为listItem设置OnItemClickListener

Question

我得到了一个简单的ListView的方法，例如：

listView.setOnItemClickListener(new OnItemClickListener() {

              @Override
              public void onItemClick(AdapterView<?> parent, View view,
                 int position, long id) {

但每当我在我的活动中这样做时，我都会得到一个nullpointerexception。

有人可以告诉我它应该是怎样的吗？谢谢！

编辑：对不起！我不想只用代码填写整个问题，我认为在这种情况下它是必要的，所以我把我的整个java代码放在下面：

import android.app.ListActivity;
import android.content.Context;
import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.Toast;

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.util.ArrayList;
import java.util.List;


public class Save extends ListActivity {
String FileName;
EditText et;
String listItem[]={};
ListView listView;

@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    this.setContentView(R.layout.activity_save);
    Button save = (Button) findViewById(R.id.SAVE); //this is my button
    final EditText title = (EditText) findViewById(R.id.save_filename); //This is unused
    final EditText maintext = (EditText) findViewById(R.id.test);
    getActionBar().setDisplayUseLogoEnabled(false);
    getActionBar().setDisplayHomeAsUpEnabled(true);
    et = (EditText) findViewById(R.id.save_filename);
    listView = (ListView) findViewById(R.id.list); //I've specified this listView only for the OnItemClickListener action

    List values = new ArrayList();
    for (int i = 0; i < listItem.length; i++) {
        values.add(listItem[i]);
    }

    ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
            android.R.layout.simple_list_item_1, values);
    setListAdapter(adapter);

    save.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            try {
                FileName = et.getText().toString() + ".txt";
                FileOutputStream fout = openFileOutput(FileName, Context.MODE_PRIVATE);
                OutputStreamWriter outsw = new OutputStreamWriter(fout);
                try {
                    outsw.write(String.valueOf(maintext));
                    outsw.flush();
                    outsw.close();
                    Toast.makeText(getBaseContext(), "Your file has been saved", Toast.LENGTH_LONG).show();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            }

            ArrayAdapter<String> adapter = (ArrayAdapter<String>) getListAdapter();
            String device;
            switch (view.getId()) {
                case R.id.SAVE:
                    List myList = new ArrayList();
                    device = et.getText().toString();
                    myList.add(device);
                    adapter.add(device);
                    et.setText("");
                    break;
            }
            adapter.notifyDataSetChanged();
        }
    });

    //I think this is wrong, I get no error in Java by the way. How could this work if there's a specific extension for my activity that define the listview? **That's my question**. 
    listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {

        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

            // ListView Clicked item index
            int itemPosition     = position;

            // ListView Clicked item value
            String  itemValue    = (String) listView.getItemAtPosition(position);

            // Show Alert
            Toast.makeText(getApplicationContext(),
                    "Position :"+itemPosition+"  ListItem : " +itemValue , Toast.LENGTH_LONG)
                    .show();
        }
    });

}

和xml：

<Button
    android:layout_weight="1"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:id="@+id/SAVE"
    android:text="@string/save_button" />

<EditText
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/save_filename"
    android:layout_alignRight="@+id/linearLayout"
    android:hint="@string/save_hint"
    android:layout_alignEnd="@+id/linearLayout"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:layout_alignParentTop="true"
    android:layout_marginBottom="5dp" />

<ListView
    android:id="@android:id/list"
    android:layout_height="wrap_content"
    android:layout_width="match_parent"
    android:layout_above="@+id/linearLayout"
    android:layout_below="@+id/save_filename">
</ListView>

对于cunfusion再次抱歉!!

