Generic Java

21/2-2003

What is generics?

• To be able to assign type variables to a class• These variables are not bound to any

specific type until the declaration• This makes it possible to write generic

functions that will work with many different types

• Analogous to parametric types

Java: The current situation

• Utility classes with simulated generics

• Ex. a container such as a Collection where all elements are treated as instances of type Object, regardless of their original types

• While retrieving the elements it is necessary to cast them to their original types

• This is inconvenient and error-prone

Motivation for GJ

• avoid having to write casts

• make it possible to catch cast errors at compile-time

• should be compatible with the unextended java language

Assigning type parameters in GJclass Pair <A,B> {

private A element1;private B element2;

public Pair (A element1, B element2) {this.element1 = element1;this.element2 = element2;

}

public A getElement1() {return element1;

}

public B getElement2() {return element2;

}}

class Test {public static void main (String[] args) {

Pair <String, Integer> shoe;shoe = new Pair <String, Integer

> (”Addidas”, new Integer (”38”));

String name = shoe.getElement1();

Integer size = shoe.getElement2();}

}

Assigning

• it is not possible to assign an object to another object of the same type, but with a different type parameter

• it will, for example, cause a compile error if you try to assign Pair <String, Integer> to Pair <String, Character>

Invariant subtyping

• Not allowed:

• Collection<object> c = new Collection<String>();

• Allowed:

• Collection<String> = new LinkedList<String>();

Translating GJ

• the translation erases the type parameters, maps type variables to their bounds, inserts the appropriate casts, and bridge methods

• the GJ-compiler translates the parameterized source code to get the generic effect with features already present in Java (but in a type-safe manner)

• the translation is homogeneous• the type parameters are not available run-time

Consistency with legacy code

• translates into normal-looking Java code

• raw types

• retrofitting

• unchecked warnings