As of JDK 5.0, 1.5.0 java.util however, the collection classes are Generic Classes with type parameters.

From Core Java, Volume I Chapter 13:

Generics are desirable because they let you write code that is safer and easier than code that is littered with Object variables and casts. Generics are particularly useful for collection classes, such as the ubiquitous ArrayList.

Generics are -at least on the surface- similar to templates in C++.     ...

Generic programming means to write code that can be reused for objects of many different types. For example, you don't want to program separate classes to collect String and File objects. And you don't have to—the single class ArrayList collects objects of any class. This is one example of generic programming.

Before JDK 5.0, generic programming in Java was always achieved with inheritance. The ArrayList (and Vector class) simply maintained an array of Object references:

public class ArrayList // before JDK 5.0 { public Object get(int i) { ... } public void add(Object o) { ... } ... private Object[] elementData; }
This approach has two problems. A cast is necessary whenever you retrieve a value: ArrayList files = new ArrayList(); ... String filename = (String) files.get(0); Moreover, there is no error checking. You can add values of any class files.add(new File("...")); This call compiles and runs without error. Elsewhere, casting the result of get to a String will cause an error.

Discuss Collections Framework Overview: Interfaces ( Collection , Set, List, Queue, etc.), AbstractClasses ( AbstractCollection, AbstractList, AbstractSequentialList, AbstractSet, AbstractQueue, AbstractMap), Concrete Classes

Concrete Collections

Example of creation and use of ArrayList here

Again, as of JDK 5.0 however, the collection classes are Generic Classes with type parameters.
Example of using the parameter for a LinkedList of String

If you use older versions of of Java, you drop the type parameter and replace the generic types with the Object type.

For the purposes of this class (CSCI 511), in past implementations of Lab 1, students used the "legacy" class java.util.Vector . Actually, Vector and ArrayList are quite similar in some ways. Both grow dynamically and if the contents are not all of the same parameter type, both hold Objects. Since these Collections are potentially filled with Objects of different types, one may need to use the Java languageinstanceof operator or Classes method isInstance(Object obj) to determine what the class really was in order to cast it back to what it had originally been.

Note also that primitive data types would have to be in their Wrapper Classes so that they would be Objects in order to be inserted.

Collections Tutorial with

• Intro
• Interfaces (nice thinks like traversing collections)

• Guide

• generics online Java tutorial

• Sun tutorial pdf

See also Java in a Nutshells link on collections

From Generics Tutorial:

Type Parameter Naming Conventions