Type checking for Java generic containers

A generic container is a type that is guaranteed by the compiler to ensure the correctness of the object type by specifying the container that contains the object, and the type error can be checked at compile-time. Assign the list<long> object longlist as follows to a list<generictest> Object gtlist , a compilation error is reported.

Public class generictest

{

&NBSP;&NBSP;&NBSP; &NBSP; public &NBSP; static &NBSP; LIST<LONG> &NBSP; longlist &NBSP; = Arrays. aslist   (1L, 2L);

public static void main (String args []){

// The following statement will be compiled with an error message

list<generictest> gtlist= longlist  ;         

}

}

If this is just a compile check, can we bypass the build check? Let's first assign the list<long> object longlist to a raw List variable Rawlist, and then converts the rawlist coercion type to the list<generictest> type. The result is compiled and the runtime does not have an error. We succeeded in tricking the compiler.

Public class generictest

{

public static list<long> longlist = Arrays. Aslist (1L, 2L);

public static void main (String args []){

List rawlist= longlist;

        // This can be compiled through, successfully fooled the compiler

List<generictest> gtlist= rawlist;

}

}

   then, if we uselist<generictest>Types of variablesGtlist,What's going to happen. NotegtlistThe container inside the reference is actuallyLongThe type object. What happens will it refer to a faulty memory? As a result, we foundString result = Gtlist.get (0). stringvalue;statement is correctly thrown at run timejava.lang.ClassCastExceptionexception. AnalysisMainThe byte code of the function, A type conversion check byte code was inserted before the statement, which caused the exception to be thrown. SeemsJavaThe designer has already taken this into account by adding a type check before the generic container object is used to prevent such a situation.

Public class generictest

{

public static list<long> Longlist = Arrays. Aslist (1L, 2L);

public Stringstringvalue ="ss";

public static void main (String args []){

List rawlist= longlist;

        // This can be compiled through, successfully fooled the compiler

List<generictest> gtlist= rawlist;

        // will be thrown at run time java.lang.ClassCastException

String result =gtlist.get (0). StringValue ;

}

}

Byte code analysis is as follows:

0 getstatic #28 <variable/GenericTest.longList>// Get class variables longlist, and put it on top of the stack

3 astore_1// put the top of the stack reference into the first local variable rawlist

4 aload_1// the first local variable rawlist , put on top of the stack

5 astore_2// Place the top of the stack reference into the second local variable Tglist, Here we can see the assignment of rawlist to tglist, without any type detection, so run through

6 Aload_2

7 Iconst_0

8 Invokeinterface #43 <java/util/List.get> count 2

checkcast #1 <variable/GenericTest>// Detecting Type Conversions , When we use objects in the container, there is type detection, which causes the java.lang.ClassCastException to be thrown

GetField #37 <variable/GenericTest.stringValue>// get instance properties and put them on top of the stack

astore_3// Place the top of the stack reference into the third local variable result

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Type checking for Java generic containers