java - IndexOutOfBoundsException 导致 Android 应用程序崩溃

Question

我正在单击上一个 Activity 的 View 列表中的一个项目。此 Activity 应该获取该对象并显示该对象的特征。当我单击 viewList 时，应用程序崩溃。这是日志猫:

04-16 19:37:09.183 22696-22696/cs4326.cook4me E/AndroidRuntime: FATAL EXCEPTION: main
Process: cs4326.cook4me, PID: 22696
java.lang.RuntimeException: Unable to start activity ComponentInfo{cs4326.cook4me/cs4326.cook4me.RecipeInstructActivity}: java.lang.IndexOutOfBoundsException: Index: 1, Size: 0
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2665)
     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
     at android.app.ActivityThread.-wrap12(ActivityThread.java)
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
     at android.os.Handler.dispatchMessage(Handler.java:102)
     at android.os.Looper.loop(Looper.java:154)
     at android.app.ActivityThread.main(ActivityThread.java:6119)
     at java.lang.reflect.Method.invoke(Native Method)
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
        Caused by: java.lang.IndexOutOfBoundsException: Index: 1, Size: 0
     at java.util.ArrayList.add(ArrayList.java:457)
     at cs4326.cook4me.RecipeInstructActivity.onCreate(RecipeInstructActivity.java:45)
     at android.app.Activity.performCreate(Activity.java:6679)
     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2618)
     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) 
     at android.app.ActivityThread.-wrap12(ActivityThread.java) 
     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) 
     at android.os.Handler.dispatchMessage(Handler.java:102) 
     at android.os.Looper.loop(Looper.java:154) 
     at android.app.ActivityThread.main(ActivityThread.java:6119) 
     at java.lang.reflect.Method.invoke(Native Method) 
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 

这是相应的 Activity 代码(错误显示在第 45 行):

package cs4326.cook4me;

import java.util.ArrayList;
import java.util.List;

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.util.Log;

public class RecipeInstructActivity extends FragmentActivity {

    private static final String TAG = "RecipeInstructActivity";
    /**
     * Identifier for the example fragment.
     */
    public static final int FRAGMENT_COMPLETE = 1;
    public static final int FRAGMENT_STEPPED = 2;

    /**
     * The adapter definition of the fragments.
     */
    private FragmentPagerAdapter _fragmentPagerAdapter;

    /**
     * The ViewPager that hosts the section contents.
     */
    private ViewPager _viewPager;

    /**
     * List of fragments.
     */
    private List<Fragment> _fragments = new ArrayList<>();

    @Override
    protected void onCreate(final Bundle savedInstanceState) {
        Log.d(TAG, "onCreate:");
        super.onCreate(savedInstanceState);

        this.setContentView(R.layout.activity_main);

        // Each fragment to our list.
        this._fragments.add(FRAGMENT_COMPLETE, new CompleteInstructionFragment());
        this._fragments.add(FRAGMENT_STEPPED, new SteppedInstructionFragment());

        // Setup the fragments, defining the number of fragments, the screens and titles.
        this._fragmentPagerAdapter = new FragmentPagerAdapter(this.getSupportFragmentManager()){
            @Override
            public int getCount() {
                return RecipeInstructActivity.this._fragments.size();
            }
            @Override
            public Fragment getItem(final int position) {
                return RecipeInstructActivity.this._fragments.get(position);
            }
            @Override
            public CharSequence getPageTitle(final int position) {
                // Define titles for each fragment.
                switch (position) {
                    case FRAGMENT_COMPLETE:
                        return "Complete Recipe";
                    case FRAGMENT_STEPPED:
                        return "Step-by-Step";
                    default:
                        return null;
                }
            }
        };

        this._viewPager = (ViewPager) this.findViewById(R.id.pager);
        this._viewPager.setAdapter(this._fragmentPagerAdapter);

        // Set the default fragment.
        this.openFragment(FRAGMENT_COMPLETE);
    }

    /**
     * Open the specified fragment.
     * @param fragment
     */
    public void openFragment(final int fragment) {
        this._viewPager.setCurrentItem(fragment);
    }

    /**
     * Get the fragment object for the specified fragment.
     * @param fragment
     * @return
     */
    public Fragment getFragment(final int fragment) {
        return this._fragments.get(fragment);
    }
}

