Question

我是Android新手。要创建幻灯片，我使用了var div = $('<div/>').html(html); div.find('<script/>').attr('src', 'other url'); div.find('<link/>').attr('href', 'other url');。以下是我的代码。

PageAdapter

我的问题出在public class MyPagerAdapter extends PagerAdapter { @Override public int getCount() { return 4; //set number of swipe screens here } @Override public Object instantiateItem(final ViewGroup collection, final int position) { LayoutInflater inflater = (LayoutInflater) collection.getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE); int resId = 0; switch (position) { case 0: resId = R.layout.activity_slide; //set which layout will show on load break; case 1: resId = R.layout.slide_2; //what layout swiping shows break; case 2: resId = R.layout.slide_3; //what layout swiping shows break; case 3: resId = R.layout.slide_4; //what layout swiping shows break; } View view = inflater.inflate(resId, null); ((ViewPager) collection).addView(view, 0); return view; } @Override public void destroyItem(final ViewGroup arg0, final int arg1, final Object arg2) { ((ViewPager) arg0).removeView((View) arg2); } @Override public boolean isViewFromObject(final View arg0, final Object arg1) { return arg0 == ((View) arg1); } }我放置了layout.slide_4，我想点击该按钮打开新活动。所以我在下面写了代码oncreate：

Button

logcat的

Button btnExplore = (Button)findViewById(R.id.imageButton);
    btnExplore.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View vw) {
            Intent i = new Intent(IntroSlide.this, StartScreen.class);
            startActivity(i);
        }
    });

我得到了01-18 14:58:58.267 22740-22740/com.totsmart.pedronapp W/dalvikvm: threadid=1: thread exiting with uncaught exception (group=0x536dd160) 01-18 14:58:58.287 22740-22740/com.totsmart.pedronapp E/AndroidRuntime: FATAL EXCEPTION: main Process: com.totsmart.pedronapp, PID: 22740 java.lang.NullPointerException at com.totsmart.pedronapp.IntroSlide$MyPagerAdapter.instantiateItem(IntroSlide.java:59) at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:870) at android.support.v4.view.ViewPager.populate(ViewPager.java:1086) at android.support.v4.view.ViewPager.populate(ViewPager.java:952) at android.support.v4.view.ViewPager$3.run(ViewPager.java:251) at android.view.Choreographer$CallbackRecord.run(Choreographer.java:771) at android.view.Choreographer.doCallbacks(Choreographer.java:574) at android.view.Choreographer.doFrame(Choreographer.java:543) at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:757) at android.os.Handler.handleCallback(Handler.java:733) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:149) at android.app.ActivityThread.main(ActivityThread.java:5045) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:515) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:794) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:610) at dalvik.system.NativeStart.main(Native Method)。我哪里错了？

Answer 1

根据我们在注释中的讨论，您的布局将在运行时加载。因此，如果未加载任何布局，并且您尝试使用该布局的任何视图，您将获得NullPointerException。

我建议在设置onClickListener之前添加一个空检查

Button btnExplore = (Button)findViewById(R.id.imageButton);
if(btnExplore != null)
{
    btnExplore.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View vw) {
            Intent i = new Intent(IntroSlide.this, StartScreen.class);
            startActivity(i);
        }
    });
}

注意：您应该将所有这些代码放在一个单独的方法中，并在每次加载新布局时调用该方法。

更新：更好的解决方案是根据各个布局划分侦听器代码，并为不同布局的视图侦听器保留不同的方法。在此之后，从switch语句中调用适当的方法。此外，您还必须在个别情况下移动以下代码。

View view = inflater.inflate(resId, null);
((ViewPager) collection).addView(view, 0);
return view;

单击imageButton时出现NullPointerException

1 个答案: