我正在创建一个引发错误的应用程序，活动未启动。我的活动课是：
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.videos_layout);
i = new FirstImage(this);
c = (ViewFlipper) findViewById(R.id.viewFlipper3);
back = (Button) findViewById(R.id.button1);
next = (Button) findViewById(R.id.button2);
t = (TextView) findViewById(R.id.textView1);
if(count == 0)
i.changeImage(R.drawable.human);
c.addView(i);
c.setOnTouchListener(this);
c.onTouchEvent(event);
addListenerOnButton();
}
private void addListenerOnButton() {
if (count == 0)
back.setEnabled(false);
next.setOnClickListener( new OnClickListener() {
public void onClick(View arg0) {
if(count==0){
i.changeImage(R.drawable.hand);
back.setEnabled(true);
count++;
addListenerOnButton();
}
else if (count==1){
i.changeImage(R.drawable.tissue);
count++;
addListenerOnButton();
}
else if (count==2){
image.setImageResource(R.drawable.cellconstfinal1);
count++;
addListenerOnButton();
}
else if (count==3){
//nucleus
image.setImageResource(R.drawable.cellconstfinal1);
count++;
addListenerOnButton();
}
else if (count==4){
//nchromosome
image.setImageResource(R.drawable.hand);
count++;
addListenerOnButton();
}
else
{ //double helix
count++;
image.setImageResource(R.drawable.handash1);
next.setEnabled(false);
addListenerOnButton();
}
}
});
back.setOnClickListener(new OnClickListener() {
public void onClick(View arg0) {
if(count==1){
image.setImageResource(R.drawable.human);
back.setEnabled(false);
count--;
addListenerOnButton();
}
else if (count==2){
image.setImageResource(R.drawable.hand);
count--;
addListenerOnButton();
}
else if (count==3){
image.setImageResource(R.drawable.tissue);
count--;
addListenerOnButton();
}
else if (count==4){
image.setImageResource(R.drawable.cellconstfinal1);
count--;
addListenerOnButton();
}
else if (count==5){
//nucleus
image.setImageResource(R.drawable.tissue);
count--;
addListenerOnButton();
}
else {
//chromosome
//count==6
image.setImageResource(R.drawable.cellconstfinal1);
count--;
addListenerOnButton();
next.setEnabled(true);
}
}
});
}
public void pageinfo(float a,float b){
t.setText(Float.toString(a)+"x"+Float.toString(b)+"y");
}
public boolean onTouch(View v, MotionEvent me) {
// TODO Auto-generated method stub
switch(me.getAction()){
case MotionEvent.ACTION_DOWN:
a=me.getX();
b= me.getY();
pageinfo(a,b);
break;
case MotionEvent.ACTION_MOVE:
a=me.getX();
b= me.getY();
pageinfo(a,b);
break;
case MotionEvent.ACTION_UP:
a=me.getX();
b= me.getY();
pageinfo(a,b);
break;
case MotionEvent.ACTION_OUTSIDE:
a=me.getX();
b= me.getY();
pageinfo(a,b);
break;
default: return false;
}
return true;
}
}
我的
FirstImage类代码：public class FirstImage extends ImageView {
float a=0;
float b=0;
TextView t;
ImageView i;
public FirstImage(Context context) {
super(context);
t= (TextView) findViewById(R.id.textView1);
i= new ImageView (context);
}
public FirstImage(Context context, AttributeSet attrs) {
super(context, attrs);
}
protected void changeImage(int id){
i.setImageResource(id);
}
stacktrace除外：
05-11 00:21:41.197: E/AndroidRuntime(1142): FATAL EXCEPTION: main
05-11 00:21:41.197: E/AndroidRuntime(1142): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.androidtablayout/com.example.androidtablayout.VideosActivity}: java.lang.NullPointerException
05-11 00:21:41.197: E/AndroidRuntime(1142): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
