view的空指针异常，listview加载数据底部添加按钮
listview数据加载后底部添加按钮：
先写一个xml文件，里面添加按钮控件，如下listbutton。xml，按钮居中对齐的哦。
<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="wrap_content" android:layout_height="match_parent" android:gravity="center_vertical|center_horizontal" android:layout_gravity="center_vertical|center_horizontal" android:orientation="horizontal" > <Button android:id="@+id/lastButton" android:layout_width="wrap_content" android:layout_height="30px" android:gravity="center_vertical" android:layout_marginTop="5px" android:text="上一页" android:onClick="lastPageDate"/> <TextView android:id="@+id/counTe" android:layout_width="wrap_content" android:layout_height="wrap_content" android:gravity="center_vertical" android:text="" /> <Button android:id="@+id/nextButton" android:layout_width="wrap_content" android:layout_height="30px" android:layout_gravity="center_vertical" android:layout_marginTop="5px" android:layout_marginLeft="10px" android:text="下一页" android:onClick="nextPageDate"/></LinearLayout>
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放listview的那个xml就不展示了，因为很简单，只是在界面里加一个listview。不过必须设置android:layout_width="fill_parent"，否则按钮无法居中。
下面就是后台代码了。
MainActivity。java。
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package com.example.dbconnectiontest;/*必须引用apache.http相关类来创建HTTP连接*/import java.io.IOException;import java.util.ArrayList;import java.util.HashMap;import java.util.List;import java.util.Map;import javax.security.auth.PrivateCredentialPermission;import org.apache.http.Header;import org.apache.http.HttpRequest;import org.apache.http.HttpResponse;import org.apache.http.client.ClientProtocolException;import org.apache.http.client.HttpClient;import org.apache.http.client.methods.HttpGet;import org.apache.http.impl.client.DefaultHttpClient;import org.apache.http.protocol.HTTP;import org.apache.http.util.EntityUtils;import com.example.dbconnectiontest.entity.people;import com.example.dbconnectiontest.jsonto.JsonToList;import android.R.integer;import android.app.Activity;import android.content.Context;import android.os.Bundle;import android.view.Menu;import android.view.View;import android.view.View.OnClickListener;import android.widget.ArrayAdapter;import android.widget.Button;import android.widget.ListView;import android.widget.SimpleAdapter;import android.widget.TextView;import android.widget.Toast;/* 必须引用java.io与java.util相关类来读写文件*/public class MainActivity extends Activity { private static final String TAG = "MainActivity"; private static final String URL2 = "http://192.168.1.101:8080/BootStarpImage/getList.action"; static int index = 1;//存放页码 static int indexCount = 5;//存放总页码 Button seButton,lastButton,nextButton;//获取数据按钮，上一页，下一页按钮 TextView counTextView;//显示页码 ListView lView;//listview View buView; //获取listview底部按钮的xml private int[] lay = new int[4];//存放实体类对象的text数组 private List data = new ArrayList();//存放获取到的数据 @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); seButton = (Button)findViewById(R.id.btnOne); seButton.setOnClickListener(new clicklist()); lView = (ListView)findViewById(R.id.listview); /** * 在listview底部添加按钮 */ buView = getLayoutInflater().inflate(R.layout.listbutton, null); lView.addFooterView(buView); /** * 必须放在lview后面，先添加listview才能找到listview上的控件，否则抛出空指针异常 */ lastButton = (Button)findViewById(R.id.lastButton); nextButton = (Button)findViewById(R.id.nextButton); counTextView = (TextView)findViewById(R.id.counTe); } /** {"people":[{"firstName": "Brett", "lastName":"McLaughlin", "email": "aaaa" }, { "firstName": "Jason", "lastName":"Hunter", "email": "bbbb"},{ "firstName": "Elliotte","lastName":"Harold", "email": "cccc" }]} */ @Override public boolean onCreateOptionsMenu(Menu menu) { // Inflate the menu; this adds items to the action bar if it is present. getMenuInflater().inflate(R.menu.main, menu); return