当我尝试在Android应用中实现登录屏幕时，我遇到了问题。用户必须使用字符串＆＃34; user＆＃34;填写登录名和密码字段。 每当我运行此代码时，我的android虚拟设备都会崩溃。它甚至没有打开和＃34;不幸的是，LoginScreen已停止＆＃34;设备屏幕上显示错误消息。这是我的代码：
我的主要活动
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" />
我的LogCat
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。 该应用程序具有以下配置
Minimum Required SDK: API 18
Target SDK: API 19
Compile With: API 19
拜托，我做错了什么？
答案 0 :(得分：0)
试试这个..
由于Button，EditText都属于fragment_main.xml，因此您必须在PlaceholderFragment.java
public static class PlaceholderFragment extends Fragment {
public PlaceholderFragment() {
}
Button ok = null;
EditText login = null;
EditText pass = null;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container,
false);
ok = (Button) rootView.findViewById(R.id.ok);
login = (EditText) rootView.findViewById(R.id.login_text);
pass = (EditText) rootView.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(getActivity(), "Redirecting...",
Toast.LENGTH_SHORT).show();
} else {
Toast.makeText(getActivity(), "Wrong Credentials",
Toast.LENGTH_SHORT).show();
}
}
}
});
return rootView;
}
}
答案 1 :(得分：0)
您正在初始化活动中fragment_main.xml中的视图。相反，您应该使用onCreateView在片段rootView.findViewById中初始化它。
findViewById在当前视图层次结构中查找视图。您已将activity_main.xml设置为“活动”。因此，如果您在Activity中初始化视图失败，则导致NUllPointerException。
public static class PlaceholderFragment extends Fragment {
public PlaceholderFragment() {
}
Button ok = null;
EditText login = null;
EditText pass = null;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container,
false);
ok = (Button) rootView.findViewById(R.id.ok);
login = (EditText) rootView.findViewById(R.id.login_text);
pass = (EditText) rootView.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(getActivity(), "Redirecting...",
Toast.LENGTH_SHORT).show();
} else {
Toast.makeText(getActivity(), "Wrong Credentials",
Toast.LENGTH_SHORT).show();
}
}
}
});
return rootView;
}
}
答案 2 :(得分：0)
添加方法
myMethod() {
//Code hear
}
To Activity和xml add
android:onClick="myMethod"
到你的按钮