我通过“加载学生列表”按钮加载列表：但是当我单击列表中的某个项而不是转到下一个活动时，它只会崩溃：NULL指针执行：我正在捆绑stID（学生Id）作为额外的意图。在toItemClick上做一个Toast给了我正确的学生ID，然后它是如何为null，我想传递给intent的上下文可能会有一些问题;但那是猜测;你们帮我指出这个错误;
package com.example.connection;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.UUID;
import android.app.Activity;
import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothDevice;
import android.bluetooth.BluetoothSocket;
import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;
import android.widget.Toast;
import android.view.View;
import android.view.View.OnClickListener;
public class SelectStudent extends Activity {
private BluetoothAdapter btAdapter = null;
private BluetoothSocket btSocket = null;
private OutputStream outStream = null;
// Well known SPP UUID
private static final UUID MY_UUID =
UUID.fromString("00001101-0000-1000-8000-00805F9B34FB");
// Insert your server's MAC address
private static String address = "01:36:05:23:4A:44";
public static String grpid;
Context obj;
Context objx;
/** Called when the activity is first created. */
public void onCreate(Bundle savedInstanceState) {
objx=this;
super.onCreate(savedInstanceState);
setContentView(R.layout.select_student);
final ListView listview = (ListView) findViewById(R.id.stdlist);
final ArrayList<String> list = new ArrayList<String>();
Button btnLogin= (Button) findViewById(R.id.LoadList);
Button btnClose =(Button) findViewById(R.id.Exit);
btnClose.setText("Close");
Button insert = (Button) findViewById(R.id.add_btn);
insert.setOnClickListener(new OnClickListener() {
public void onClick(View arg0) {
Intent intent = new Intent(SelectStudent.this, InsertMarks.class);
startActivity(intent);
}
});
btAdapter = BluetoothAdapter.getDefaultAdapter();
btnClose.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
System.exit(0);
}
});
btnLogin.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
try
{
// Set up a pointer to the remote node (Machine/PC) using it's address.
BluetoothDevice device = btAdapter.getRemoteDevice(address);
// -- Notes -- //
// Two things are needed to make a connection:
// A MAC address, which we got above.
// A Service ID or UUID.
// UUID for SPP.
try {
btSocket = device.createRfcommSocketToServiceRecord(MY_UUID);
} catch (IOException e) {
//AlertBox("Fatal Error", "In onResume() and socket create failed: " + e.getMessage() + ".");
}
// During message pass no devise discovery is allowed.
btAdapter.cancelDiscovery();
// Establish the connection. This will block until it connects.
try {
btSocket.connect();
//out.append("\n...Connection established and data link opened...");
} catch (IOException e) {
try {
btSocket.close();
} catch (IOException e2) {
//AlertBox("Fatal Error", "In onResume() and unable to close socket during connection failure" + e2.getMessage() + ".");
}
}
// Create a data stream so we can talk to server.
//out.append("\n...Sending message to server...");
String message = "2\n";
//out.append("\n\n...The message that we will send to the server is: "+message);
try {
outStream = btSocket.getOutputStream();
} catch (IOException e) {
//AlertBox("Fatal Error", "In onResume() and output stream creation failed:" + e.getMessage() + ".");
}
byte[] msgBuffer = message.getBytes();
try {
outStream.write(msgBuffer);
} catch (IOException e) {
String msg = "In onResume() and an exception occurred during write: " + e.getMessage();
if (address.equals("00:00:00:00:00:00"))
msg = msg + ".\n\nUpdate your server address from 00:00:00:00:00:00 to the correct address on line 37 in the java code";
msg = msg + ".\n\nCheck that the SPP UUID: " + MY_UUID.toString() + " exists on server.\n\n";
//AlertBox("Fatal Error", msg);
}
InputStream inStream;
try {
inStream = btSocket.getInputStream();
BufferedReader bReader=new BufferedReader(new InputStreamReader(inStream));
String lineRead=bReader.readLine();
Toast.makeText(objx, lineRead, Toast.LENGTH_SHORT).show();
bReader.close();
inStream.close();
do
{
try
{
String id,temp;
int length;
id=lineRead.substring(0, lineRead.indexOf("*"));
length=lineRead.indexOf("*");
temp=lineRead.substring(length+1);
lineRead=temp;
if(!id.equals(""))
list.add(id);
}
catch(Exception exp)
{
Toast.makeText(getApplication(), exp.getMessage(), Toast.LENGTH_SHORT).show();
}
final StableArrayAdapter adapter = new StableArrayAdapter(objx,
android.R.layout.simple_list_item_1, list);
listview.setAdapter(adapter);
}
while(lineRead.contains("*"));
//finish();
} catch (IOException e) {
e.printStackTrace();
}
//out.append("\n...In onPause()...");
if (outStream != null) {
try {
outStream.flush();
} catch (IOException e) {
// AlertBox("Fatal Error", "In onPause() and failed to flush output stream: " + e.getMessage() + ".");
}
}
try {
btSocket.close();
} catch (IOException e2) {
// AlertBox("Fatal Error", "In onPause() and failed to close socket." + e2.getMessage() + ".");
