Question

我正在尝试更改像Fragment（用于SlidingTabLayout）部分的TextViews等元素。我可以从Tab1类访问TextView：

public class Tab1 extends Fragment {

public static TextView serverName;

@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.tab_1,container,false);
    serverName = (TextView) view.findViewById(R.id.serverName);
    serverName.setText("This works, but I can't change text from outside the Tab1 class");
    return view;
}

但是当我想从任何地方访问serverName TextView时，我总是得到null值。在这里，我试图从包含滑动选项卡的活动中更改文本（Tab1是其中的一部分）：

public class Dashboard2 extends AppCompatActivity {

Toolbar toolbar;
ViewPager pager;
ViewPagerAdapter adapter;
SlidingTabLayout tabs;
CharSequence tabsTitles[] = {"Info", "Options"};

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_dashboard2);
    InitializeToolbarAndTabs();
    Tab1.serverName.setText("This doesn't work");
}

private void InitializeToolbarAndTabs()
{
    toolbar = (Toolbar) findViewById(R.id.tool_bar);
    setSupportActionBar(toolbar);

    adapter =  new ViewPagerAdapter(getSupportFragmentManager(), tabsTitles, tabsTitles.length);

    pager = (ViewPager) findViewById(R.id.pager);
    pager.setAdapter(adapter);

    tabs = (SlidingTabLayout) findViewById(R.id.tabs);
    tabs.setDistributeEvenly(true);

    tabs.setCustomTabColorizer(new SlidingTabLayout.TabColorizer() {
        @Override
        public int getIndicatorColor(int position) {
            return getResources().getColor(R.color.tabsScrollColor);
        }
    });

    tabs.setViewPager(pager);
}

}

来自Android Studio的日志：

java.lang.RuntimeException: Unable to start activity ComponentInfo{ASD.Dashboard2}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3119)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3218)
        at android.app.ActivityThread.access$1000(ActivityThread.java:198)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1676)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:145)
        at android.app.ActivityThread.main(ActivityThread.java:6837)
        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:1404)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
        at ASD.Dashboard2.onCreate(Dashboard2.java:54)
        at android.app.Activity.performCreate(Activity.java:6500)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1120)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3072)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3218)
        at android.app.ActivityThread.access$1000(ActivityThread.java:198)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1676)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:145)
        at android.app.ActivityThread.main(ActivityThread.java:6837)
        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:1404)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)

尝试调用虚方法＆＃39; void android.widget.TextView.setText（java.lang.CharSequence）＆＃39;在null对象引用上 - 如何解决这个问题？

Answer 1

不确定您在做什么，但您始终可以通过FragmentManage.findFragmentByTag()或FragmentManager.findFragmentById（）找到您的片段。一旦找到，只需访问您的领域。

Answer 2

我认为你没有初始化Tab1片段，至少我看不到它。

通过静态声明从Activity访问片段变量是一个可怕的想法，在片段类中使用void。

Answer 3

很抱歉，您获得的错误与访问或不是Fragment的静态对象无关。您收到的错误是因为当您拨打Tab1.serverName.setText("This doesn't work");时，您的片段仍然没有给视图充气，或者仍未向TextView收费（尚未到达serverName = (TextView) view.findViewById(R.id.serverName);）。片段以异步方式充入视图，因此即使您在布局中将其声明为标记，片段的视图仍可能未完全加载。

如果您确定要确保片段的视图已完全加载，请使用protected void onResumeFragments()方法：

@Override
protected void onResumeFragments() {
    super.onResumeFragments();
    Tab1.serverName.setText("This doesn't work");
}

无论如何，我强烈建议您不要静态访问片段对象，而是使用findFragmentById()或findFragmentByTag()方法然后访问给定片段内的公共方法。

希望它有所帮助。

Android：无法访问Fragment类之外的静态元素

3 个答案: