java - NullPointEreException 选项卡布局 GridView

Question

我正在尝试在选项卡布局中制作一个简单的 Android gridView。但是，我的应用程序最终崩溃了。

这是我的 Activity ，其中包含选项卡:

EventsActivity.java

package com.grace.main;

import android.app.ActionBar;
import android.app.Fragment;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.GridView;

public class EventsActivity extends FragmentActivity implements ActionBar.TabListener {
    //needs R string
    private static final String STATE_SELECTED_NAVIGATION_ITEM = "selected_navigation_item";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_events);

        //Set up the action bar to show tabs
        final ActionBar actionBar = getActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        //Add a tab to the action bar
        //needs R.string values
        actionBar.addTab(actionBar.newTab().setText("Tab 1").setTabListener(this));
        actionBar.addTab(actionBar.newTab().setText("Tab 2").setTabListener(this));
        actionBar.addTab(actionBar.newTab().setText("Tab 3").setTabListener(this));
    }

    @Override
    public void onRestoreInstanceState(Bundle savedInstanceState){
        //restores the previously serialised current tab position
        if (savedInstanceState.containsKey(STATE_SELECTED_NAVIGATION_ITEM)){
            getActionBar().setSelectedNavigationItem(savedInstanceState.getInt(STATE_SELECTED_NAVIGATION_ITEM));
        }
    }

    @Override
    public void onSaveInstanceState(Bundle outState){
        //Serialise the current tab position
        outState.putInt(STATE_SELECTED_NAVIGATION_ITEM, getActionBar().getSelectedNavigationIndex());
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.events, menu);
        return true;
    }

    @Override
    public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
        //Uses container view
        Fragment fragment = new DummySectionFragment();
        Bundle args = new Bundle();
        args.putInt(DummySectionFragment.ARG_SECTION_NUMBER, tab.getPosition()+1);
        fragment.setArguments(args);
        getFragmentManager().beginTransaction().replace(R.id.container, fragment).commit();
    }

    @Override
    public void onTabUnselected(ActionBar.Tab tab,FragmentTransaction fragmentTransaction) {
        //Do nothing
    }

    @Override
    public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
        //Do nothing
    }

    //dummy test
    public static class DummySectionFragment extends Fragment {
        public static final String ARG_SECTION_NUMBER = "placeholder_text";
        GridView gridView;
        static final String[] numbers1 = new String[] {

            "1", "2", "3", "4", "5"
        };
        static final String[] numbers2 = new String[] {

            "6", "7", "8", "9", "10"
        };
        static final String[] numbers3 = new String[] {

            "11", "12", "13", "14", "15"
        };

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,Bundle savedInstanceState) {
            GridView gridview = (GridView)getActivity() .findViewById(R.id.gridview);

            switch (getArguments().getInt(ARG_SECTION_NUMBER)) {
                case 1:
                    ArrayAdapter<String> adapter1 = new ArrayAdapter<String>(getActivity(),android.R.layout.simple_list_item_1, numbers1);
                    gridView.setAdapter(adapter1);
                    break;
                case 2:
                    ArrayAdapter<String> adapter2 = new ArrayAdapter<String>(getActivity(),android.R.layout.simple_list_item_1, numbers2);
                    gridView.setAdapter(adapter2);
                    break;
                case 3:
                    ArrayAdapter<String> adapter3 = new ArrayAdapter<String>(getActivity(),android.R.layout.simple_list_item_1, numbers3);
                    gridView.setAdapter(adapter3);
                    break;
                default:
                    //textView.setText("HELLO");
                    break;
            }
            return gridview;
        }
      }
}

这是我的布局 xml:Activity_events.xml

<FrameLayout xmlns:tools="http://schemas.android.com/tools"
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent" 
    tools:context="capstone.DGW.Main.EventsActivity"
    tools:ignore="MergeRootFrame" >

    <GridView
        android:id="@+id/gridview"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:columnWidth="50dp"
        android:gravity="center"
        android:numColumns="auto_fit"
        android:stretchMode="columnWidth" >

    </GridView>
</FrameLayout>

我的 LogCat 错误是:

05-06 10:35:47.637: E/AndroidRuntime(16838):    Caused by: java.lang.NullPointerException

05-06 10:35:47.637: E/AndroidRuntime(16838):    at com.grace.main.EventsActivity$DummySectionFragment.onCreateView(EventsActivity.java:99)

05-06 10:35:47.637: E/AndroidRuntime(16838):    at android.app.Fragment.performCreateView(Fragment.java:1700)

05-06 10:35:47.637: E/AndroidRuntime(16838):    at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:890)

05-06 10:35:47.637: E/AndroidRuntime(16838):    at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1062)

05-06 10:35:47.637: E/AndroidRuntime(16838):    at android.app.BackStackRecord.run(BackStackRecord.java:684)

05-06 10:35:47.637: E/AndroidRuntime(16838):    at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1447)

05-06 10:35:47.637: E/AndroidRuntime(16838):    at android.app.Activity.performStart(Activity.java:5240)

05-06 10:35:47.637: E/AndroidRuntime(16838):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2168)

05-06 10:35:47.637: E/AndroidRuntime(16838):    ... 11 more

如有任何帮助，我们将不胜感激!

