Generics d h A Li and the ArrayList

(1)

Chapter 14

Generics d h A Li and the ArrayList

Class

Introduction to Generics Introduction to Generics

• Beginning with version 5.0, Java allows class and method definitions that include

parameters for types

• Such definitions are called generics

– Generic programming with a type parameter enables code to be written that applies to any class

The ArrayList Class The ArrayList Class

i i l i h d d J lib i

• ArrayList is a class in the standard Java libraries

– Unlike arrays, which have a fixed length once they have been created an ArrayList is an object that can grow been created, an ArrayList is an object that can grow and shrink while your program is running

• In general, an In general, an ArrayList ArrayList serves the same purpose serves the same purpose as an array, except that an ArrayList can change length while the program is running g p g g

The ArrayList Class The ArrayList Class

• The class ArrayList is implemented using an array as a private instance variable y p

– When this hidden array is full, a new larger hidden

array is created and the data is transferred to this

new array

(2)

The ArrayList Class The ArrayList Class

Wh l i i d f

• Why not always use an ArrayList instead of an array?

1 A A Li t i l ffi i t th

1. An ArrayList is less efficient than an array

2. It does not have the convenient square bracket notation 3 The base type of an ArrayList must be a class type 3. The base type of an ArrayList must be a class type

(or other reference type): it cannot be a primitive type – This last point is less of a problem now that Java provides p p p

automatic boxing and unboxing of primitives

Using the ArrayList Class Using the ArrayList Class

• In order to make use of the ArrayList class, it must first be imported from the package

java.util

• An ArrayList is created and named in the same way as object of any class, except that you specify the base type as follows:

ArrayList<BaseType> aList = new ArrayList<BaseType>();

Using the ArrayList Class Using the ArrayList Class

• An initial capacity can be specified when creating an

ArrayList as well

A Li t

– The following code creates an ArrayList that stores objects of the base type String with an initial capacity of 20 items

ArrayList<String> list = new ArrayList<String>(20);

Specifying an initial capacity does not limit the size to – Specifying an initial capacity does not limit the size to

which an ArrayList can eventually grow

• Note that the base type of an ArrayList is Note that the base type of an ArrayList is specified as a type parameter

Using the ArrayList Class Using the ArrayList Class

• The add method is used to set an element for the first time in an ArrayList y

list.add("something");

– The method name The method name add add is overloaded is overloaded – There is also a two argument version that

allows an item to be added at any currently

used index position or at the first unused

position

(3)

Using the ArrayList Class Using the ArrayList Class

• The size method is used to find out how many indices already have elements in the

A Li t ArrayList

int howMany = list.size();

Th h d i d l i i

• The set method is used to replace any existing element, and the get method is used to access the value of any existing element

the value of any existing element

list.set(index, "something else");

String thing = list get(index);

String thing = list.get(index);

Tip: Summary of Adding to an ArrayList

h h d i ll d l

• The add method is usually used to place an element in an ArrayList position for the

( )

first time (at an ArrayList index)

• The simplest p add method has a single g

parameter for the element to be added, and adds an element at the next unused index, in , order

Tip: Summary of Adding to an ArrayList

l b dd d l d

• An element can be added at an already occupied list position by using the two‐

parameter version of add

• This causes the new element to be placed at p the index specified, and every other member of the ArrayList y to be moved up by one p y position

Tip: Summary of Adding to an ArrayList

Th i f dd l b d

• The two‐argument version of add can also be used to add an element at the first unused position (if that position is known)

position is known)

• Any individual element can be changed using the set method

set method

– However, set can only reset an element at an index that already contains an element

already contains an element

• In addition, the method size can be used to

(4)

Methods in the Class ArrayList Methods in the Class ArrayList

h l f i l i i l

• The tools for manipulating arrays consist only of the square brackets and the instance

variable length

• ArrayLists, however, come with a y , , selection of powerful methods that can do many of the things for which code would have y g to be written in order to do them using arrays

Some Methods in the Class ArrayList (Part 1 of 11)

Some Methods in the Class ArrayList (Part 2 of 11)

Some Methods in the Class

ArrayList (Part 3 of 11)

(5)

Some Methods in the Class ArrayList (Part 4 of 11)

Some Methods in the Class ArrayList (Part 5 of 11)

Some Methods in the Class ArrayList (Part 6 of 11)

Some Methods in the Class

ArrayList (Part 7 of 11)

(6)

Some Methods in the Class ArrayList (Part 8 of 11)

Some Methods in the Class ArrayList (Part 9 of 11)

Some Methods in the Class ArrayList (Part 10 of 11)

Some Methods in the Class

ArrayList (Part 11 of 11)

(7)

Why are Some Parameters of Type Base_Type and Others of type Object

Wh l ki t th th d il bl i th A Li t

• When looking at the methods available in the ArrayList class, there appears to be some inconsistency

– In some cases, when a parameter is naturally an object of the base In some cases, when a parameter is naturally an object of the base type, the parameter type is the base type

– However, in other cases, it is the type Object

• This is because the Arra List class implements a number

• This is because the ArrayList class implements a number of interfaces, and inherits methods from various ancestor classes

– These interfaces and ancestor classes specify that certain parameters have type Object

The "For Each" Loop The For Each Loop

• The ArrayList class is an example of a collection class

• Starting with version 5.0, Java has added a new kind of for loop called a for each or new kind of for loop called a for‐each or enhanced for loop

– This kind of loop has been designed to cycle through all the elements in a collection (like an ArrayList )

A for‐each Loop Used with an ArrayList (Part 1 of 3)

A for‐each Loop Used with an

ArrayList (Part 2 of 3)

(8)

A for‐each Loop Used with an ArrayList (Part 3 of 3)

Golf Score Program (Part 1 of 6) Golf Score Program (Part 1 of 6)

Golf Score Program (Part 2 of 6)

Golf Score Program (Part 2 of 6) Golf Score Program (Part 3 of 6) Golf Score Program (Part 3 of 6)

(9)

Golf Score Program (Part 4 of 6) Golf Score Program (Part 4 of 6)

Golf Score Program (Part 5 of 6) Golf Score Program (Part 5 of 6)

Golf Score Program (Part 6 of 6)

Golf Score Program (Part 6 of 6) Tip: Use trimToSize to Save Memory Tip: Use trimToSize to Save Memory

A A Li t t ti ll i it it h

• An ArrayList automatically increases its capacity when needed

– However, the capacity may increase beyond what a program requires However, the capacity may increase beyond what a program requires – In addition, although an ArrayList grows automatically when

needed, it does not shrink automatically

• If an Arra List has a large amount of excess capacity an

• If an ArrayList has a large amount of excess capacity, an

invocation of the method trimToSize will shrink the

capacity of the ArrayList y down to the size needed

(10)

Pitfall: The clone method Makes a Shallow Copy

Wh d f A Li t i d d

• When a deep copy of an ArrayList is needed, using the clone method is not sufficient

– Invoking Invoking clone clone on an on an ArrayList ArrayList object produces a object produces a shallow copy, not a deep copy

• In order to make a deep copy, it must be possible to

k d f b f h b

make a deep copy of objects of the base type

– Then a deep copy of each element in the ArrayList can be created and placed into a new ArrayList object be created and placed into a new ArrayList object

The Vector Class The Vector Class

h d d lib i h l d

• The Java standard libraries have a class named Vector that behaves almost exactly the same as the class ArrayList

• In most situations, either class could be used ,

– However the ArrayList class is newer, and is becoming the preferred class g p

Parameterized Classes and Generics Parameterized Classes and Generics

Th l A Li t i t i d l

• The class ArrayList is a parameterized class

• It has a parameter, denoted by Base_Type, that can be replaced by any reference type to obtain a can be replaced by any reference type to obtain a class for ArrayLists with the specified base type

• Starting with version 5.0, Java allows class definitions Starting with version 5.0, Java allows class definitions with parameters for types