05-11 00:21:41.197: E/AndroidRuntime(1142): at android.app.ActivityThread.startActivityNow(ActivityThread.java:2503)
05-11 00:21:41.197: E/AndroidRuntime(1142): at android.app.LocalActivityManager.moveToState(LocalActivityManager.java:127)
05-11 00:21:41.197: E/AndroidRuntime(1142): at android.app.LocalActivityManager.startActivity(LocalActivityManager.java:339)
05-11 00:21:41.197: E/AndroidRuntime(1142): at android.widget.TabHost$IntentContentStrategy.getContentView(TabHost.java:651)
05-11 00:21:41.197: E/AndroidRuntime(1142): at android.widget.TabHost.setCurrentTab(TabHost.java:323)
05-11 00:21:41.197: E/AndroidRuntime(1142): at android.widget.TabHost$2.onTabSelectionChanged(TabHost.java:129)
05-11 00:21:41.197: E/AndroidRuntime(1142): at android.widget.TabWidget$TabClickListener.onClick(TabWidget.java:453)
05-11 00:21:41.197: E/AndroidRuntime(1142): at android.view.View.performClick(View.java:2408)
05-11 00:21:41.197: E/AndroidRuntime(1142): at android.view.View$PerformClick.run(View.java:8816)
05-11 00:21:41.197: E/AndroidRuntime(1142): at android.os.Handler.handleCallback(Handler.java:587)
05-11 00:21:41.197: E/AndroidRuntime(1142): at android.os.Handler.dispatchMessage(Handler.java:92)
05-11 00:21:41.197: E/AndroidRuntime(1142): at android.os.Looper.loop(Looper.java:123)
05-11 00:21:41.197: E/AndroidRuntime(1142): at android.app.ActivityThread.main(ActivityThread.java:4627)
05-11 00:21:41.197: E/AndroidRuntime(1142): at java.lang.reflect.Method.invokeNative(Native Method)
05-11 00:21:41.197: E/AndroidRuntime(1142): at java.lang.reflect.Method.invoke(Method.java:521)
05-11 00:21:41.197: E/AndroidRuntime(1142): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
05-11 00:21:41.197: E/AndroidRuntime(1142): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
05-11 00:21:41.197: E/AndroidRuntime(1142): at dalvik.system.NativeStart.main(Native Method)
05-11 00:21:41.197: E/AndroidRuntime(1142): Caused by: java.lang.NullPointerException
05-11 00:21:41.197: E/AndroidRuntime(1142): at com.example.androidtablayout.VideosActivity.onCreate(VideosActivity.java:45)
05-11 00:21:41.197: E/AndroidRuntime(1142): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
05-11 00:21:41.197: E/AndroidRuntime(1142): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
05-11 00:21:41.197: E/AndroidRuntime(1142): ... 18 more
最佳答案
您应该始终在onCreate方法中初始化视图：
//...
FirstImage i;
//...
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.videos_layout);
i = new FirstImage(this);
//...
同样在
FirstImage类中，在构造函数中初始化那些Views：public FirstImage(Context context) {
super(context);
TextView t= (TextView) findViewById(R.id.textView1); // WHERE do you expect to find this view, in which layout?
ImageView i= new ImageView (context); //NEVER pass a null Context
}
编辑：
我的建议是遵循一些有关自定义
Views的教程。您不能将其他Views添加到ImageView子类中（也不应该）。即使在ImageView中构建FirstImage，changeImage方法也将抛出NullPointerException当前代码的构建方式。一个简单的自定义视图：
public class FirstImage extends LinearLayout {
float a=0;
float b=0;
TextView t;
ImageView i;
public FirstImage(Context context) {
super(context);
t = (TextView) findViewById(R.id.textView1); // What is with this TextView?!?
i = new ImageView (context);
LinearLayout.LayoutParams lp = LinearLayout.LayoutParams(LinearLayout.LayoutParams.WRAP_CONTENT, LinearLayout.LayoutParams.WRAP_CONTENT);
addView(i, lp);
}
protected void changeImage(int id){
i.setImageResource(id);
}