这是父 viewList Activity 的代码:

package cs4326.cook4me;

import android.content.Intent;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.util.Log;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;

import com.google.firebase.database.ChildEventListener;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.Query;

import java.util.Comparator;
import java.util.TreeSet;

public class RecipesActivity extends AppCompatActivity implements AdapterView.OnItemClickListener{
    private static final String TAG = "RecipesActivity";
    private DatabaseReference databaseReference;
    private ListView mainListView ;
    private ArrayAdapter<String> listAdapter ;
    private TreeSet<Recipe> allRecipeData;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_recipes);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        //Initialize reference to database
        databaseReference = FirebaseDatabase.getInstance().getReference();
        //Initialize recipe data set
        allRecipeData = new TreeSet<Recipe>(new Comparator<Recipe>() {
            @Override
            public int compare(Recipe o1, Recipe o2) {
                return o1.getTitle().compareToIgnoreCase(o2.getTitle());
            }
        });
        // Create ArrayAdapter
        listAdapter = new ArrayAdapter<String>(this, R.layout.row_layout);
        //Create query for recipes
        Query recipesReference = databaseReference.child("recipes").orderByChild("title");
        recipesReference.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                // Get Recipe object and use the values to update the UI
                Recipe recipe = dataSnapshot.getValue(Recipe.class);
                listAdapter.add(recipe.getTitle());
                allRecipeData.add(recipe);
                Log.d(TAG, "loadRecipe:onChildAdded");
            }

            @Override
            public void onChildChanged(DataSnapshot dataSnapshot, String previousName) {
                // Get Recipe object and use the values to update the UI
                Recipe recipe = dataSnapshot.getValue(Recipe.class);
                listAdapter.remove(previousName);
                listAdapter.add(recipe.getTitle());
                Log.d(TAG, "loadRecipe:onChildChanged");
            }

            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {
                // Get Recipe object and use the values to update the UI
                Recipe recipe = dataSnapshot.getValue(Recipe.class);
                listAdapter.remove(recipe.getTitle());
                allRecipeData.remove(recipe);
                Log.d(TAG, "loadRecipe:onRemoved");
            }

            @Override
            public void onChildMoved(DataSnapshot dataSnapshot, String olden) {
                // Get Recipe object and use the values to update the UI
                Recipe recipe = dataSnapshot.getValue(Recipe.class);
                listAdapter.remove(olden);
                listAdapter.add(recipe.getTitle());
                Log.d(TAG, "loadRecipe:onChildMoved");
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                // Getting Recipe failed, log a message
                Log.w(TAG, "loadRecipe:onCancelled", databaseError.toException());
            }
        });

        // Find the ListView resource.
        mainListView = (ListView) findViewById( R.id.listViewRecipes );
        //Add list items!
        mainListView.setAdapter( listAdapter );

        //TODO: Make it so clicking / tapping on each item redirects to Recipe
        mainListView.setOnItemClickListener(this);


        //Floating login button
        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent transfer = new Intent(RecipesActivity.this, LoginActivity.class);
                startActivity(transfer);
            }
        });

        //If you type, it will automatically start searching
        setDefaultKeyMode(DEFAULT_KEYS_SEARCH_LOCAL);
        //Floating search button
        FloatingActionButton floatingSearch = (FloatingActionButton) findViewById(R.id.search_recipes);
        floatingSearch.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                onSearchRequested();
            }
        });

        //Redirects back to Main Menu if press up button
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    }

    public void onItemClick(AdapterView<?> l, View v, int position, long id) {
        Log.d(TAG, "You clicked " + listAdapter.getItem(position));
        String nameOf = listAdapter.getItem(position);
        Recipe selectedRecipe = new Recipe();
        // Then you start a new Activity via Intent
        Intent specialTransfer = new Intent(RecipesActivity.this, RecipeInstructActivity.class);
        for (Recipe r : allRecipeData) {
            if (r.getTitle().equals(nameOf)) {
                selectedRecipe = r;
            }
        }
        //Pass selected recipe 
        specialTransfer.putExtra("recipe_object", selectedRecipe);
        startActivity(specialTransfer);

    }

}

这是 AndroidManifest.XML:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="cs4326.cook4me">

    <uses-permission android:name="android.permission.INTERNET" />

    <!-- To auto-complete the email text field in the login form with the user's emails -->
    <uses-permission android:name="android.permission.GET_ACCOUNTS" />
    <uses-permission android:name="android.permission.READ_PROFILE" />
    <uses-permission android:name="android.permission.READ_CONTACTS" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/icon"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/Theme.AppCompat.Light">
        <activity
            android:name=".MainActivity"
            android:label="@string/title_activity_main"
            android:theme="@style/AppTheme.NoActionBar">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name=".LoginActivity"
            android:label="@string/app_name"
            android:theme="@style/AppTheme.NoActionBar" >
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="cs4326.cook4me.MainActivity" />
        </activity>

