我是android编程的新手。我试图弄清楚当你按下按钮时，它将显示旋转器中所选选项的Toast消息。 Eclipse没有给我一个错误消息，但只要我尝试在模拟器中运行它，程序就会崩溃。谢谢你的帮助。另外，我在代码的末尾添加了LogCat。
import android.os.Build;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemSelectedListener;
import android.widget.ArrayAdapter;
import android.widget.NumberPicker;
import android.widget.Spinner;
import android.widget.Toast;
public class MainActivity extends Activity
{
private NumberPicker np = (NumberPicker) findViewById(R.id.btnGetMonthFromSpinner);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
loadDisplay();
}
@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;
}
private void loadDisplay()
{
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH)
{
// For API 14 or older
String[] data = getResources().getStringArray(R.array.months);
NumberPicker np = (NumberPicker) findViewById(R.id.pick_number);
np.setMaxValue(data.length-1);
np.setMinValue(0);
np.setWrapSelectorWheel(true);
np.setDisplayedValues(data);
}
else
{
// For API 13 or older
Spinner spinner = (Spinner) findViewById(R.id.listofnumbers);
// Create an ArrayAdapter using the string array and a default spinner layout
ArrayAdapter<CharSequence> adapter = ArrayAdapter.createFromResource(this, R.array.numberlist, android.R.layout.simple_spinner_item);
// Specify the layout to use when the list of choices appears
adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
// Apply the adapter to the spinner
spinner.setAdapter(adapter);
spinner.setOnItemSelectedListener(new SpinnerOnItemSelectedListener());
}
}
public void showSelectedItem(View view)
{
// Called from onClick event in Button definition.
// The Button only displays with NumberPicker layouts, not the Spinner.
String[] entries = np.getDisplayedValues();
Toast.makeText(getApplicationContext(), entries[np.getValue()], Toast.LENGTH_SHORT).show();
}
public class SpinnerOnItemSelectedListener implements OnItemSelectedListener
{
public void onItemSelected(AdapterView<?> parent, View view, int pos, long id)
{
// Either of the Toast statements will work. Spinner inherits the getSelectedItem()
// from the AdapterView class, it returns an Object which I cast to a String.
// We can't use the magical 'this' keyword in makeText() because we are in another
// class, so we use getApplicationContext() instead.
Toast.makeText(getApplicationContext(), (CharSequence) parent.getItemAtPosition(pos), Toast.LENGTH_SHORT).show();
//Toast.makeText(getApplicationContext(), (String)spinner.getSelectedItem(), Toast.LENGTH_SHORT).show();
}
public void onNothingSelected(AdapterView<?> parent)
{
// Do nothing.
}
}
}
logcat的---------------------------------------------- --------------
01-15 06:58:23.238: D/AndroidRuntime(5105): Shutting down VM
01-15 06:58:23.238: W/dalvikvm(5105): threadid=1: thread exiting with uncaught exception (group=0x40a71930)
01-15 06:58:23.258: E/AndroidRuntime(5105): FATAL EXCEPTION: main
01-15 06:58:23.258: E/AndroidRuntime(5105): java.lang.RuntimeException: Unable to start activity ComponentInfo{ca.mohawk.you.lab3/ca.mohawk.you.lab3.MainActivity}: java.lang.ClassCastException: android.widget.Button cannot be cast to android.widget.NumberPicker
01-15 06:58:23.258: E/AndroidRuntime(5105): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
01-15 06:58:23.258: E/AndroidRuntime(5105): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
01-15 06:58:23.258: E/AndroidRuntime(5105): at android.app.ActivityThread.access$600(ActivityThread.java:141)
01-15 06:58:23.258: E/AndroidRuntime(5105): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
01-15 06:58:23.258: E/AndroidRuntime(5105): at android.os.Handler.dispatchMessage(Handler.java:99)
01-15 06:58:23.258: E/AndroidRuntime(5105): at android.os.Looper.loop(Looper.java:137)
01-15 06:58:23.258: E/AndroidRuntime(5105): at android.app.ActivityThread.main(ActivityThread.java:5041)
01-15 06:58:23.258: E/AndroidRuntime(5105): at java.lang.reflect.Method.invokeNative(Native Method)
01-15 06:58:23.258: E/AndroidRuntime(5105): at java.lang.reflect.Method.invoke(Method.java:511)
01-15 06:58:23.258: E/AndroidRuntime(5105): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
01-15 06:58:23.258: E/AndroidRuntime(5105): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
01-15 06:58:23.258: E/AndroidRuntime(5105): at dalvik.system.NativeStart.main(Native Method)
01-15 06:58:23.258: E/AndroidRuntime(5105): Caused by: java.lang.ClassCastException: android.widget.Button cannot be cast to android.widget.NumberPicker
01-15 06:58:23.258: E/AndroidRuntime(5105): at ca.mohawk.you.lab3.MainActivity.onCreate(MainActivity.java:24)
01-15 06:58:23.258: E/AndroidRuntime(5105): at android.app.Activity.performCreate(Activity.java:5104)
01-15 06:58:23.258: E/AndroidRuntime(5105): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
01-15 06:58:23.258: E/AndroidRuntime(5105): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
01-15 06:58:23.258: E/AndroidRuntime(5105): ... 11 more
01-15 06:58:26.948: I/Process(5105): Sending signal. PID: 5105 SIG: 9
答案 0 :(得分：1)
您需要在致电View后初始化所有 setContentView()，以便NumberPicker为null。更改
private NumberPicker np = (NumberPicker) findViewById(R.id.btnGetMonthFromSpinner);
到
NumberPicker np;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// here or anywhere after here
np = (NumberPicker) findViewById(R.id.btnGetMonthFromSpinner);
loadDisplay();
}
但是，您在loadDisplay()中初始化了另一个具有相同名称的选择器。不确定你是否打算这样做。