– These classes that have type parameters are called

t i d l i d fi iti i l

parameterized class or generic definitions, or, simply, generics

Nonparameterized ArrayList and Vector Classes

h d l

• The ArrayList and Vector classes

discussed here have a type parameter for the base type

• There are also ArrayList y and Vector classes with no parameter whose base type is Object j

– These classes are left over from earlier versions of

Java

(11)

Generics Generics

Cl d h d h

• Classes and methods can have a type parameter

– A type parameter can have any reference type (i.e., any class type) plugged in for the type parameter

class type) plugged in for the type parameter

– When a specific type is plugged in, this produces a specific class type or method yp

– Traditionally, a single uppercase letter is used for a type parameter, but any non‐keyword identifier may be used

Generics Generics

A l d fi iti ith t t i t d i

• A class definition with a type parameter is stored in a file and compiled just like any other class

• Once a parameterized class is compiled it can be

• Once a parameterized class is compiled, it can be used like any other class

– However, the class type plugged in for the type parameter , yp p gg yp p must be specified before it can be used in a program – Doing this is said to instantiate the generic class

Sample<String> object = Sample<String> object =

new Sample<String>();

A Class Definition with a Type

Parameter Class Definition with a Type Parameter Class Definition with a Type Parameter

A l th t i d fi d ith t f t i

• A class that is defined with a parameter for a type is called a generic class or a parameterized class

– The type parameter is included in angular brackets after The type parameter is included in angular brackets after the class name in the class definition heading

– Any non‐keyword identifier can be used for the type parameter b t b con ention the parameter starts ith parameter, but by convention, the parameter starts with an uppercase letter

– The type parameter can be used like other types used in

the definition of a class

(12)

Tip: Compile with the -Xlint Option Tip: Compile with the -Xlint Option

h i f ll h b

• There are many pitfalls that can be

encountered when using type parameters

• Compiling with the -Xlint option will provide more informative diagnostics of any

p g y

problems or potential problems in the code

javac –Xlint Sample.java javac Xlint Sample.java

A Generic Ordered Pair Class (Part 1 of 4)

A Generic Ordered Pair Class (Part 2 of 4)

A Generic Ordered Pair Class

(Part 3 of 4)

(13)

A Generic Ordered Pair Class (Part 4 of 4)

Using Our Ordered Pair Class (Part 1 of 3)

Using Our Ordered Pair Class (Part 2 of 3)

Using Our Ordered Pair Class

(Part 3 of 3)

(14)

Pitfall: A Generic Constructor Name Has No Type Parameter

• Although the class name in a parameterized class definition has a type

• Although the class name in a parameterized class definition has a type parameter attached, the type parameter is not used in the heading of the constructor definition

public Pair<T>() public Pair<T>()

• A constructor can use the type parameter as the type for a parameter of the constructor, but in this case, the angular brackets are not used

public Pair(T first T second) public Pair(T first, T second)

• However, when a generic class is instantiated, the angular brackets are used

P i St i i

Pair<String> pair =

new Pair<STring>("Happy", "Day");

Pitfall: A Primitive Type Cannot be Plugged in for a Type Parameter

• The type plugged in for a type parameter must always be a reference type y yp

– It cannot be a primitive type such as int, double , or char

double , or char

– However, now that Java has automatic boxing, this is not a big restriction

is not a big restriction

– Note: reference types can include arrays

Pitfall: A Type Parameter Cannot Be Used

E h T N C B U d

Everywhere a Type Name Can Be Used

Withi th d fi iti f t i d l

• Within the definition of a parameterized class definition, there are places where an ordinary class name would be allowed, but a type parameter is not name would be allowed, but a type parameter is not allowed

• In particular, the type parameter cannot be used in simple expressions using new to create a new object

– For instance, the type parameter cannot be used as a constructor name or like a constructor:

constructor name or like a constructor:

T object = new T();

T[] a = new T[10];

Pitfall: An Instantiation of a Generic Class Cannot be an Array Base Type

• Arrays such as the following are illegal:

Pair<String>[] a = g []

new Pair<String>[10];

