我是android的初学者，我正在开发一个有两个屏幕的应用程序，当我尝试从一个屏幕切换到另一个屏幕时，应用程序崩溃了，谁能告诉我为什么会这样，是因为第二课的声明吗？我在清单文件中添加了第二个类，这似乎不是问题所在。主要课程是这样的：
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
final DatabaseH dh =new DatabaseH(this);
final EditText ed1=(EditText)findViewById(R.id.editText1);
Button b1=(Button)findViewById(R.id.button1);
Button b2=(Button)findViewById(R.id.button2);
final String[] itemsn = new String[]{"Cat","Dog","Mouse","fish"};
final ListView listView = (ListView) findViewById(R.id.listView1);
final TextView tv1 = (TextView)findViewById(R.id.textView1);
final ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1);
listView.setAdapter(adapter);
b1.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
dh.additem(ed1.getText().toString());
}
});
b2.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
//String[] values = new String[dh.getAllContacts().size()];
//values=dh.getAllContacts().toArray(values);
// TODO Auto-generated method stu
adapter.clear();
adapter.addAll(dh.getAllItems());
//tv1.setText(dh.getAllContacts().toString());
}});
listView.setOnItemLongClickListener(new AdapterView.OnItemLongClickListener() {
@Override
public boolean onItemLongClick(AdapterView<?> arg0, View arg1,
int arg2, long arg3) {
//deleting the item at clicked position
tv1.setText(listView.getItemAtPosition(arg2).toString());
dh.deleteContact(listView.getItemAtPosition(arg2).toString());
adapter.clear();
adapter.addAll(dh.getAllItems());
// TODO Auto-generated method stub
return false;
}
});
Button b3=(Button)findViewById(R.id.admin);
b3.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
try{
Intent myIntent = new Intent(v.getContext(), dbManager.class);
startActivityForResult(myIntent, 0);}
catch(Exception e){
tv1.setText(e.toString());
}
}
});
}
@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;
}
public boolean onOptionsItemSelected(MenuItem item) {
switch(item.getItemId()){
case R.id.action_settings:
break;
}
return super.onOptionsItemSelected(item);
}
}
第二类：
public class dbManager extends Activity {@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.db_manager);
final DatabaseH dh1 = null;
final Spinner sp1=(Spinner)findViewById(R.id.spinner1);
final EditText itn=(EditText)findViewById(R.id.itemname);
final EditText cos=(EditText)findViewById(R.id.cost);
final EditText avai=(EditText)findViewById(R.id.status);
final RatingBar rtb1=(RatingBar)findViewById(R.id.rating);
Button submit=(Button)findViewById(R.id.sub);
Button add =(Button)findViewById(R.id.Addnew);
List<String> list1 = new ArrayList<String>();
list1.addAll(dh1.getAllItems());
ArrayAdapter<String> dataAdapter = new ArrayAdapter<String>(this,android.R.layout.simple_spinner_item, list1);
dataAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
sp1.setAdapter(dataAdapter);
submit.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
dh1.addSubitem(itn.getText().toString(),cos.getText().toString(),avai.getText().toString(),sp1.getSelectedItem().toString(),rtb1.getRating());
}
});
}
}
这是我的 logcat o/p：
07-31 12:29:38.227: W/dalvikvm(528): threadid=1: thread exiting with uncaught exception (group=0x40014760)
07-31 12:29:38.246: E/AndroidRuntime(528): FATAL EXCEPTION: main
07-31 12:29:38.246: E/AndroidRuntime(528): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.treedb/com.example.treedb.dbManager}: java.lang.NullPointerException
07-31 12:29:38.246: E/AndroidRuntime(528): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1736)
07-31 12:29:38.246: E/AndroidRuntime(528): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1752)
07-31 12:29:38.246: E/AndroidRuntime(528): at android.app.ActivityThread.access$1500(ActivityThread.java:123)
07-31 12:29:38.246: E/AndroidRuntime(528): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:993)
07-31 12:29:38.246: E/AndroidRuntime(528): at android.os.Handler.dispatchMessage(Handler.java:99)
07-31 12:29:38.246: E/AndroidRuntime(528): at android.os.Looper.loop(Looper.java:126)
07-31 12:29:38.246: E/AndroidRuntime(528): at android.app.ActivityThread.main(ActivityThread.java:3997)
07-31 12:29:38.246: E/AndroidRuntime(528): at java.lang.reflect.Method.invokeNative(Native Method)
07-31 12:29:38.246: E/AndroidRuntime(528): at java.lang.reflect.Method.invoke(Method.java:491)
07-31 12:29:38.246: E/AndroidRuntime(528): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841)
07-31 12:29:38.246: E/AndroidRuntime(528): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599)
07-31 12:29:38.246: E/AndroidRuntime(528): at dalvik.system.NativeStart.main(Native Method)
07-31 12:29:38.246: E/AndroidRuntime(528): Caused by: java.lang.NullPointerException
07-31 12:29:38.246: E/AndroidRuntime(528): at com.example.treedb.dbManager.onCreate(dbManager.java:30)
07-31 12:29:38.246: E/AndroidRuntime(528): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1048)
07-31 12:29:38.246: E/AndroidRuntime(528): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1700)
07-31 12:29:38.246: E/AndroidRuntime(528): ... 11 more