Question

我正在使用Android 2.1，并希望为MapsAvtivity添加一个后退按钮。我已经尝试了此页面上的内容[Display back button on action bar但应用程序崩溃了。我已经用另一个论坛推荐的AppCompatActivity替换了扩展FragmentActivity，但应用程序仍然崩溃。我知道它与动作栏有关，因为如果我删除它，应用程序就可以了。似乎操作栏为空。我被卡住并且已经多次启动了应用程序。这是错误：

 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

一些事情。确保您的acttivity扩展AppCompatActivity，确保您使用的样式有一个操作栏（您在清单中设置它）。

android:theme="@style/AppTheme"

确保使用正确的操作栏（ActionBar或SupportActionBar）

getSupportActionBar().setDisplayHomeAsUpEnabled(true);

或

getActionBar().setDisplayHomeAsUpEnabled(true);

最后，添加onOptionsItemSelected方法，以便后退按钮知道该怎么做。

public boolean onOptionsItemSelected(MenuItem item) {

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

    return super.onOptionsItemSelected(item);
}

添加后退按钮活动

1 个答案: