Contents - Cultural View

Generics in Java 127
Pair grade440 = new Pair("mike", "A");
Pair marks440 = new Pair("mike", 100);
System.out.println("grade: " + grade440);
System.out.println("marks: " + marks440);
Generic method definitions
Here is an example of a generic method using the generic class above:
public Pair twice(T value)
{
}
return new Pair(value,value);
In many cases the user of the method need not indicate the type parameters, as they can be inferred:
Pair pair = twice("Hello");
The parameters can be explicitly added if needed:
Pair pair = this.twice("Hello");
Note that you cannot use native types, ex:
Pair pair; // this fails. You have to use Integer instead.
Generics in throws clause
Although exceptions themselves cannot be generic, generic parameters can appear in a throws clause:
public void throwMeConditional
{
}
(boolean conditional, T exception) throws T
if(conditional)
throw exception;
Type erasure
Generics are checked at compile-time for type-correctness. The generic type information is then removed in a
process called type erasure. For example, List will be converted to the non-generic type List, which can
contain arbitrary objects. The compile-time check guarantees that the resulting code is type-correct.
As a result of type erasure, type parameters cannot be determined at run-time. For example, when an ArrayList is
examined at runtime, there is no general way to determine whether, before type erasure, it was an
ArrayList or an ArrayList. Many people are dissatisfied with this restriction [5] . There are partial
approaches. For example, individual elements may be examined to determine the type they belong to; for example, if
an ArrayList contains an Integer, that ArrayList was presumably parameterized with Integer. Reflection can also
determine the type parameter. However, no approach works in all cases.
Demonstrating this point, the following code outputs "Equal":
ArrayList li = new ArrayList();
ArrayList lf = new ArrayList();
if (li.getClass() == lf.getClass()) // evaluates to true