true; } class clicklist implements OnClickListener{ @Override public void onClick(View v) { String urlString = URL2 + "?index=1"; String string = httpToServiceSelectData(urlString); listToData(string); /** * 查询总页码 */// urlString = "";// string = httpToServiceSelectData(urlString); setPageButton(v);此处必须传入v及view，否则后续报出关于view的空指针 } } /** * 请求数据 传入一个url * @param urlString */ private String httpToServiceSelectData(String urlString) { HttpClient client=new DefaultHttpClient(); //创建一个GET请求 HttpGet httpGet=new HttpGet(urlString); //向服务器发送请求并获取服务器返回的结果 HttpResponse response; try { response = client.execute(httpGet); int staCode = response.getStatusLine().getStatusCode(); if(staCode == 200){ //返回的结果可能放到InputStream，http Header中等。 String string = EntityUtils.toString(response.getEntity(),HTTP.UTF_8); return string;//返回读取到的数据，用其它方法进行转换成实体 }else if(staCode == 404) Toast.makeText(getApplicationContext(), "服务器异常，请稍候再试。", Toast.LENGTH_SHORT).show(); else if (staCode == 500) Toast.makeText(getApplicationContext(), "读取数据失败，请稍候查询。", Toast.LENGTH_SHORT).show(); } catch (ClientProtocolException e1) { e1.printStackTrace(); } catch (IllegalStateException e1) { e1.printStackTrace(); } catch (IOException e1) { e1.printStackTrace(); } catch (Exception e) { e.printStackTrace(); } return null; } /** * 将查询到的数据添加到listview中 * @param string 返回的字符串（转成过的） */ private void listToData(String string) { JsonToList jToList = new JsonToList(); List<people> li = jToList.returnListView(string);此处调用的是一个java类中的方法，自己写的，代码就几行。要的在下面 for (people ent : li) { Map<String, String> map = new HashMap<String, String>(); map.put("id", ent.getId()+""); map.put("firstName", ent.getFirstName()); map.put("lastName", ent.getLastName()); map.put("email", ent.getEmail()); data.add(map); } SimpleAdapter sAdapter = new SimpleAdapter(MainActivity.this, data, R.layout.listview, new String[]{"id","firstName","lastName","email"}, new int[]{R.id.ids,R.id.firstName,R.id.lastName,R.id.email}); sAdapter.notifyDataSetChanged();//增加或删除操作进行后调用该方法，更改listview中的数据，否则会一直往listview下方添加 lView.setAdapter(sAdapter); } //设置页码信息，以及按钮的是否可点击 private void setPageButton(View view) { counTextView.setText(index + "/" + indexCount);//设置页码数 if(index == 1) lastButton.setClickable(false); else lastButton.setClickable(true); if(index == indexCount) nextButton.setClickable(false); else nextButton.setClickable(true); } @Override protected void onDestroy() { super.onDestroy(); } //点击过按钮过后，加载数据时按钮不可用 public void setButtonAll(Button lastButton,Button nextButton) { lastButton.setClickable(false); nextButton.setClickable(false); } /** * 上一页按钮点击事件 * @param view */ public void lastPageDate(View view) {// setButtonAll(lastButton,nextButton); if (index > 1) { index--;// String url = "";// String string = httpToServiceSelectData(url);// listToData(string);//设置数据 setPageButton(view); } } /** * 下一页按钮点击事件 * @param view */ public void nextPageDate(View view) {// setButtonAll(lastButton,nextButton); if (index < indexCount) { index++;// String url = "";// String string = httpToServiceSelectData(url);// listToData(string);//设置数据 Toast.makeText(MainActivity.this, index+"", Toast.LENGTH_SHORT).show();//此处的index必须+“”，转换为字符串，否则报错 System.out.println(index++); setPageButton(view); } }}?
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JsonToList。java
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public class JsonToList { public List<people> returnListView(String string) { java.lang.reflect.Type type = new com.google.gson.reflect.TypeToken<List<people>>(){}.getType(); Gson gson = new Gson(); List<people> list = gson.fromJson(string, type); return list; }}?
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下面来看一下关于view的错误。例如上面的setPageButton(View view)方法变为setPageButton()就会报错。
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