我正在编写一个带有两个按钮的简单加法减法程序。一个按钮递增1，另一个递减1.如果我删除onclicklistener方法，程序工作正常。我无法弄清楚我做错了什么。
这是我的代码：
package com.newboston.thefirst;
import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.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.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;
import android.os.Build;
public class MainActivity extends ActionBarActivity {
int counter;
Button add, sub;
TextView display;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
counter=0;
add= (Button) findViewById(R.id.buttonAdd);
sub= (Button) findViewById(R.id.buttonSub);
display= (TextView) findViewById(R.id.textView1);
add.setOnClickListener(new OnClickListener() {
public void onClick(View v) {
// TODO Auto-generated method stub
counter++;
display.setText("Your total is "+counter);
}
});
sub.setOnClickListener(new OnClickListener() {
public void onClick(View v) {
// TODO Auto-generated method stub
counter--;
display.setText("Your total is "+counter);
}
});
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment())
.commit();
}
}
/* @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;
}
}
}
这些是我得到的错误：
03-24 03:31:38.592: I/Process(361): Sending signal. PID: 361 SIG: 9
03-24 03:38:44.952: I/dalvikvm(389): Could not find method
android.content.pm.PackageManager.getActivityLogo, referenced from method
android.support.v7.internal.widget.ActionBarView.<init>
03-24 03:38:44.952: W/dalvikvm(389): VFY: unable to resolve virtual method 312:
Landroid/content/pm/PackageManager;.getActivityLogo
(Landroid/content/ComponentName;)Landroid/graphics/drawable/Drawable;
03-24 03:38:44.952: D/dalvikvm(389): VFY: replacing opcode 0x6e at 0x008b
03-24 03:38:44.962: I/dalvikvm(389): Could not find method
android.content.pm.ApplicationInfo.loadLogo, referenced from method
android.support.v7.internal.widget.ActionBarView.<init>
03-24 03:38:44.962: W/dalvikvm(389): VFY: unable to resolve virtual method 308:
Landroid/content/pm/ApplicationInfo;.loadLogo
(Landroid/content/pm/PackageManager;)Landroid/graphics/drawable/Drawable;
03-24 03:38:44.962: D/dalvikvm(389): VFY: replacing opcode 0x6e at 0x0099
03-24 03:38:44.972: D/dalvikvm(389): VFY: dead code 0x008e-0092 in
Landroid/support/v7/internal/widget/ActionBarView;.<init>
(Landroid/content/Context;Landroid/util/AttributeSet;)V
03-24 03:38:44.972: D/dalvikvm(389): VFY: dead code 0x009c-00a0 in
Landroid/support/v7/internal/widget/ActionBarView;.<init>
(Landroid/content/Context;Landroid/util/AttributeSet;)V
03-24 03:38:45.122: D/AndroidRuntime(389): Shutting down VM
03-24 03:38:45.122: W/dalvikvm(389): threadid=1: thread exiting with uncaught exception
(group=0x4001d800)
03-24 03:38:45.132: E/AndroidRuntime(389): FATAL EXCEPTION: main
03-24 03:38:45.132: E/AndroidRuntime(389): java.lang.RuntimeException: Unable to start
activity ComponentInfo{com.newboston.thefirst/com.newboston.thefirst.MainActivity}:
java.lang.NullPointerException
03-24 03:38:45.132: E/AndroidRuntime(389): at
android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
03-24 03:38:45.132: E/AndroidRuntime(389): at
android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
03-24 03:38:45.132: E/AndroidRuntime(389): at
android.app.ActivityThread.access$2300(ActivityThread.java:125)
03-24 03:38:45.132: E/AndroidRuntime(389): at
android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
03-24 03:38:45.132: E/AndroidRuntime(389): at
android.os.Handler.dispatchMessage(Handler.java:99)
03-24 03:38:45.132: E/AndroidRuntime(389): at android.os.Looper.loop(Looper.java:123)
03-24 03:38:45.132: E/AndroidRuntime(389): at
android.app.ActivityThread.main(ActivityThread.java:4627)
03-24 03:38:45.132: E/AndroidRuntime(389): at
java.lang.reflect.Method.invokeNative(Native Method)
03-24 03:38:45.132: E/AndroidRuntime(389): at
java.lang.reflect.Method.invoke(Method.java:521)
03-24 03:38:45.132: E/AndroidRuntime(389): at
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
03-24 03:38:45.132: E/AndroidRuntime(389): at
com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
03-24 03:38:45.132: E/AndroidRuntime(389): at dalvik.system.NativeStart.main(Native
Method)
03-24 03:38:45.132: E/AndroidRuntime(389): Caused by: java.lang.NullPointerException
03-24 03:38:45.132: E/AndroidRuntime(389): at
com.newboston.thefirst.MainActivity.onCreate(MainActivity.java:31)
03-24 03:38:45.132: E/AndroidRuntime(389): at
android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
03-24 03:38:45.132: E/AndroidRuntime(389): at
android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
03-24 03:38:45.132: E/AndroidRuntime(389): ... 11 more
修改
XML代码
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.newboston.thefirst.MainActivity$PlaceholderFragment" >
<TextView
android:id="@+id/textView1"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="Your total is 0"
android:textSize="30sp"
android:gravity="center"
/>
<Button
android:layout_width="200sp"
android:layout_height="wrap_content"
android:layout_centerHorizontal="true"
android:layout_marginTop="71dp"
android:gravity="center"
android:text="Subtract one"
android:id="@+id/buttonSub" />
<Button
android:layout_width="200sp"
android:layout_height="wrap_content"
android:layout_below="@+id/textView1"
android:layout_centerHorizontal="true"
android:layout_marginTop="110dp"
android:gravity="center"
android:text="Add one"
android:id="@+id/buttonAdd" />
</RelativeLayout>
答案 0 :(得分：0)
你正在比较一个空值，这就是为什么抛出空指针的原因：
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment())
.commit();
}
你应该试试
if(savedInstanceState != null){
// leave it blank if you don't intend
// to do anything if savedInstanceState is not null
}else{
// your codes here ..
}