}
//bReader.close();
//inStream.close();
}
catch(Exception exp)
{
Toast.makeText(getApplication(), "Error Found!"+exp.getMessage(), Toast.LENGTH_SHORT).show();
}
}
});
listview.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, final View view,
int position, long id) {
try
{
final String item = (String) parent.getItemAtPosition(position);
//InsertMarks.stdid=position;
Intent i= new Intent(SelectStudent.this,InsertMarks.class);
i.putExtra("stID", item);
startActivity(i);
//Toast.makeText(getApplication(), "Working"+item, Toast.LENGTH_SHORT).show();
//String app = SelectStudent.this.toString();
//Toast.makeText(getApplication(), "Current Context: "+app, Toast.LENGTH_LONG).show();
//next();
}
catch(Exception exp)
{
//Toast.makeText(objx, exp.getMessage(), Toast.LENGTH_LONG).show();
}
};
});
}
private class StableArrayAdapter extends ArrayAdapter<String> {
HashMap<String, Integer> mIdMap = new HashMap<String, Integer>();
public StableArrayAdapter(Context context, int textViewResourceId,
List<String> objects) {
super(context, textViewResourceId, objects);
for (int i = 0; i < objects.size(); ++i) {
mIdMap.put(objects.get(i), i);
}
}
@Override
public long getItemId(int position) {
String item = getItem(position);
return mIdMap.get(item);
}
@Override
public boolean hasStableIds() {
return true;
}
}
}
我的XML布局：
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<TextView
android:id="@+id/textView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_alignParentTop="true"
android:layout_marginLeft="20dp"
android:layout_marginTop="15dp"
android:text="@string/SelectStudent"
android:textAppearance="?android:attr/textAppearanceLarge" />
<ListView
android:id="@+id/stdlist"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_above="@+id/Exit"
android:layout_alignLeft="@+id/textView1"
android:layout_alignRight="@+id/Exit"
android:layout_below="@+id/textView1"
android:layout_marginTop="16dp"
android:scrollbarAlwaysDrawVerticalTrack="true" >
</ListView>
<Button
android:id="@+id/LoadList"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/Exit"
android:layout_alignBottom="@+id/Exit"
android:layout_alignParentLeft="true"
android:text="@string/LoadList" />
<Button
android:id="@+id/Exit"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_marginBottom="17dp"
android:layout_toRightOf="@+id/LoadList"
android:text="@string/Next" />
<Button
android:id="@+id/add_btn"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/stdlist"
android:layout_toRightOf="@+id/stdlist"
android:text="@string/add" />
</RelativeLayout>
我需要一种方法来转移到下一个活动，学生ID作为额外的传递：
01-07 19:28:28.888: E/AndroidRuntime(4354): FATAL EXCEPTION: main
01-07 19:28:28.888: E/AndroidRuntime(4354): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.connection/com.example.connection.InsertMarks}: java.lang.NullPointerException
01-07 19:28:28.888: E/AndroidRuntime(4354): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2084)
01-07 19:28:28.888: E/AndroidRuntime(4354): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2111)
01-07 19:28:28.888: E/AndroidRuntime(4354): at android.app.ActivityThread.access$600(ActivityThread.java:134)
01-07 19:28:28.888: E/AndroidRuntime(4354): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1251)
01-07 19:28:28.888: E/AndroidRuntime(4354): at android.os.Handler.dispatchMessage(Handler.java:99)
01-07 19:28:28.888: E/AndroidRuntime(4354): at android.os.Looper.loop(Looper.java:137)
01-07 19:28:28.888: E/AndroidRuntime(4354): at android.app.ActivityThread.main(ActivityThread.java:4666)
01-07 19:28:28.888: E/AndroidRuntime(4354): at java.lang.reflect.Method.invokeNative(Native Method)
01-07 19:28:28.888: E/AndroidRuntime(4354): at java.lang.reflect.Method.invoke(Method.java:511)
01-07 19:28:28.888: E/AndroidRuntime(4354): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:809)
01-07 19:28:28.888: E/AndroidRuntime(4354): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:576)
01-07 19:28:28.888: E/AndroidRuntime(4354): at dalvik.system.NativeStart.main(Native Method)
01-07 19:28:28.888: E/AndroidRuntime(4354): Caused by: java.lang.NullPointerException
01-07 19:28:28.888: E/AndroidRuntime(4354): at com.example.connection.InsertMarks.onCreate(InsertMarks.java:48)
01-07 19:28:28.888: E/AndroidRuntime(4354): at android.app.Activity.performCreate(Activity.java:4510)
01-07 19:28:28.888: E/AndroidRuntime(4354): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1050)
01-07 19:28:28.888: E/AndroidRuntime(4354): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2048)
01-07 19:28:28.888: E/AndroidRuntime(4354): ... 11 more
答案 0 :(得分：0)
看着LogCat，它说NullPointerException活动中有InsertMarks，我猜你没有恢复从SelectStudent传递到InsertMarks的值}。
你应该这样做（在InsertMarks中）：
String yourStudent = getIntent().getStringExtra("stID");
但我无法确定，因为你没有发布InsertMarks代码。