\nWhen a generic type is instantiated, the compiler translates those types by a technique called type erasure<\/I> \u2014 a process where the compiler removes all information related to type parameters and type arguments within a class or method. Type erasure enables Java applications that use generics to maintain binary compatibility with Java libraries and applications that were created before generics. <\/P>
\nFor instance,
Box<String><\/CODE> is translated to typeBox<\/CODE>, which is called the raw type<\/I> \u2014 a raw type is a generic class or interface name without any type arguments. This means that you can’t find out what type ofObject<\/CODE> a generic class is using at runtime. The following operations are not possible: <\/P>
\npublic class MyClass<E> {
\n public static void myMethod(Object item) {
\n if (item instanceof E<\/B>) { \/\/Compiler error
\n …
\n }
\n E item2 = new E();<\/B> \/\/Compiler error
\n E[] iArray = new E[10];<\/B> \/\/Compiler error
\n E obj = (E)new Object();<\/B> \/\/Unchecked cast warning
\n }
\n}
\n<\/PRE><\/BLOCKQUOTE>The operations shown in bold are meaningless at runtime because the compiler removes all information about the actual type argument (represented by the type parameterE<\/CODE>) at compile time.
\nType erasure exists so that new code may continue to interface with legacy code. Using a raw type for any other reason is considered bad programming practice and should be avoided whenever possible. <\/P>
\nWhen mixing legacy code with generic code, you may encounter warning messages similar to the following: <\/P>
\nNote: WarningDemo.java uses unchecked or unsafe operations.
\nNote: Recompile with -Xlint:unchecked for details.
\n<\/PRE><\/BLOCKQUOTE>This can happen when using an older API that operates on raw types, as shown in the followingWarningDemo<\/CODE><\/A> program:
\npublic class WarningDemo {
\n public static void main(String[] args){
\n Box<Integer> bi;
\n bi = createBox();
\n }<\/p>\n\/**
\n * Pretend that this method is part of an old library,
\n * written before generics. It returns
\n * Box instead of Box<T>.
\n *\/
\n static Box createBox(){
\n return new Box();
\n }
\n}
\n<\/PRE><\/BLOCKQUOTE>Recompiling with-Xlint:unchecked<\/CODE> reveals the following additional information:
\nWarningDemo.java:4: warning: [unchecked] unchecked conversion
\nfound : Box
\nrequired: Box<java.lang.Integer>
\n bi = createBox();
\n ^
\n1 warning
\n<\/PRE><\/BLOCKQUOTE><\/BLOCKQUOTE>
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