我有4个图像作为按钮，当选择了正确的按钮时，会出现一个正常工作的箭头按钮。
我的问题是我尝试更改每个按钮的后台资源，以便在单击此箭头时更改，但我在此行获取空指针异常：
happybutton.setBackgroundResource(R.drawable.mum1);
我在java onCreate中声明了nextArrow按钮 -
nextArrow = (Button) findViewById(R.id.nextArrow);
nextArrow.setOnClickListener(onClickListener);
类：
protected void onCreate (Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.expressions);
Button happybutton = (Button) findViewById(R.id.happybutton);
happybutton.setOnClickListener(onClickListener);
Button sadbutton = (Button)findViewById(R.id.sadbutton);
sadbutton.setOnClickListener(onClickListener);
Button worriedbutton = (Button) findViewById(R.id.worriedbutton);
worriedbutton.setOnClickListener(onClickListener);
Button excitedbutton = (Button) findViewById(R.id.excitedbutton);
excitedbutton.setOnClickListener(onClickListener);
//Button nextArrow = (Button) findViewById(R.id.nextArrow);
//nextArrow.setOnClickListener(onClickListener);
tick = (ImageView) findViewById(R.id.tick);
cross = (ImageView)findViewById(R.id.cross);
cross2 = (ImageView) findViewById(R.id.cross2);
cross3 = (ImageView) findViewById(R.id.cross3);
nextArrow = (Button) findViewById(R.id.nextArrow);
nextArrow.setOnClickListener(onClickListener);
tryagain = MediaPlayer.create(Expressions.this,R.raw.tryagain);
correct = MediaPlayer.create(Expressions.this,R.raw.correct);
}
private OnClickListener onClickListener = new OnClickListener(){
public void onClick(final View view){
switch(view.getId()){
case R.id.happybutton:
tick.setVisibility(View.VISIBLE);
nextArrow.setVisibility(View.VISIBLE);
correct.start();
break;
case R.id.sadbutton:
cross.setVisibility(View.VISIBLE);
tryagain.start();
break;
case R.id.worriedbutton:
cross3.setVisibility(View.VISIBLE);
tryagain.start();
break;
case R.id.excitedbutton:
cross2.setVisibility(View.VISIBLE);
tryagain.start();
break;
case R.id.nextArrow:
happybutton.setBackgroundResource(R.drawable.mum1);
worriedbutton.setBackgroundResource(R.drawable.mum2);
sadbutton.setBackgroundResource(R.drawable.mum3);
excitedbutton.setBackgroundResource(R.drawable.mum4);
break;
}
}
logcat的：
07-31 16:17:11.312: E/AndroidRuntime(7818): FATAL EXCEPTION: main
07-31 16:17:11.312: E/AndroidRuntime(7818): java.lang.NullPointerException
07-31 16:17:11.312: E/AndroidRuntime(7818): at
com.example.Autism_App.Expressions$1.onClick(Expressions.java:84)
07-31 16:17:11.312: E/AndroidRuntime(7818): at android.view.View.performClick(View.java:4383)
07-31 16:17:11.312: E/AndroidRuntime(7818): at android.view.View$PerformClick.run(View.java:18097)
07-31 16:17:11.312: E/AndroidRuntime(7818): at android.os.Handler.handleCallback(Handler.java:725)
07-31 16:17:11.312: E/AndroidRuntime(7818): at android.os.Handler.dispatchMessage(Handler.java:92)
07-31 16:17:11.312: E/AndroidRuntime(7818): at android.os.Looper.loop(Looper.java:176)
07-31 16:17:11.312: E/AndroidRuntime(7818): at android.app.ActivityThread.main(ActivityThread.java:5279)
07-31 16:17:11.312: E/AndroidRuntime(7818): at java.lang.reflect.Method.invokeNative(Native Method)
07-31 16:17:11.312: E/AndroidRuntime(7818): at java.lang.reflect.Method.invoke(Method.java:511)
07-31 16:17:11.312: E/AndroidRuntime(7818): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
07-31 16:17:11.312: E/AndroidRuntime(7818): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
07-31 16:17:11.312: E/AndroidRuntime(7818): at dalvik.system.NativeStart.main(Native Method)
的xml：
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<TextView
android:id="@+id/question"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="29dp"
android:text="Which face is the happy face?"
