Question

我是Android开发的新手，我已经按照本教程使用Navigation Drawer创建了一个Android滑动菜单：http://www.androidhive.info/2013/11/android-sliding-menu-using-navigation-drawer/

我在应用程序的主页面（HomeFragment）中放置了一个按钮，我想将按钮链接到另一个活动/页面（NewActivity）。目前我遇到了很多错误。

MainActivity：

package info.androidhive.slidingmenu;

import info.androidhive.slidingmenu.adapter.NavDrawerListAdapter;
import info.androidhive.slidingmenu.model.NavDrawerItem;

import java.util.ArrayList;

import android.app.Activity;
import android.app.Fragment;
import android.app.FragmentManager;
import android.content.res.Configuration;
import android.content.res.TypedArray;
import android.os.Bundle;
import android.support.v4.app.ActionBarDrawerToggle;
import android.support.v4.widget.DrawerLayout;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ListView;

public class MainActivity extends Activity {
    private DrawerLayout mDrawerLayout;
    private ListView mDrawerList;
    private ActionBarDrawerToggle mDrawerToggle;

    // nav drawer title
    private CharSequence mDrawerTitle;

    // used to store app title
    private CharSequence mTitle;

    // slide menu items
    private String[] navMenuTitles;
    private TypedArray navMenuIcons;

    private ArrayList<NavDrawerItem> navDrawerItems;
    private NavDrawerListAdapter adapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mTitle = mDrawerTitle = getTitle();

        // load slide menu items
        navMenuTitles = getResources().getStringArray(R.array.nav_drawer_items);

        // nav drawer icons from resources
        navMenuIcons = getResources()
                .obtainTypedArray(R.array.nav_drawer_icons);

        mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
        mDrawerList = (ListView) findViewById(R.id.list_slidermenu);

        navDrawerItems = new ArrayList<NavDrawerItem>();

        // adding nav drawer items to array
        // Home
        navDrawerItems.add(new NavDrawerItem(navMenuTitles[0], navMenuIcons.getResourceId(0, -1)));
        // Find People
        navDrawerItems.add(new NavDrawerItem(navMenuTitles[1], navMenuIcons.getResourceId(1, -1)));
        // Photos
        navDrawerItems.add(new NavDrawerItem(navMenuTitles[2], navMenuIcons.getResourceId(2, -1)));
        // Communities, Will add a counter here
        navDrawerItems.add(new NavDrawerItem(navMenuTitles[3], navMenuIcons.getResourceId(3, -1), true, "22"));
        // Pages
        navDrawerItems.add(new NavDrawerItem(navMenuTitles[4], navMenuIcons.getResourceId(4, -1)));
        // What's hot, We  will add a counter here
        navDrawerItems.add(new NavDrawerItem(navMenuTitles[5], navMenuIcons.getResourceId(5, -1), true, "50+"));


        // Recycle the typed array
        navMenuIcons.recycle();

        mDrawerList.setOnItemClickListener(new SlideMenuClickListener());

        // setting the nav drawer list adapter
        adapter = new NavDrawerListAdapter(getApplicationContext(),
                navDrawerItems);
        mDrawerList.setAdapter(adapter);

        // enabling action bar app icon and behaving it as toggle button
        getActionBar().setDisplayHomeAsUpEnabled(true);
        getActionBar().setHomeButtonEnabled(true);

        mDrawerToggle = new ActionBarDrawerToggle(this, mDrawerLayout,
                R.drawable.ic_drawer, //nav menu toggle icon
                R.string.app_name, // nav drawer open - description for accessibility
                R.string.app_name // nav drawer close - description for accessibility
        ) {
            public void onDrawerClosed(View view) {
                getActionBar().setTitle(mTitle);
                // calling onPrepareOptionsMenu() to show action bar icons
                invalidateOptionsMenu();
            }

            public void onDrawerOpened(View drawerView) {
                getActionBar().setTitle(mDrawerTitle);
                // calling onPrepareOptionsMenu() to hide action bar icons
                invalidateOptionsMenu();
            }
        };
        mDrawerLayout.setDrawerListener(mDrawerToggle);

        if (savedInstanceState == null) {
            // on first time display view for first nav item
            displayView(0);
        }
    }