Answer 1

这里的问题来自于您使用ListActivity而不是Activity。因此，ListView的ID不会添加到R文件中，因此findById调用无法调用它。如果使用Activity，则不会发生这种情况。使用ListActivity时，您可以使用内置的getListView（）方法来获取ListView对象。 此外，当我运行您给定的代码时，我得到一个错误，即listView必须由ID列表定义。这来自您定义ID时XML文件中的错误。

下面我发布了您给定XML的两个部分

<EditText
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/save_filename"

<ListView
    android:id="@android:id/list"
    android:layout_height="wrap_content"
    android:layout_width="match_parent"
    android:layout_above="@+id/linearLayout"
    android:layout_below="@+id/save_filename">
</ListView>

请注意，在定义ID时，顶部显示android:id="@+id/save_filename"，而底部显示android:id="@android:id/list". Also when you use save_file_name it is done like this android：layout_below =＆＃34; @ + id / save_filename＆＃34; This is incorrect. To define an id you use @ + id`。这应该在第一次引用元素时完成。它告诉android在R.java中为它生成一个值。 XML中的错误可能会导致您的R.java文件无法生成并导致不同的问题，但我认为ID的额外优势是可以原谅的。

由于您为布局定义了id，我收到了错误。相反它应该看起来更像这个

android:id="@+id/android:list"

注意+来创建一个新的id，因为我使用的是ListActivity，我还必须将它定义为android：list而不是list。 我将XML更改为：

<EditText
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/save_filename"
    android:layout_alignRight="@+id/linearLayout"
    android:hint="save here"
    android:layout_alignEnd="@id/linearLayout"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:layout_alignParentTop="true"
    android:layout_marginBottom="5dp" />

<ListView
    android:id="@+id/android:list"
    android:layout_height="wrap_content"
    android:layout_width="match_parent"
    android:layout_above="@id/linearLayout"
    android:layout_below="@id/save_filename">
</ListView>

这是一些更正的代码，我注意到我改变了什么。

package com.example.sosandbox;

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.util.ArrayList;
import java.util.List;

import android.app.ListActivity;
import android.content.Context;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.Toast;


public class Save extends ListActivity {
String FileName;
EditText et;
//Added some test values
String listItem[]={"1","2","3","4","5"};
ListView listView;

@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    this.setContentView(R.layout.activity_save);
    Button save = (Button) findViewById(R.id.SAVE); //this is my button
//    This is unused so I commented it out
//    final EditText title = (EditText) findViewById(R.id.save_filename); //This is unused
    final EditText maintext = (EditText) findViewById(R.id.save_filename);
    getActionBar().setDisplayUseLogoEnabled(false);
    getActionBar().setDisplayHomeAsUpEnabled(true);
    et = (EditText) findViewById(R.id.save_filename);
//    This is your issue. 
//    listView = (ListView) findViewById(R.id.list);
//    Change it to this because you are using a listActivity
    listView = getListView();//I've specified this listView only for the OnItemClickListener action

    List values = new ArrayList();
//    This won't run because you initilize listItem to an empty array or 0 so 0 < 0 evaluates to false and the loop doesn't run
    for (int i = 0; i < listItem.length; i++) {
        values.add(listItem[i]);
    }

    ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
            android.R.layout.simple_list_item_1, values);
    setListAdapter(adapter);

    save.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            try {
                FileName = et.getText().toString() + ".txt";
                FileOutputStream fout = openFileOutput(FileName, Context.MODE_PRIVATE);
                OutputStreamWriter outsw = new OutputStreamWriter(fout);
                try {
                    outsw.write(String.valueOf(maintext));
                    outsw.flush();
                    outsw.close();
                    Toast.makeText(getBaseContext(), "Your file has been saved", Toast.LENGTH_LONG).show();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            }

            ArrayAdapter<String> adapter = (ArrayAdapter<String>) getListAdapter();
            String device;
            switch (view.getId()) {
                case R.id.SAVE:
                    List myList = new ArrayList();
                    device = et.getText().toString();
                    myList.add(device);
                    adapter.add(device);
                    et.setText("");
                    break;
            }
            adapter.notifyDataSetChanged();
        }
    });

    //I think this is wrong, I get no error in Java by the way. How could this work if there's a specific extension for my activity that define the listview? **That's my question**. 
    listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {

        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {

            // ListView Clicked item index
            int itemPosition     = position;

            // ListView Clicked item value
            String  itemValue    = (String) listView.getItemAtPosition(position);

            // Show Alert
            Toast.makeText(getApplicationContext(),
                    "Position :"+itemPosition+"  ListItem : " +itemValue , Toast.LENGTH_LONG)
                    .show();
        }
    });

}
}