Answer 1

好的。所以我导入了你的代码并让它工作。

有几件事导致了困惑，因此我将粘贴工作代码并解释我更改的内容。

import android.app.ActionBar;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.GridView;

public class EventsActivity extends FragmentActivity implements ActionBar.TabListener {
    //needs R string
    private static final String STATE_SELECTED_NAVIGATION_ITEM = "selected_navigation_item";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_events_container);

        //Set up the action bar to show tabs
        final ActionBar actionBar = getActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        //Add a tab to the action bar
        //needs R.string values
        actionBar.addTab(actionBar.newTab().setText("Tab 1").setTabListener(this));
        actionBar.addTab(actionBar.newTab().setText("Tab 2").setTabListener(this));
        actionBar.addTab(actionBar.newTab().setText("Tab 3").setTabListener(this));
    }

    @Override
    public void onRestoreInstanceState(Bundle savedInstanceState){
        //restores the previously serialised current tab position
        if (savedInstanceState.containsKey(STATE_SELECTED_NAVIGATION_ITEM)){
            getActionBar().setSelectedNavigationItem(savedInstanceState.getInt(STATE_SELECTED_NAVIGATION_ITEM));
        }
    }

    @Override
    public void onSaveInstanceState(Bundle outState){
        //Serialise the current tab position
        outState.putInt(STATE_SELECTED_NAVIGATION_ITEM, getActionBar().getSelectedNavigationIndex());
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        //getMenuInflater().inflate(R.menu.events, menu);
        return true;
    }

    @Override
    public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
        //Uses container view
         android.support.v4.app.Fragment fragment = new DummySectionFragment();
        Bundle args = new Bundle();
        args.putInt(DummySectionFragment.ARG_SECTION_NUMBER, tab.getPosition()+1);
        fragment.setArguments(args);
        getSupportFragmentManager().beginTransaction().replace(R.id.events_container, fragment).commit();

        //getSupportFragmentManager().beginTransaction()
        //.add(R.id.assessment_container, fragment).commit();

    }

    @Override
    public void onTabUnselected(ActionBar.Tab tab,FragmentTransaction fragmentTransaction) {
        //Do nothing
    }

    @Override
    public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
        //Do nothing
    }

    //dummy test
    public static class DummySectionFragment extends Fragment {
        public static final String ARG_SECTION_NUMBER = "placeholder_text";
        GridView gridView;
        static final String[] numbers1 = new String[] {

            "1", "2", "3", "4", "5"
        };
        static final String[] numbers2 = new String[] {

            "6", "7", "8", "9", "10"
        };
        static final String[] numbers3 = new String[] {

            "11", "12", "13", "14", "15"
        };

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,Bundle savedInstanceState) {

            View rootView = inflater.inflate(R.layout.fragment_events,
                    container, false);

            GridView gridview = (GridView)rootView.findViewById(R.id.gridview);

            switch (getArguments().getInt(ARG_SECTION_NUMBER)) {
                case 1:
                    ArrayAdapter<String> adapter1 = new ArrayAdapter<String>(getActivity(),android.R.layout.simple_list_item_1, numbers1);
                    gridview.setAdapter(adapter1);
                    break;
                case 2:
                    ArrayAdapter<String> adapter2 = new ArrayAdapter<String>(getActivity(),android.R.layout.simple_list_item_1, numbers2);
                    gridview.setAdapter(adapter2);
                    break;
                case 3:
                    ArrayAdapter<String> adapter3 = new ArrayAdapter<String>(getActivity(),android.R.layout.simple_list_item_1, numbers3);
                    gridview.setAdapter(adapter3);
                    break;
                default:
                    //textView.setText("HELLO");
                    break;
            }
            return rootView;
        }
      }
}

根据我的经验， fragment 和 Activity 有着特殊的关系。我总是用两个布局 xml 文件来设计 fragment/Activity 关系；一个简单的 Activity 容器和一个显示大部分内容的 fragment 容器。

这是一个简单的 Activity 布局，名为activity_events_layout.xml

  <FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/events_container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"

    />

记下 id/events_container。该引用在 fragment 事务期间使用。现在进入布局fragment_events.xml:

<FrameLayout xmlns:tools="http://schemas.android.com/tools"
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent" 
    tools:context="capstone.DGW.Main.EventsActivity"
    tools:ignore="MergeRootFrame" >

    <GridView
        android:id="@+id/gridview"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:columnWidth="50dp"
        android:gravity="center"
        android:numColumns="auto_fit"
        android:stretchMode="columnWidth" >

    </GridView>
</FrameLayout>

这是包含 Gridview 的 fragment 。

现在回到代码。创建 fragment 时，首先需要膨胀布局，然后可以在该 View 上调用 findViewById 来访问 subview 。然后为 onCreateView 方法返回膨胀的 View 。

另一件事，我一直在 support.v4.app.Fragment 和 supportFragmentManager 方面取得成功，所以我也对其进行了修改。