假设
A延伸X您需要对对象执行一些操作。执行操作的方法可能是： pre $ public void doMagic（List< ;? extends X> e）
或者，您可以选择致电它
public void doMagic2（List< X> e）您要求魔术完成：
列表< X> ; listOfA = new ArrayList< X>（）; listOfA.add（new A（））; C c = new C（）; c.doMagic（listOfA）; //按预期工作 c.doMagic2（listOfA）; //按预期工作你能描述为什么一种方法比另一种方法更受欢迎，何时应该使用
如果是 List< lt; code> ;？它会接受 X 列表和 X 。一个优于另一个，？扩展X 将允许您在处理各种不同的子类时处理情况。
一个例子是一个函数例如计算文件的大小（以字节为单位）：
public int getTotalSizeInBytes（List< ;? extends File> files）{ $ b现在这个函数可以计算大小而不管文件类型可以是 plainTextFile ， excelFile ， wordFile ， databaseFile 等。Suppose
A extends XYou need to perform some operation on object either A. Method performing the operation may be:
public void doMagic(List<? extends X> e)Alternatively, you may chose to call it
public void doMagic2(List<X> e)You ask for magic to be done by:
List<X> listOfA = new ArrayList<X>(); listOfA.add(new A()); C c = new C(); c.doMagic(listOfA); // works as expected c.doMagic2(listOfA); // works as expectedCan you describe why one approach is preferred over another and when it should be used please?
解决方案In case of List<X> as you can guess, it would work with objects of type X only.
In case of List<? extends X>, it would accept a list of X and all the sub-classes of X or all the implementations of X if X was an interface.
The benefits of one over the other, well, ? extends X would allow you to take care of situation when you want to handle all sorts of different sub classes.
An example would be a function that calculates, say, size in bytes of files:
public int getTotalSizeInBytes(List<? extends File> files) { ... }Now this function can go about calculating the size regardless of the file type which can be plainTextFile, excelFile, wordFile, databaseFile etc.
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