COMP 121

Week 8: Generic Collections

Objectives To understand type variables and how they are

used in generic programming To be able to implement and use generic classes

and methods To introduce generic collections and the Java

Collection hierarchy

Type Variables Generic programming: creation of programming

constructs that can be used with many different types In Java, achieved with inheritance or with type variables

For example: Type variables: Java's ArrayList (e.g. ArrayList<String>) Inheritance: Using Object to enable a data structure to store

any type of object A Generic class is declared with a type variable

In an ArrayList, the type variable denotes the element type

Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

public class ArrayList<E> { public ArrayList() { . . . } public void add(E element) { . . . } . . . }

Type Variables (cont’d) Must be instantiated

The actual type must be supplied when you use the generic class

Can be instantiated with class or interface types ArrayList<BankAccount>ArrayList<Measurable>

Cannot use a primitive type as a type variable ArrayList<double> // Wrong!

Use corresponding wrapper class instead ArrayList<Double>

Supplied type replaces type variable in class interface Type variables make generic code safer and easier to

read

Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

Instantiating a Generic Class

GenericClassName<Type1, Type2, . . .>

Example:

ArrayList<BankAccount>HashMap<String, Integer>

Purpose:To supply specific types for the type variables of a generic class.

Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

Generic Methods

Generic method: a method with a type variable Can be defined inside ordinary and generic

classes The type variables of a generic method

are specified between the modifiers and the method return type

No need to instantiate the type variables

Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

Generic Methods (cont’d) Non-generic method to print all the String

elements in an arraypublic static void print(String[] a) { for (String e : a) System.out.print(e + " "); System.out.println(); }

Generic method to print all the elements in an array

public static <E> void print(E[] a) { for (E e : a) System.out.print(e + " "); System.out.println(); }

Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

Conventions for Type Variable Names

Type Variable Name Meaning

E Element type in a collection

K Key type in a map

V Value type in a map

T General type

S, U Additional general types

Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

Generic Collections Generics allow you to define a collection that

contains references to objects of a specific type ArrayList<String> myList = new ArrayList<String>();

specifies that myList is a List of String where String is a type parameter

Only references to objects of type String can be stored in myList, and all items retrieved would be of type String

A type parameter is analogous to a method parameter

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Creating a Generic Collection

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

The Collection Hierarchy

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Common Features of Collections Collection interface specifies a set of common

methods Fundamental features include:

Collections grow as needed Collections hold references to objects that can be

accessed using an Iterator object Collections have at least two constructors (one to

create an empty collection and one to make a copy of an existing collection)

Collections override equals, hashCode, and toString (inherited from Object) in a reasonable way that includes the elements they contain

Collections are considered “unordered,” meaning that there is no guarantee about where in a collection an added element will be placed

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Some Collection Methods

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Summary In Java, generic programming can be

achieved with inheritance or with type variables

A generic class has one or more type variables

Type variables can be instantiated with class or interface types

Type variables make generic code safer and easier to read

Type variables of a generic class follow the class name and are enclosed in angle brackets

Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

Summary (continued) Type variables can be used for fields, method

parameters, and return values Type variables of a generic method are

specified between the modifiers and the return type

Type variables do not need to be instantiated when calling a generic method

Generics are used in the Java Collection framework to define a collection that contains references to objects of a specific type

Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.

Any Questions?