Introduction to Java Generics

ParametricPolymorphism.java

import java.util.ArrayList;

public class ParametricPolymorphism {

    public static void main(String[] args) {

        // Java Generics are an implementation of parametric polymorphism. They can be complicated subject.
        // We will only focus on their simple use case with collections (lists, sets, stacks, etc)

        // Old Java (1.4 and older):
        ArrayList l1 = new ArrayList();
        l1.add("hello");
        String s1 = (String) l1.get(0);  // Note the cast

        // New-ish (since 1.5):
        ArrayList<String> l2 = new ArrayList <String>();
        l2.add("hello");
        String s2 = l2.get(0);

        // Why?
        // Type-safety! The compiler can catch problems

        // This blows up at runtime
        NonGenericStack ngStack = new NonGenericStack();
        ngStack.push("String");
        ngStack.push(123);
        String str1 = (String) ngStack.pop();

        // This is a compile error
        GenericStack<String> gStack = new GenericStack<String>();
        gStack.push("String");
        gStack.push(123);                     // compiler will complain!!
        String str2 = (String) ngStack.pop();

        // We love the compiler. Its purpose in life is to make our lives easier.
    }

    static class NonGenericStack {
        private ArrayList contents = new ArrayList();

        public void push(Object element) {
            contents.add(element);
        }

        public Object pop() {
            int top = contents.size()-1;
            Object result = contents.get(top);
            contents.remove(top);
            return result;
        }
    }

    static class GenericStack<E> {
        private ArrayList<E> contents = new ArrayList<E>();

        public void push(E element) {
            contents.add(element);
        }

        public E pop() {
            int top = contents.size()-1;
            E result = contents.get(top);
            contents.remove(top);
            return result;
        }
    }

}