.invoke(Native Method)
                                                                                        at java.lang.reflect.Method.invoke(Method.java:372)
                                                                                        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
                                                                                        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694

Answer 1

几件事。确保您的操作扩展AppCompatactive，确保您使用的样式有一个操作栏（您在清单中设置此项）。

android:theme="@style/AppTheme"

确保您使用的是正确的操作栏（ActionBar或SupportActionBar）

getSupportActionBar().setDisplayHomeAsUpEnabled(true);

或

getActionBar().setDisplayHomeAsUpEnabled(true);

最后，添加onOptionsItemSelec方法，以便您的后退按钮知道该做什么。

public boolean onOptionsItemSelected(MenuItem item) {

    switch (item.getItemId()) {
        case android.R.id.home:
            finish();
            return true;
    }

    return super.onOptionsItemSelected(item);
}