        <activity
            android:name=".RegisterActivity"
            android:label="Register User"
            android:parentActivityName=".LoginActivity" >
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="cs4326.cook4me.LoginActivity" />
        </activity>

        <activity
            android:name=".ProfileActivity"
            android:label="User profile"
            android:parentActivityName=".LoginActivity" />
        <activity
            android:name=".SettingsActivity"
            android:label="@string/title_activity_settings"
            android:parentActivityName=".MainActivity" >
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="cs4326.cook4me.MainActivity" />
        </activity>
        <!--
        <activity
            android:name=".RecipeFragment"
            android:label="@string/title_activity_recipe_fragment"
            android:theme="@style/AppTheme.NoActionBar" />
        <activity
            android:name=".ProfileFragment"
            android:label="@string/title_activity_profile_fragment"
            android:theme="@style/AppTheme.NoActionBar"></activity>
        -->
        <activity
            android:name=".RecipesActivity"
            android:label="@string/title_activity_recipes"
            android:parentActivityName=".MainActivity"
            android:theme="@style/AppTheme.NoActionBar">
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="cs4326.cook4me.MainActivity" />
            <meta-data android:name="android.app.default_searchable"
                android:value=".SearchRecipeActivity" />
        </activity>
        <activity
            android:name=".CookingTerminologyActivity"
            android:label="@string/title_activity_cooking_terminology"
            android:parentActivityName=".MainActivity"
            android:theme="@style/Theme.AppCompat.Light.NoActionBar">
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="cs4326.cook4me.MainActivity" />
        </activity>
        <activity
            android:name=".TermActivity"
            android:label="@string/title_activity_term"
            android:parentActivityName=".CookingTerminologyActivity"
            android:theme="@style/Theme.AppCompat.Light.NoActionBar">
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="cs4326.cook4me.CookingTerminologyActivity" />
        </activity>

        <activity
            android:name=".SearchRecipeActivity"
            android:label="@string/title_activity_search_recipe"
            android:parentActivityName=".RecipesActivity"
            android:theme="@style/Theme.AppCompat.Light.NoActionBar">
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="cs4326.cook4me.RecipesActivity" />
            <intent-filter>
                <action android:name="android.intent.action.SEARCH" />
            </intent-filter>
            <meta-data android:name="android.app.searchable"
                android:resource="@xml/searchable"/>
        </activity>

        <activity
            android:name=".RecipeInstructActivity"
            android:parentActivityName=".RecipesActivity">
        <meta-data
            android:name="android.support.PARENT_ACTIVITY"
            android:value="cs4326.cook4me.RecipeInstructActivity" />

        </activity>
    </application>

</manifest>

Answer 1

问题 1:静态初始化

private List<Fragment> _fragments = new ArrayList<>(); //be careful doing this!

字段初始化在 Android Activity 和 fragment 中效果不佳，因为它们的生命周期是受控制的。虽然在这种情况下它不会导致问题，但如果您在字段初始化中使用 findViewById(int id) 或其他此类方法，则可能会遇到麻烦。在 Activity 和 Fragment 的生命周期真正清晰之前，最好避免将代码放入 onCreate() 或 onResume() 回调中(根据需要)。

问题 2:滚动我们自己的 FragmentManager

在此代码中， fragment 被缓存在列表中。没有必要这样做 - 这是 FragmentManager 缓存 fragment 的工作。

此外，方法 FragmentPagerAdapter#getItem() 应该用于实例化，而不是从列表中检索。这并不是你的错——我认为这个方法的命名很糟糕。它的真正含义更像是“createItem()”，它应该看起来更像这样:

    this._fragmentPagerAdapter = new FragmentPagerAdapter(this.getSupportFragmentManager()){
        @Override
        public int getCount() {
            return RecipeInstructActivity.this._fragments.size();
        }
        @Override
        public Fragment getItem(final int position) {
            switch (position) {
                case FRAGMENT_COMPLETE:
                    return new CompleteInstructionFragment();
                case FRAGMENT_STEPPED:
                    return new StepByStepFramgment();
        }

如果可能的话，最好将所有内容留给 ViewPager。如果您不能并且需要自己操作 fragment ，那么当您添加 fragment 时，请使用重载，该重载允许您在完成事务时向 fragment 添加标签，然后当您需要 fragment 的句柄时，您可以使用 findFragmentByTag 来检索它。