单击按钮时出现空指针异常 [英] Null pointer exception on button click
本文介绍了单击按钮时出现空指针异常的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
我有4张图像作为按钮，选择正确的按钮后，会出现一个箭头按钮，该按钮工作正常.
I have 4 images as buttons and when the correct button is selected, an arrow button appears which is working fine.
我的问题是，我试图更改每个按钮的背景资源，以在单击此箭头时进行更改，但是我在此行遇到了空指针异常:
My problem is that I'm trying to change the background resource of each button to change when this arrow is clicked but I'm getting a null pointer exception at this line :
happybutton.setBackgroundResource(R.drawable.mum1);
我已经在我的java onCreate中声明了nextArrow按钮-
I have declared the nextArrow button in my java onCreate -
nextArrow = (Button) findViewById(R.id.nextArrow);
nextArrow.setOnClickListener(onClickListener);
班级:
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:
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)
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>
我错过了一些明显的东西吗?
Am I missing something really obvious?
推荐答案
您正在初始化局部变量，而不是实例字段
You are initializing a local variable not the instance field
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，并将替换该名称的实例字段作为此方法的持续时间.您初始化此变量，以便调用其setOnClickListener方法可以正常工作.但是变量在onCreate()方法的末尾消失，其他方法仅看到永不初始化的同名实例字段.
When you say Button happybutton you are creating a new variable whose scope is confined to the onCreate() method, and will supersede the instance field of that name for the duration of this method. You initialize this variable, so calling its setOnClickListener method works fine. But the variable disappears at the end of the onCreate() method, and other methods see only the never-initialized instance field of the same name.
相反，只需初始化您可能已经在onCreate()外部声明的实例字段
Instead, simply initialize the instance field which you your presumably already declared outside of onCreate()
Button happybutton;
protected void onCreate (Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.expressions);
happybutton = (Button) findViewById(R.id.happybutton);
happybutton.setOnClickListener(onClickListener);
这可能是一个细微的错误，因为用局部变量代替实例字段是完全有效的代码，不会触发任何编译器投诉.
This can be a fairly subtle mistake, since superceding an instance field with a local variable is entirely valid code which will not trigger any compiler complaints.
这篇关于单击按钮时出现空指针异常的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
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