android:textSize="80sp" />
<Button
android:id="@+id/happybutton"
android:layout_width="200dp"
android:layout_height="200dp"
android:layout_alignLeft="@+id/excitedbutton"
android:layout_alignParentBottom="true"
android:layout_marginBottom="43dp"
android:background="@drawable/happyface"/>
<Button
android:id="@+id/nextArrow"
android:layout_width="100dp"
android:layout_height="100dp"
android:layout_alignParentBottom="true"
android:layout_alignParentRight="true"
android:layout_marginRight="65dp"
android:visibility="invisible"
android:background="@drawable/nextarrow" />
<Button
android:id="@+id/worriedbutton"
android:layout_width="200dp"
android:layout_height="200dp"
android:layout_alignBaseline="@+id/happybutton"
android:layout_alignBottom="@+id/happybutton"
android:layout_alignLeft="@+id/sadbutton"
android:background="@drawable/worriedface" />
<Button
android:id="@+id/sadbutton"
android:layout_width="200dp"
android:layout_height="200dp"
android:layout_below="@+id/question"
android:layout_marginRight="113dp"
android:layout_marginTop="29dp"
android:layout_toLeftOf="@+id/nextArrow"
android:background="@drawable/sadface" />
<Button
android:id="@+id/excitedbutton"
android:layout_width="200dp"
android:layout_height="200dp"
android:layout_alignBaseline="@+id/sadbutton"
android:layout_alignBottom="@+id/sadbutton"
android:layout_alignLeft="@+id/question"
android:layout_marginLeft="116dp"
android:background="@drawable/excitedface" />
<ImageView
android:id="@+id/cross2"
android:layout_width="200dp"
android:layout_height="200dp"
android:layout_alignLeft="@+id/excitedbutton"
android:layout_alignTop="@+id/excitedbutton"
android:src="@drawable/cross"
android:visibility="invisible" />
<ImageView
android:id="@+id/cross"
android:layout_width="200dp"
android:layout_height="200dp"
android:layout_alignLeft="@+id/sadbutton"
android:layout_alignTop="@+id/sadbutton"
android:src="@drawable/cross"
android:visibility="invisible" />
<ImageView
android:id="@+id/tick"
android:layout_width="200dp"
android:layout_height="200dp"
android:layout_alignLeft="@+id/excitedbutton"
android:layout_alignTop="@+id/worriedbutton"
android:src="@drawable/tick"
android:visibility="invisible" />
<ImageView
android:id="@+id/cross3"
android:layout_width="200dp"
android:layout_height="200dp"
android:layout_alignLeft="@+id/worriedbutton"
android:layout_alignTop="@+id/worriedbutton"
android:src="@drawable/cross"
android:visibility="invisible" />
</RelativeLayout>
我错过了一些非常明显的东西吗？
答案 0 :(得分：5)
您正在初始化本地变量而不是实例字段
protected void onCreate (Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.expressions);
Button happybutton = (Button) findViewById(R.id.happybutton);
happybutton.setOnClickListener(onClickListener);
当您说Button happybutton时，您正在创建一个 new 变量，其范围仅限于onCreate（）方法，并且在此方法的持续时间内将取代该名称的实例字段。您初始化此变量，因此调用其setOnClickListener方法可以正常工作。但是变量在onCreate（）方法的末尾消失，而其他方法只看到同名的从未初始化的实例字段。
相反，只需初始化您可能已在onCreate（）之外声明的实例字段
Button happybutton;
protected void onCreate (Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.expressions);
happybutton = (Button) findViewById(R.id.happybutton);
happybutton.setOnClickListener(onClickListener);
这可能是一个相当微妙的错误，因为使用局部变量取代实例字段是完全有效的代码，不会触发任何编译器投诉。
答案 1 :(得分：0)
我相信，班级成员happybutton未初始化。
答案 2 :(得分：0)
按钮应该是类变量，如下所示：
public class MyClass {
Button happybutton;
Button sadbutton;
Button worriedbutton;
Button excitedbutton;
protected void onCreate (Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.expressions);
happybutton = (Button) findViewById(R.id.happybutton);
happybutton.setOnClickListener(onClickListener);
sadbutton = (Button)findViewById(R.id.sadbutton);
sadbutton.setOnClickListener(onClickListener);
worriedbutton = (Button) findViewById(R.id.worriedbutton);
worriedbutton.setOnClickListener(onClickListener);
excitedbutton = (Button) findViewById(R.id.excitedbutton);
excitedbutton.setOnClickListener(onClickListener);
...