Although this is a reasonable thing to want to do – Although this is a reasonable thing to want to do,

it is not allowed given the way that Java implements generic classes

implements generic classes

(15)

Using Our Ordered Pair Class and Automatic Boxing (Part 1 of 3)

Using Our Ordered Pair Class and Automatic Boxing (Part 2 of 3)

Using Our Ordered Pair Class and Automatic Boxing (Part 3 of 3)

Pitfall: A Class Definition Can Have More Than One Type Parameter

• A generic class definition can have any number of type parameters yp p

– Multiple type parameters are listed in angular

brackets just as in the single type parameter case,

but are separated by commas

(16)

Multiple Type Parameters (Part 1 of 4)

Multiple Type Parameters (Part 2 of 4)

Multiple Type Parameters (Part 3 of 4)

Multiple Type Parameters (Part 4

of 4)

(17)

Pitfall: A Generic Class Cannot Be an Exception Class

i i d i l

• It is not permitted to create a generic class with Exception , Error , Throwable , or any descendent class of Throwable

– A generic class cannot be created whose objects are throwable

public class Gex<T> extends Exception

h b l ll l

– The above example will generate a compiler error message

Using a Generic Class with Two Type Parameters (Part 1 of 2)

Using a Generic Class with Two

Type Parameters (Part 2 of 2) Bounds for Type Parameters Bounds for Type Parameters

S ti it k t t i t th ibl

• Sometimes it makes sense to restrict the possible types that can be plugged in for a type parameter T

– For instance to ensure that only classes that implement For instance, to ensure that only classes that implement the Comparable interface are plugged in for T, define a class as follows:

public class RClass<T extends Comparable>

– "extends Comparable" serves as a bound on the type parameter T

– Any attempt to plug in a type for T which does not implement the – Any attempt to plug in a type for T which does not implement the

Comparable interface will result in a compiler error message

(18)

Bounds for Type Parameters Bounds for Type Parameters

A b d b l ( h h

• A bound on a type may be a class name (rather than an interface name)

– Then only descendent classes of the bounding class may be plugged in Then only descendent classes of the bounding class may be plugged in for the type parameters

public class ExClass<T extends Class1>

• A bounds expression may contain multiple interfaces and up

• A bounds expression may contain multiple interfaces and up to one class

• If there is more than one type parameter, the syntax is as If there is more than one type parameter, the syntax is as follows:

public class Two<T1 extends Class1, T2 extends Class2 & Comparable>

Class2 & Comparable>

A Bounded Type Parameter A Bounded Type Parameter

Tip: Generic Interfaces Tip: Generic Interfaces

• An interface can have one or more type parameters

p

• The details and notation are the same as they are for classes with type parameters

are for classes with type parameters

Generic Methods Generic Methods

Wh i l i d fi d th t t

• When a generic class is defined, the type parameter can be used in the definitions of the methods for that generic class

that generic class

• In addition, a generic method can be defined that has its own type parameter that is not the type parameter of any class

– A generic method can be a member of an ordinary class or a member of a generic class that has some other type a member of a generic class that has some other type parameter

– The type parameter of a generic method is local to that th d t t th l

method, not to the class

(19)

Generic Methods Generic Methods

Th b l d (i l

• The type parameter must be placed (in angular brackets) after all the modifiers, and before the returned type

returned type

public static <T> T genMethod(T[] a)

• When one of these generic methods is invoked the

• When one of these generic methods is invoked, the method name is prefaced with the type to be

plugged in enclosed in angular brackets plugged in, enclosed in angular brackets

String s = NonG.<String>genMethod(c);

Inheritance with Generic Classes Inheritance with Generic Classes

A i l b d fi d d i d l f

• A generic class can be defined as a derived class of an ordinary class or of another generic class

– As in ordinary classes an object of the subclass type would As in ordinary classes, an object of the subclass type would also be of the superclass type

• Given two classes: A and B , and given G: a generic

l h l h b d

class, there is no relationship between G<A> and G<B>

This is true regardless of the relationship between class A – This is true regardless of the relationship between class A