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我面臨一個問題，因爲我嘗試在我的Android應用程序中實現登錄屏幕。用戶必須使用字符串「user」填寫登錄名和密碼字段。 每當我運行此代碼時，我的android虛擬設備崩潰。它甚至沒有打開，並且「不幸的是，LoginScreen已停止」錯誤消息出現在設備屏幕上。這是我的代碼：setOnClickListener崩潰我的Android應用程序
我Main_Activity
package com.example.loginscreen;
import android.app.Activity;
import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
public class MainActivity extends Activity {
Button ok = null;
EditText login = null;
EditText pass = null;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if (savedInstanceState == null) {
getFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}
ok = (Button) findViewById(R.id.ok);
login = (EditText) findViewById(R.id.login_text);
pass = (EditText) findViewById(R.id.pass_text);
ok.setOnClickListener(new View.OnClickListener() {
public void onClick(View view) {
String loginStr = login.getText().toString();
String passStr = pass.getText().toString();
if (loginStr != null && loginStr.equals("user")) {
if (passStr != null && passStr.equals("user")) {
Toast.makeText(getApplicationContext(), "Redirecting...",
Toast.LENGTH_SHORT).show();
} else {
Toast.makeText(getApplicationContext(), "Wrong Credentials",
Toast.LENGTH_SHORT).show();
}
}
}
});
}
@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;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
/**
* A placeholder fragment containing a simple view.
*/
public static class PlaceholderFragment extends Fragment {
public PlaceholderFragment() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container,
false);
return rootView;
}
}
}
fragment_main
<EditText
android:id="@+id/login_text"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="117dp"
android:ems="10"
android:inputType="textPersonName" >
<requestFocus />
</EditText>
<EditText
android:id="@+id/pass_text"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/login_text"
android:layout_centerHorizontal="true"
android:layout_marginTop="72dp"
android:ems="10"
android:inputType="textPassword" />
<Button
android:id="@+id/ok"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/pass_text"
android:layout_centerHorizontal="true"
android:layout_marginTop="56dp"
android:text="@string/ok" />
我logcat的
04-20 11:50:39.501: D/AndroidRuntime(2471): Shutting down VM
04-20 11:50:39.501: W/dalvikvm(2471): threadid=1: thread exiting with uncaught exception (group=0xb1a21ba8)
04-20 11:50:39.511: E/AndroidRuntime(2471): FATAL EXCEPTION: main
04-20 11:50:39.511: E/AndroidRuntime(2471): Process: com.example.loginscreen, PID: 2471
04-20 11:50:39.511: E/AndroidRuntime(2471): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.loginscreen/com.example.loginscreen.MainActivity}: java.lang.NullPointerException
04-20 11:50:39.511: E/AndroidRuntime(2471): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
04-20 11:50:39.511: E/AndroidRuntime(2471): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
04-20 11:50:39.511: E/AndroidRuntime(2471): at android.app.ActivityThread.access$800(ActivityThread.java:135)
04-20 11:50:39.511: E/AndroidRuntime(2471): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
04-20 11:50:39.511: E/AndroidRuntime(2471): at android.os.Handler.dispatchMessage(Handler.java:102)
04-20 11:50:39.511: E/AndroidRuntime(2471): at android.os.Looper.loop(Looper.java:136)
04-20 11:50:39.511: E/AndroidRuntime(2471): at android.app.ActivityThread.main(ActivityThread.java:5017)
04-20 11:50:39.511: E/AndroidRuntime(2471): at java.lang.reflect.Method.invokeNative(Native Method)
04-20 11:50:39.511: E/AndroidRuntime(2471): at java.lang.reflect.Method.invoke(Method.java:515)
04-20 11:50:39.511: E/AndroidRuntime(2471): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-20 11:50:39.511: E/AndroidRuntime(2471): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-20 11:50:39.511: E/AndroidRuntime(2471): at dalvik.system.NativeStart.main(Native Method)
04-20 11:50:39.511: E/AndroidRuntime(2471): Caused by: java.lang.NullPointerException
04-20 11:50:39.511: E/AndroidRuntime(2471): at com.example.loginscreen.MainActivity.onCreate(MainActivity.java:38)
04-20 11:50:39.511: E/AndroidRuntime(2471): at android.app.Activity.performCreate(Activity.java:5231)
04-20 11:50:39.511: E/AndroidRuntime(2471): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-20 11:50:39.511: E/AndroidRuntime(2471): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
04-20 11:50:39.511: E/AndroidRuntime(2471): ... 11 more
我在Virtual Box上使用Eclipse + ADT 22.3版本。 應用程序有以下配置
Minimum Required SDK: API 18
Target SDK: API 19
Compile With: API 19
請，我究竟做錯了什麼？
'產生的原因：在com。示例顯示java.lang.NullPointerException。 loginscreen.MainActivity.onCreate（MainActivity.java:38）'。這意味着行38. – keyser