Java泛型-此语法的用途是什么？

Question

以下这部分代码<String, Void, Bitmap>是什么意思？我什至不知道这个语法叫什么。

private class DownloadImageTask extends AsyncTask<String, Void, Bitmap> {

}

这是原始代码（可从此处找到：http : //developer.android.com/guide/components/processes-and-
threads.html
）：

public void onClick(View v) {
    new DownloadImageTask().execute("http://example.com/image.png");
}

private class DownloadImageTask extends AsyncTask<String, Void, Bitmap> {
    /** The system calls this to perform work in a worker thread and
      * delivers it the parameters given to AsyncTask.execute() */
    protected Bitmap doInBackground(String... urls) {
        return loadImageFromNetwork(urls[0]);
    }

    /** The system calls this to perform work in the UI thread and delivers
      * the result from doInBackground() */
    protected void onPostExecute(Bitmap result) {
        mImageView.setImageBitmap(result);
    }
}

Answer 1

AsyncTask<String, Void, Bitmap>

告诉AsyncTask使用3种不同的类型进行描述，当您使用AsyncTask时，将String作为第一个参数，将Void作为第二个参数，将Bitmap作为第三个参数。

从Java5开始，这在Java中称为泛型。请阅读本教程以了解有关泛型的更多信息。这是有关android
AsyncTasktask如何使用泛型的javadoc。

更新： 从AsyncTask javadoc

1) Params, the type of the parameters sent to the task upon execution.
2) Progress, the type of the progress units published during the background computation.
3) Result, the type of the result of the background computation.