我是Android编程的初学者。我正在编写一个简单的代码，在主要活动中单击按钮后，它会打开第二个活动。但是应用程序不能在模拟器中打开，在logcat中我不断获得空指针异常。请帮我纠正这个。
MainActivity.java
package com.example.whyprocastinate;
import android.support.v7.app.ActionBarActivity;
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;
public class MainActivity extends ActionBarActivity {
Button b1;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
b1 = (Button) findViewById(R.id.button1);
this.setClickListener();
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}
}
private void setClickListener(){
b1.setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View v) {
startSecondActivity();
}
});
}
private void startSecondActivity(){
Intent intent = new Intent(this, EnterTask.class);
this.startActivity(intent);
}
@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;
}
}
}`
这是logcat显示的内容
07-09 02:18:27.673: W/dalvikvm(1082): threadid=1: thread exiting with uncaught exception (group=0xb2b02ba8)
07-09 02:18:27.693: E/AndroidRuntime(1082): FATAL EXCEPTION: main
07-09 02:18:27.693: E/AndroidRuntime(1082): Process: com.example.whyprocastinate, PID: 1082
07-09 02:18:27.693: E/AndroidRuntime(1082): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.whyprocastinate/com.example.whyprocastinate.MainActivity}: java.lang.NullPointerException
07-09 02:18:27.693: E/AndroidRuntime(1082): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
07-09 02:18:27.693: E/AndroidRuntime(1082): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
07-09 02:18:27.693: E/AndroidRuntime(1082): at android.app.ActivityThread.access$800(ActivityThread.java:135)
07-09 02:18:27.693: E/AndroidRuntime(1082): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
07-09 02:18:27.693: E/AndroidRuntime(1082): at android.os.Handler.dispatchMessage(Handler.java:102)
07-09 02:18:27.693: E/AndroidRuntime(1082): at android.os.Looper.loop(Looper.java:136)
07-09 02:18:27.693: E/AndroidRuntime(1082): at android.app.ActivityThread.main(ActivityThread.java:5017)
07-09 02:18:27.693: E/AndroidRuntime(1082): at java.lang.reflect.Method.invokeNative(Native Method)
07-09 02:18:27.693: E/AndroidRuntime(1082): at java.lang.reflect.Method.invoke(Method.java:515)
07-09 02:18:27.693: E/AndroidRuntime(1082): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
07-09 02:18:27.693: E/AndroidRuntime(1082): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
07-09 02:18:27.693: E/AndroidRuntime(1082): at dalvik.system.NativeStart.main(Native Method)
07-09 02:18:27.693: E/AndroidRuntime(1082): Caused by: java.lang.NullPointerException
07-09 02:18:27.693: E/AndroidRuntime(1082): at com.example.whyprocastinate.MainActivity.setClickListener(MainActivity.java:33)
07-09 02:18:27.693: E/AndroidRuntime(1082): at com.example.whyprocastinate.MainActivity.onCreate(MainActivity.java:24)
07-09 02:18:27.693: E/AndroidRuntime(1082): at android.app.Activity.performCreate(Activity.java:5231)
07-09 02:18:27.693: E/AndroidRuntime(1082): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
07-09 02:18:27.693: E/AndroidRuntime(1082): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
07-09 02:18:27.693: E/AndroidRuntime(1082): ... 11 more
07-09 02:18:31.023: I/Process(1082): Sending signal. PID: 1082 SIG: 9`
MainActivity.java中的第33行是
b1.setOnClickListener(new View.OnClickListener(){
Fragement_main.xml
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
tools:context="com.example.whyprocastinate.MainActivity$PlaceholderFragment" >
<TextView
android:id="@+id/heading"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerInParent="true"
android:layout_marginTop="10dp"
android:textSize="23sp"
android:text="@string/heading" />
<Button
android:id="@+id/button1"
android:text="+"
android:layout_width="40dp"
android:layout_height="55dp"
android:layout_marginTop="0dp"
android:layout_alignParentRight="true"
android:layout_alignBottom="@+id/heading"
android:background="@drawable/button1"
/>
</RelativeLayout>
＆＃39; EnterTask＆＃39;是第二个活动。这是一项空白活动，我还没有在其中添加任何代码。
答案 0 :(得分：0)
Try This Code:-
package com.example.whyprocastinate;
import android.support.v7.app.ActionBarActivity;
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;
public class MainActivity extends ActionBarActivity {
Button b1;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
b1 = (Button) findViewById(R.id.button1);
this.setClickListener();
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}
}
private void setClickListener(){
b1.setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View v) {
startSecondActivity();
}
});
}
private void startSecondActivity(){
Intent intent = new Intent(this, EnterTask.class);
this.startActivity(intent);
}
@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);
Button button = (Button) rootView.findViewById(R.id.button1);
button.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
Toast.makeText(getActivity(), "Hello", Toast.LENGTH_SHORT).show();
}
});
return rootView;
}
}
}