我不知道发生了什么事。之前工作正常..可能是我搞砸了什么...... 现在，当我尝试运行此文件时，我收到以下错误： java.lang.RuntimeException：无法启动活动ComponentInfo {com.example.exp2 / com.example.exp2.MainActivity}：java.lang.NullPointerException
MainActivity.java
package com.example.exp2;
import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.content.Intent;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;
import android.os.Build;
public class MainActivity extends ActionBarActivity {
EditText usr;
EditText pass;
Button b1;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.fragment_main);
b1=(Button)findViewById(R.id.button1);//login button
usr=(EditText)findViewById(R.id.editText1);
pass=(EditText)findViewById(R.id.editText2);
b1.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
String username=usr.getText().toString();
String pswds=pass.getText().toString();
if(username.equalsIgnoreCase("santhej")&& pswds.equalsIgnoreCase("kallada"))
{
//Code For Successful Login
Toast.makeText(getApplicationContext(), "Successful Login", Toast.LENGTH_SHORT).show();
Intent in=new Intent(getApplicationContext(), Menu_Activity.class);
in.putExtra("username",username);
startActivity(in);
}
else
{
Toast.makeText(getApplicationContext(), pswds, 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.xml
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.exp2.MainActivity$PlaceholderFragment" >
<EditText
android:id="@+id/editText1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentRight="true"
android:layout_alignTop="@+id/welcome"
android:ems="10" />
<TextView
android:id="@+id/welcome"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/textView2"
android:layout_alignParentTop="true"
android:layout_marginTop="103dp"
android:text="Username"
android:textAppearance="?android:attr/textAppearanceMedium" />
<TextView
android:id="@+id/textView2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/editText1"
android:layout_marginTop="16dp"
android:layout_toLeftOf="@+id/editText1"
android:text="Password:"
android:textAppearance="?android:attr/textAppearanceMedium" />
<EditText
android:id="@+id/editText2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/editText1"
android:layout_alignTop="@+id/textView2"
android:ems="10"
android:inputType="textPassword" >
<requestFocus />
</EditText>
<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/textView2"
android:layout_centerHorizontal="true"
android:layout_marginTop="26dp"
android:text="Login" />
</RelativeLayout>
logcat的
08-24 04:50:29.854: D/AndroidRuntime(3445): Shutting down VM
08-24 04:50:29.854: W/dalvikvm(3445): threadid=1: thread exiting with uncaught exception (group=0x40a71930)
08-24 04:50:29.873: E/AndroidRuntime(3445): FATAL EXCEPTION: main
08-24 04:50:29.873: E/AndroidRuntime(3445): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.exp2/com.example.exp2.MainActivity}: java.lang.NullPointerException
08-24 04:50:29.873: E/AndroidRuntime(3445): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
08-24 04:50:29.873: E/AndroidRuntime(3445): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
08-24 04:50:29.873: E/AndroidRuntime(3445): at android.app.ActivityThread.access$600(ActivityThread.java:141)
08-24 04:50:29.873: E/AndroidRuntime(3445): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
08-24 04:50:29.873: E/AndroidRuntime(3445): at android.os.Handler.dispatchMessage(Handler.java:99)
08-24 04:50:29.873: E/AndroidRuntime(3445): at android.os.Looper.loop(Looper.java:137)
08-24 04:50:29.873: E/AndroidRuntime(3445): at android.app.ActivityThread.main(ActivityThread.java:5041)
08-24 04:50:29.873: E/AndroidRuntime(3445): at java.lang.reflect.Method.invokeNative(Native Method)
08-24 04:50:29.873: E/AndroidRuntime(3445): at java.lang.reflect.Method.invoke(Method.java:511)
08-24 04:50:29.873: E/AndroidRuntime(3445): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
08-24 04:50:29.873: E/AndroidRuntime(3445): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
08-24 04:50:29.873: E/AndroidRuntime(3445): at dalvik.system.NativeStart.main(Native Method)
08-24 04:50:29.873: E/AndroidRuntime(3445): Caused by: java.lang.NullPointerException
08-24 04:50:29.873: E/AndroidRuntime(3445): at com.example.exp2.MainActivity.onCreate(MainActivity.java:34)
08-24 04:50:29.873: E/AndroidRuntime(3445): at android.app.Activity.performCreate(Activity.java:5104)
08-24 04:50:29.873: E/AndroidRuntime(3445): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
08-24 04:50:29.873: E/AndroidRuntime(3445): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
08-24 04:50:29.873: E/AndroidRuntime(3445): ... 11 more
答案 0 :(得分：0)
public class MainActivity extends Activity {
EditText usr;
EditText pass;
Button b1;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.fragment_main);
b1=(Button)findViewById(R.id.button1);//login button
usr=(EditText)findViewById(R.id.editText1);
pass=(EditText)findViewById(R.id.editText2);
b1.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
String username=usr.getText().toString();
String pswds=pass.getText().toString();
if(username.equalsIgnoreCase("santhej")&& pswds.equalsIgnoreCase("kallada"))
{
//Code For Successful Login
Toast.makeText(getApplicationContext(), "Successful Login", Toast.LENGTH_SHORT).show();
Intent in=new Intent(getApplicationContext(), Menu_Activity.class);
in.putExtra("username",username);
startActivity(in);
}
else
{
Toast.makeText(getApplicationContext(), pswds, 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);
}