安卓中的 ImageView

最新推荐文章于 2024-07-14 14:10:13 发布

原创最新推荐文章于 2024-07-14 14:10:13 发布 · 1.3k 阅读

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#image #dialog #access #object

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本文分享了一位初学者阅读《Android.Apps.for.Absolute.Beginners》的心得体会，对比中文教材，该书不仅详细介绍了每一步操作流程，还深入浅出地讲解了相关概念。文章特别提到了书中关于final变量用法的例子，帮助读者更好地理解其作用。

在读《Android.Apps.for.Absolute.Beginners》，非常适合初学者。虽然是英文的，可是看了快一本了感觉所需的单词量还不到四级的要求。相比看中文的安卓开发初级，似乎作者都是急着挣稿费的，很多地方只会教你怎么做而讲不出原理。可英文书不但详尽的的把每一个步骤都列出来或直接上截图，并且还经常点拨一下难懂的概念。就像下面所列举的一样(p142)：

@Override
public boolean onOptionsItemSelected(MenuItem item) {
LinearLayout bkgr= (LinearLayout)findViewById(R.id.uilayout);
final ImageView image=(ImageView)findViewById(R.id.imageView1);
AlertDialog.Builder builder=new AlertDialog.Builder(this);

Here is the code to customize our dialog:
builder.setTitle("Pick an Image!")
.setMessage("Please Select Image One or Image Two:")
.setCancelable(false)
.setPositiveButton("IMAGE 1", new DialogInterface.OnClickListener()
{
public void onClick(DialogInterface dialog, int id) {
image.setImageResource(R.drawable.image1);
}
})
.setNegativeButton("IMAGE 2", new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int id) {
image.setImageResource(R.drawable.image2);
}
});

We work with the image object, which we know can’t be reassigned a value because it is final. This is to deal with situations where the event listener is used after the onOptionsItemSelected() method has terminated. In this case, a non-final image variable would not be around to take a new assignment, whereas a final variable is frozen in memory for access at all times (of course, this may never happen, but Java was built this way just to be sure).

“a non-final image variable would not be around to take a new assignment”这句话是否可理解为：一个非final类型的image变量将不能被分配新值？可是这不正是final的特性么？该怎么理解？