Java 应用程序崩溃，请单击McClick

当我点击某个项目时，我的应用程序崩溃。我的代码在eclipse中没有错误，但当我点击其中一个项目时（不幸的是，应用程序已经停止工作），LogCat抛出了一堆错误，其中一个是：02-14 04:33:20.010:E/MessageQueue JNI（774）：java.lang.NullPointerException。关于如何解决这个问题有什么想法吗

package com.carouseldemo.main;

import com.carouseldemo.controls.Carousel;

import com.carouseldemo.controls.CarouselAdapter; 
import com.carouseldemo.controls.CarouselAdapter.OnItemClickListener;
/**import com.carouseldemo.controls.CarouselAdapter.OnItemSelectedListener;*/
import com.carouseldemo.controls.CarouselItem;


import android.app.Activity;
import android.os.Bundle;
import android.view.Gravity;
import android.view.LayoutInflater;
import android.view.MenuInflater;
import android.view.Menu;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import android.widget.Toast;




public class MainActivity extends Activity {

    /** Called when the activity is first created. */
    @Override
    public boolean onCreateOptionsMenu(Menu menu){
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.testmenu, menu);
        return true;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        Carousel carousel = (Carousel)findViewById(R.id.carousel);

        carousel.setOnItemClickListener(new OnItemClickListener(){

            public void onItemClick(CarouselAdapter<?> parent, View view,
                    int position, long id) {    
                LayoutInflater inflater = getLayoutInflater();
                View layout = inflater.inflate(R.layout.toast_layout,
                                               (ViewGroup) findViewById(R.id.toast_layout_root));

                TextView text = (TextView) layout.findViewById(R.id.selected_item);
                text.setText("This is a custom toast");

                Toast toast = new Toast(getApplicationContext());
                toast.setGravity(Gravity.CENTER_VERTICAL, 0, 0);
                toast.setDuration(Toast.LENGTH_LONG);
                toast.setView(layout);
                toast.show();


                Toast.makeText(MainActivity.this, 
                        String.format("%s has been clicked", 
                        ((CarouselItem)parent.getChildAt(position)).getName()), 
                        Toast.LENGTH_SHORT).show();z
    }



        });


       /** carousel.setOnItemSelectedListener(new OnItemSelectedListener(){

            public void onItemSelected(CarouselAdapter<?> parent, View view,
                    int position, long id) {

                final TextView txt = (TextView)(findViewById(R.id.selected_item));

                switch(position){
                case 0:
                    txt.setText("The cat (Felis catus), also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests. Cats have been associated with humans for at least 9,500 years, and are currently the most popular pet in the world. Owing to their close association with humans, cats are now found almost everywhere in the world.");
                    break;
                case 1:
                    txt.setText("The hippopotamus (Hippopotamus amphibius), or hippo, from the ancient Greek for \"river horse\" (ἱπποπόταμος), is a large, mostly herbivorous mammal in sub-Saharan Africa, and one of only two extant species in the family Hippopotamidae (the other is the Pygmy Hippopotamus.) After the elephant, the hippopotamus is the third largest land mammal and the heaviest extant artiodactyl.");
                    break;
                case 2:
                    txt.setText("A monkey is a primate, either an Old World monkey or a New World monkey. There are about 260 known living species of monkey. Many are arboreal, although there are species that live primarily on the ground, such as baboons. Monkeys are generally considered to be intelligent. Unlike apes, monkeys usually have tails. Tailless monkeys may be called \"apes\", incorrectly according to modern usage; thus the tailless Barbary macaque is called the \"Barbary ape\".");
                    break;
                case 3:
                    txt.setText("A mouse (plural: mice) is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse (Mus musculus). It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles. They are known to invade homes for food and occasionally shelter.");
                    break;
                case 4:
                    txt.setText("The giant panda, or panda (Ailuropoda melanoleuca, literally meaning \"black and white cat-foot\") is a bear native to central-western and south western China.[4] It is easily recognized by its large, distinctive black patches around the eyes, over the ears, and across its round body. Though it belongs to the order Carnivora, the panda's diet is 99% bamboo.");
                    break;
                case 5:
                    txt.setText("Rabbits (or, colloquially, bunnies) are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world. There are eight different genera in the family classified as rabbits, including the European rabbit (Oryctolagus cuniculus), cottontail rabbits (genus Sylvilagus; 13 species), and the Amami rabbit (Pentalagus furnessi, an endangered species on Amami ÅŒshima, Japan)");
                    break;
                }
                            }

            public void onNothingSelected(CarouselAdapter<?> parent) {          } 


        }
        );*/

    }
     }

---

日志中的第50行似乎是指这一行：

Toast.makeText(MainActivity.this, 
    String.format("%s has been clicked",    
    ((CarouselItem)parent.getChildAt(position)).getName()), 
    Toast.LENGTH_SHORT).show();

我猜它从这个部分得到了一个NullPointerException：

((CarouselItem)parent.getChildAt(position)).getName())

我建议使用调试器运行它，在这一行添加一个断点，并查看所有内容以查看失败的地方。甚至可以在上面一行的变量中捕获CarouselItem，以便于分析：

CarouselItem item = (CarouselItem) parent.getChildAt(position);

getChildAt（i）将给出父项的第i个可见子视图，而不是表示第i个项的子视图

让我用另一种方式解释。假设AdapterView有100项。您可能期望parent.getChildAt（99）将给出表示最后一项的视图：这是错误的。对于性能问题，AdapterView为当前需要显示的项目创建子视图。例如，如果您在列表顶部滚动，并且显示了5项，则AdapterView可能有5或6个子视图。在这种情况下，parent.getChildAt（99）将提供null引用

实际上，单击的子视图已经通过onItemClicked方法给出：第二个'view'参数。您可以按如下方式修改代码

Toast.makeText(MainActivity.this, 
    String.format("%s has been clicked",    
    ((CarouselItem)view).getName()), 
    Toast.LENGTH_SHORT).show();

编辑：

此外，下面的代码片段也是可疑的

        LayoutInflater inflater = getLayoutInflater();
        View layout = inflater.inflate(R.layout.toast_layout,
                                       (ViewGroup) findViewById(R.id.toast_layout_root));


        TextView text = (TextView) layout.findViewById(R.id.selected_item);
        text.setText("This is a custom toast");

在这里，您将toast_layout.xml扩展到一个视图中，并将其添加到另一个已附加到应用程序窗口的视图（R.id.toast_layout_root）。这将导致toast_layout.xml中的视图显示在主应用程序窗口中

---

但是，您可以使用膨胀视图作为toast的布局，并将其呈现为一个单独的窗口。

您可以指出哪一行是第50行吗请检查（（CarouselItem）parent.getChildAt（position））.getName（）leeds是否与NPE连接。您好，Stefan，是的，那么我该如何修复它。您好，Jiyong，我尝试使用您的代码，但效果相同。我的应用程序崩溃了？请你多提些建议好吗。感谢您使用此代码会给您提供与以前相同的错误消息，还是新的错误消息？text.setText（“这是一个自定义toast”）；是我代码的第50行。那么Jiyong，你认为我应该怎么做？嗯，你的toast_layout.xml中似乎不存在R.id.selected_项。谢谢你，Ben，我试过了，但没有找到任何东西。我会继续寻找的同时，更多的建议是非常欢迎请。非常感谢。

        LayoutInflater inflater = getLayoutInflater();
        View layout = inflater.inflate(R.layout.toast_layout,
                                       (ViewGroup) findViewById(R.id.toast_layout_root));


        TextView text = (TextView) layout.findViewById(R.id.selected_item);
        text.setText("This is a custom toast");