    /**
     * Slide menu item click listener
     * */
    private class SlideMenuClickListener implements
            ListView.OnItemClickListener {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position,
                long id) {
            // display view for selected nav drawer item
            displayView(position);
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // toggle nav drawer on selecting action bar app icon/title
        if (mDrawerToggle.onOptionsItemSelected(item)) {
            return true;
        }
        // Handle action bar actions click
        switch (item.getItemId()) {
        case R.id.action_settings:
            return true;
        default:
            return super.onOptionsItemSelected(item);
        }
    }

    /* *
     * Called when invalidateOptionsMenu() is triggered
     */
    @Override
    public boolean onPrepareOptionsMenu(Menu menu) {
        // if nav drawer is opened, hide the action items
        boolean drawerOpen = mDrawerLayout.isDrawerOpen(mDrawerList);
        menu.findItem(R.id.action_settings).setVisible(!drawerOpen);
        return super.onPrepareOptionsMenu(menu);
    }

    /**
     * Diplaying fragment view for selected nav drawer list item
     * */
    private void displayView(int position) {
        // update the main content by replacing fragments
        Fragment fragment = null;
        switch (position) {
        case 0:
            fragment = new HomeFragment();
            break;
        case 1:
            fragment = new FindPeopleFragment();
            break;
        case 2:
            fragment = new PhotosFragment();
            break;
        case 3:
            fragment = new CommunityFragment();
            break;
        case 4:
            fragment = new PagesFragment();
            break;
        case 5:
            fragment = new WhatsHotFragment();
            break;

        default:
            break;
        }

        if (fragment != null) {
            FragmentManager fragmentManager = getFragmentManager();
            fragmentManager.beginTransaction()
                    .replace(R.id.frame_container, fragment).commit();

            // update selected item and title, then close the drawer
            mDrawerList.setItemChecked(position, true);
            mDrawerList.setSelection(position);
            setTitle(navMenuTitles[position]);
            mDrawerLayout.closeDrawer(mDrawerList);
        } else {
            // error in creating fragment
            Log.e("MainActivity", "Error in creating fragment");
        }
    }

    @Override
    public void setTitle(CharSequence title) {
        mTitle = title;
        getActionBar().setTitle(mTitle);
    }

    /**
     * When using the ActionBarDrawerToggle, you must call it during
     * onPostCreate() and onConfigurationChanged()...
     */

    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
        // Sync the toggle state after onRestoreInstanceState has occurred.
        mDrawerToggle.syncState();
    }

    @Override
    public void onConfigurationChanged(Configuration newConfig) {
        super.onConfigurationChanged(newConfig);
        // Pass any configuration change to the drawer toggls
        mDrawerToggle.onConfigurationChanged(newConfig);
    }

}

HomeFragment：

    package info.androidhive.slidingmenu;

import android.app.Fragment;
import android.content.Intent;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.Button;

public class HomeFragment extends Fragment implements OnClickListener{
    private Button button;
    public HomeFragment(){}

    @Override

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

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

        Button b = (Button) rootView.findViewById(R.id.button1);
        b.setOnClickListener(this);

        return rootView;
    }

    public void onClick(View v) {

        switch (v.getId()) {
        case R.id.button1:

            Intent intent = new Intent(getActivity(), NewActivity.class);
            startActivity(intent);

            System.out.println("Hi its me");
            break;
        }
    }

}

NewActivity：

  package info.androidhive.slidingmenu;

import android.os.Bundle;
import android.app.Activity;

public class NewActivity extends Activity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // Get the view from new_activity.xml
        setContentView(R.layout.new_activity);
    }
}

AndroidManifest：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="info.androidhive.slidingmenu"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="11"
        android:targetSdkVersion="17" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="info.androidhive.slidingmenu.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

      <activity android:name=".NewActivity" >
    </activity> 

    </application>

</manifest>

fragment_home.xml：

 <?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

    <TextView
        android:id="@+id/txtLabel"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:text="Home View"
        android:textSize="16dp" />

    <ImageView
        android:id="@+id/imageView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/txtLabel"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="10dp"
        android:src="@drawable/ic_home" />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/imageView1"
        android:layout_alignParentTop="true"
        android:layout_marginTop="56dp"
        android:text="Button"
        android:text="Button" />

</RelativeLayout>

logcat的：

12-03 20:28:26.823: E/Trace(1373): error opening trace file: No such file or directory (2)
12-03 20:28:27.134: D/dalvikvm(1373): GC_FOR_ALLOC freed 84K, 8% free 2608K/2816K, paused 38ms, total 46ms
12-03 20:28:27.134: I/dalvikvm-heap(1373): Grow heap (frag case) to 3.269MB for 635812-byte allocation
12-03 20:28:27.184: D/dalvikvm(1373): GC_FOR_ALLOC freed 2K, 7% free 3226K/3440K, paused 49ms, total 49ms
12-03 20:28:27.276: D/dalvikvm(1373): GC_CONCURRENT freed <1K, 6% free 3261K/3440K, paused 4ms+62ms, total 89ms
12-03 20:28:27.284: D/AndroidRuntime(1373): Shutting down VM
12-03 20:28:27.284: W/dalvikvm(1373): threadid=1: thread exiting with uncaught exception (group=0x40a71930)
12-03 20:28:27.314: E/AndroidRuntime(1373): FATAL EXCEPTION: main
12-03 20:28:27.314: E/AndroidRuntime(1373): java.lang.RuntimeException: Unable to start activity ComponentInfo{info.androidhive.slidingmenu/info.androidhive.slidingmenu.MainActivity}: java.lang.NullPointerException
12-03 20:28:27.314: E/AndroidRuntime(1373):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at android.os.Handler.dispatchMessage(Handler.java:99)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at android.os.Looper.loop(Looper.java:137)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at android.app.ActivityThread.main(ActivityThread.java:5041)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at java.lang.reflect.Method.invokeNative(Native Method)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at java.lang.reflect.Method.invoke(Method.java:511)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at dalvik.system.NativeStart.main(Native Method)
12-03 20:28:27.314: E/AndroidRuntime(1373): Caused by: java.lang.NullPointerException
12-03 20:28:27.314: E/AndroidRuntime(1373):     at info.androidhive.slidingmenu.HomeFragment.onCreateView(HomeFragment.java:23)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at android.app.Fragment.performCreateView(Fragment.java:1695)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:885)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1057)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at android.app.BackStackRecord.run(BackStackRecord.java:682)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1435)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at android.app.Activity.performStart(Activity.java:5113)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2153)
12-03 20:28:27.314: E/AndroidRuntime(1373):     ... 11 more

已更新：

现在一切正常，上面的代码也会更新。我通过使用＆＃34; intent＆＃34;解决了这个问题。在HomeFragment中，我另外向宣言添加了一个新活动。

Answer 1

请看这里的第二行：

12-03 20:28:27.314: E/AndroidRuntime(1373): Caused by: java.lang.NullPointerException
12-03 20:28:27.314: E/AndroidRuntime(1373):     at info.androidhive.slidingmenu.HomeFragment.onCreateView(HomeFragment.java:23)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at android.app.Fragment.performCreateView(Fragment.java:1695)
12-03 20:28:27.314: E/AndroidRuntime(1373):     at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:885)

NullPointerException中有一个HomeFragment.java，第23行：

    Button b = (Button) rootView.findViewById(R.id.button1);
    b.setOnClickListener(this);  // line 23

唯一可能导致此问题的是b为null。如果R.layout.fragment_home不包含ID为button1的元素，则会发生这种情况。

这就是你需要解决的问题。

如果您在此之后遇到其他问题，请遵循相同的技巧：

获取stacktrace
查找堆栈跟踪中来自您自己代码的第一行
找出线路上的错误并进行必要的更正

Android活动 - 从一个屏幕到另一个屏幕

1 个答案: