Strong integration Conversion

 Problems with strong conversion of Collections

See the followingCode:

Private list <jobinfo> mjoblist;

Mjoblist = collections. synchronizedlist (New arraylist <jobinfo> ());

This method is correct, because the returned value of the synchronizedlist method is the list set.

Private arraylist <jobinfo> mjoblist;

Mjoblist = (arraylist <jobinfo>) collections. synchronizedlist (New arraylist <jobinfo> ());

This statement looks correct and will actually report an exception"Caused by: Java. Lang. classcastexception: Java. util. Collections $ synchronizedrandomaccesslist"

 

Analysis:

1. What does the <t> List <t> of a function mean?

What I know is that a collection of the list type is returned, and the type of the object stored in it is t, but what does the previous <t> mean?

In this generic method, the type parameter is T, which is located after all the modifiers of the function and before the return value is placed in angle brackets. This T is the type parameter of the function, t in list <t> is the type parameter of this set.

List <t>: Return Value of parameterized type.

 

Next let's take a look at the source code of the synchronizedlist method:

Public static <t> List <t> synchronizedlist (list <t> List ){
If (list = NULL ){
Throw new nullpointerexception ();
}
If (list instanceof randomaccess ){
Return new synchronizedrandomaccesslist <t> (list );
}
Return new synchronizedlist <t> (list );
}

Mjoblist = (arraylist <jobinfo>) collections. synchronizedlist (New arraylist <jobinfo> (); in this line of code, a parameter of the arraylist type is passed to the synchronizedlist method. arraylist implements the randomaccess interface, so

Will Execute return New synchronizedrandomaccesslist <t> (list); this line of code, let's look at the UML class diagram, where synchronizedrandomaccesslist, synchronizedlist, synchronizedcollection is a static internal class of the collections tool class.

 

From this figure, we can see that synchronizedrandomaccesslist is a sub-class of list, which is not of the arraylist type. Therefore, the strong conversion of the above Code is incorrect. In addition, we can see that this is a packaging mode, write the package mode later.