Question

我正在向Treehouse提出挑战，他们已经提供了一些初始项目文件。问题是他们已经2岁了，很多代码已经过时了。我尽可能地改变了，但显然仍有一些问题。

我尝试运行此代码，它说它无法在空对象引用上调用setLayoutManager（）。如果我之前初始化它，该对象如何为null？

    private void populate() {
        StaggeredGridLayoutManager lm = new StaggeredGridLayoutManager(2, StaggeredGridLayoutManager.VERTICAL);
        mAlbumList.setLayoutManager(lm);

这是文件中的所有代码。

package com.teamtreehouse.albumcover;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.support.v7.widget.RecyclerView;
import android.support.v7.widget.StaggeredGridLayoutManager;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;

import butterknife.BindView;
import butterknife.ButterKnife;


public class AlbumListActivity extends Activity {

    @BindView(R.id.album_list) RecyclerView mAlbumList;

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

        ButterKnife.bind(this);
        populate();
    }

    private void initTransitions() {
        getWindow().setExitTransition(null);
        getWindow().setReenterTransition(null);
    }

    interface OnVHClickedListener {
        void onVHClicked(AlbumVH vh);
    }

    static class AlbumVH extends RecyclerView.ViewHolder implements View.OnClickListener {
        private final OnVHClickedListener mListener;
        @BindView(R.id.album_art)
        ImageView albumArt;

        public AlbumVH(View itemView, OnVHClickedListener listener) {
            super(itemView);
            ButterKnife.bind(this, itemView);
            itemView.setOnClickListener(this);
            mListener = listener;
        }

        @Override
        public void onClick(View v) {
            mListener.onVHClicked(this);
        }
    }

    private void populate() {
        StaggeredGridLayoutManager lm = new StaggeredGridLayoutManager(2, StaggeredGridLayoutManager.VERTICAL);
        mAlbumList.setLayoutManager(lm);

        final int[] albumArts = {
                R.drawable.mean_something_kinder_than_wolves,
                R.drawable.cylinders_chris_zabriskie,
                R.drawable.broken_distance_sutro,
                R.drawable.playing_with_scratches_ruckus_roboticus,
                R.drawable.keep_it_together_guster,
                R.drawable.the_carpenter_avett_brothers,
                R.drawable.please_sondre_lerche,
                R.drawable.direct_to_video_chris_zabriskie };

        RecyclerView.Adapter adapter = new RecyclerView.Adapter<AlbumVH>() {
            @Override
            public AlbumVH onCreateViewHolder(ViewGroup parent, int viewType) {
                View albumView = getLayoutInflater().inflate(R.layout.album_grid_item, parent, false);
                return new AlbumVH(albumView, new OnVHClickedListener() {
                    @Override
                    public void onVHClicked(AlbumVH vh) {
                        int albumArtResId = albumArts[vh.getLayoutPosition() % albumArts.length];
                        Intent intent = new Intent(AlbumListActivity.this, AlbumDetailActivity.class);
                        intent.putExtra(AlbumDetailActivity.EXTRA_ALBUM_ART_RESID, albumArtResId);

                        startActivity(intent);
                    }
                });
            }

            @Override
            public void onBindViewHolder(AlbumVH holder, int position) {
                holder.albumArt.setImageResource(albumArts[position % albumArts.length]);
            }

            @Override
            public int getItemCount() {
                return albumArts.length * 4;
            }

        };
        mAlbumList.setAdapter(adapter);
    }

}

这是一个例外：

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.jimulabs.googlemusicmock, PID: 30956
                  java.lang.RuntimeException: Unable to start activity ComponentInfo{com.jimulabs.googlemusicmock/com.teamtreehouse.albumcover.AlbumListActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setLayoutManager(android.support.v7.widget.RecyclerView$LayoutManager)' on a null object reference
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2778)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
                      at android.app.ActivityThread.-wrap11(Unknown Source:0)
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)
                      at android.os.Handler.dispatchMessage(Handler.java:106)
                      at android.os.Looper.loop(Looper.java:164)
                      at android.app.ActivityThread.main(ActivityThread.java:6494)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
                   Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setLayoutManager(android.support.v7.widget.RecyclerView$LayoutManager)' on a null object reference
                      at com.teamtreehouse.albumcover.AlbumListActivity.populate(AlbumListActivity.java:59)
                      at com.teamtreehouse.albumcover.AlbumListActivity.onCreate(AlbumListActivity.java:27)
                      at android.app.Activity.performCreate(Activity.java:6999)
                      at android.app.Activity.performCreate(Activity.java:6990)
                      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1214)
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2731)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856) 
                      at android.app.ActivityThread.-wrap11(Unknown Source:0) 
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589) 
                      at android.os.Handler.dispatchMessage(Handler.java:106) 
                      at android.os.Looper.loop(Looper.java:164) 
                      at android.app.ActivityThread.main(ActivityThread.java:6494) 
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

Answer 1

这个问题的一个简单解决方案是，初始化你的mAlbumLayout并将其作为参数传递给populate（）方法。这样可以防止它变为空。

NullPointerException：尝试在空对象引用

1 个答案: