轉自http://blog.csdn.net/hz0324/article/details/9296861
如果在onCreate()上面聲明+賦值,會出現NullPointerException;如果在onCreate()裏面賦值,會出現onCreate()中定義的class,method等無法訪問該控件。
因此只能按聲明在onCreate()上,賦值在onCreate()內的方式解決這個問題。
- package com.example.animprogressbar;
- import android.app.Activity;
- import android.os.Bundle;
- import android.view.View;
- import android.view.View.OnClickListener;
- import android.view.animation.AlphaAnimation;
- import android.view.animation.AnimationSet;
- import android.widget.ImageView;
- public class MainActivity extends Activity {
- //關於爲什麼要在onCreate()上面聲明控件,但是賦值爲null?如果直接在這裏findViewById會造成空指針報錯,因爲主layout還沒畫出
- private ImageView iv = null;
- @Override
- protected void onCreate(Bundle savedInstanceState) {
- super.onCreate(savedInstanceState);
- setContentView(R.layout.activity_main);
- //爲什麼不在這裏聲明iv呢?因爲在這裏聲明會造成下面onClick()函數中無法訪問iv,從而報錯
- iv = (ImageView) findViewById(R.id.imageview);
- iv.setOnClickListener(new OnClickListener() {
- @Override
- public void onClick(View v) {
- AnimationSet animSet = new AnimationSet(true);
- AlphaAnimation alpha = new AlphaAnimation(0, 1);
- animSet.setDuration(3000);
- animSet.addAnimation(alpha);
- MainActivity.this.iv.startAnimation(animSet);
- }
- });
- }
- }