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Object type and wrapper Object type and wrapper classesclasses

Object methodsObject methods Generic classesGeneric classes Interfaces and iteratorsInterfaces and iterators

Generic ProgrammingGeneric Programming

Data StructuresData Structuresand Other Objectsand Other ObjectsUsing JavaUsing Java

Java’s Object TypeJava’s Object Type

All types (except primitive types) are All types (except primitive types) are Object types.Object types.

Recall use of Object type (clone and equals Recall use of Object type (clone and equals methods)methods)

An Object variable is capable of holding a An Object variable is capable of holding a reference to any kind of object.reference to any kind of object.

Widening ConversionWidening Conversion

An assignment x = y is widening if the data An assignment x = y is widening if the data type of x is capable of referring to a wider type of x is capable of referring to a wider variety of things than the type of y.variety of things than the type of y.

int y = 42;int y = 42;double x;double x;

x = y;x = y;

Widening ConversionWidening Conversion

String s = new String(“Engage!”);String s = new String(“Engage!”);

Object obj;Object obj;

obj = s;obj = s;

““Engage!”Engage!”

String sString s Object objObject obj

Narrowing ConversionsNarrowing Conversions

Narrowing conversions generally require an Narrowing conversions generally require an explicit typecast.explicit typecast.

int x;int x;

double y = 3.14;double y = 3.14;

x = (int) y;x = (int) y;

Narrowing ConversionsNarrowing Conversions

String s = new String(“Engage!”);String s = new String(“Engage!”);

Object obj;Object obj;

obj = s;obj = s;

s = new String(“Make it so.”);s = new String(“Make it so.”);

““Engage!”Engage!”

String sString s Object objObject obj

““Make it so.”Make it so.”

……s = (String) obj;s = (String) obj;

Wrapper ClassesWrapper Classes

Problem: primitive types are not Object Problem: primitive types are not Object types:types: byte, short, int, long, float, double, char, byte, short, int, long, float, double, char,

booleanboolean


Solution: create special classes in which Solution: create special classes in which each object holds a primitive type:each object holds a primitive type:

bytebyte

shortshort

intint

longlong

floatfloat

doubledouble

charchar

booleanboolean

ByteByte

ShortShort

IntegerInteger

LongLong

FloatFloat

DoubleDouble

CharacterCharacter

BooleanBoolean


Each wrapper class has Each wrapper class has ConstructorConstructor intValue methodintValue method

int i = 42;int i = 42;int j;int j;Integer example;Integer example;example = new Integer(i);example = new Integer(i);j = example.intValue( );j = example.intValue( );

boxingboxing

unboxingunboxing

Autoboxing and Auto-unboxingAutoboxing and Auto-unboxing

int i = 42;int i = 42;

int j;int j;

Integer example;Integer example;

example = i;example = i;

j = example;j = example;

AutoboxingAutoboxing

Auto-unboxingAuto-unboxing

Advantage of WrappersAdvantage of Wrappers

The wrapper object is a full Java object, so The wrapper object is a full Java object, so we can use it in ways that we can’t use the we can use it in ways that we can’t use the primitive types (e.g. for generics, as we will primitive types (e.g. for generics, as we will see).see).

Disadvantage of WrappersDisadvantage of Wrappers

Even simple operations take longer because Even simple operations take longer because of boxing and unboxing:of boxing and unboxing:

Integer x = 5;Integer x = 5;

Integer y = 12;Integer y = 12;

Integer z = x + y;Integer z = x + y;

AutoboxAutobox Auto-unboxAuto-unbox Auto-unboxAuto-unbox

Multi-type operationsMulti-type operations

3 solutions3 solutions

1.1. Overloaded methodsOverloaded methods

2.2. Use Object typeUse Object type

3.3. Use generic typesUse generic types

1. Overloaded Methods1. Overloaded Methods

static Integer middle(Integer[] data) {static Integer middle(Integer[] data) {

if (data.length == 0){if (data.length == 0){

return null;return null;

} else {} else {

return data[data.length/2];return data[data.length/2];

}}

}}

1. Overloaded Methods1. Overloaded Methods

static Character middle(Character[] data) {static Character middle(Character[] data) {



} else {} else {


}}

}}

It does the job, but it’s a lot of work It does the job, but it’s a lot of work creating methods for every possible type:creating methods for every possible type:

i = middle(ia);i = middle(ia);

2. Object Methods2. Object Methods

static Object middle(Object[] data) {static Object middle(Object[] data) {



} else {} else {


}}

}}Also does the job, but the call is awkward, Also does the job, but the call is awkward, and certain type errors can’t be found by and certain type errors can’t be found by the compiler:the compiler:

i = (Integer) middle(ia);i = (Integer) middle(ia);

3. Generic Classes3. Generic Classes

Depends on an unspecified underlying data Depends on an unspecified underlying data type that is eventually identified when the type that is eventually identified when the class is used in an application program.class is used in an application program.

Enhances type checking capabilitiesEnhances type checking capabilities The type used to The type used to instantiateinstantiate the generic the generic

type must be a class (not a primitive type…type must be a class (not a primitive type…for primitives, use wrapper class).for primitives, use wrapper class).

Steps to Convert Collection Class to Generic ClassSteps to Convert Collection Class to Generic Class

1.1. Change the class nameChange the class name If you used type in the name, remove itIf you used type in the name, remove it

2.2. Add <E> to all class references (including Add <E> to all class references (including the class name) within the class itselfthe class name) within the class itself

3.3. Change type of the underlying elementChange type of the underlying element Change to E, which stands for “element”Change to E, which stands for “element”

4.4. Change static methods to generic staticChange static methods to generic static Add <E> after keyword “static”Add <E> after keyword “static”

Steps to Convert Collection Class to Generic ClassSteps to Convert Collection Class to Generic Class

5.5. Take care when creating any new E objectsTake care when creating any new E objects Might need to use Object constructorMight need to use Object constructor

6.6. Modify equality testsModify equality tests Most == or != must be changed to use Most == or != must be changed to use equalsequals

methodmethod

7.7. Deal with null references appropriatelyDeal with null references appropriately

8.8. Set unused reference variables to nullSet unused reference variables to null

9.9. Don’t forget to update documentation!Don’t forget to update documentation!

ExamplesExamples

ArrayBagArrayBag ArrayBag2ArrayBag2 NodeNode

Java InterfacesJava Interfaces

An interface is a formal outline of the available An interface is a formal outline of the available methods for some class. Interfaces form a methods for some class. Interfaces form a contract between the class and the outside contract between the class and the outside world, and this contract is enforced at build world, and this contract is enforced at build time by the compiler. If your class claims to time by the compiler. If your class claims to implement an interface, all methods defined by implement an interface, all methods defined by that interface must appear in its source code that interface must appear in its source code before the class will successfully compile.before the class will successfully compile.

ExampleExample

AudioClip

Writing a Class to Implement an InterfaceWriting a Class to Implement an Interface

1.1. Read interface documentationRead interface documentation

2.2. Tell the compiler you are implementing an Tell the compiler you are implementing an interfaceinterface

public class MP3Player implements AudioClippublic class MP3Player implements AudioClip

3.3. Implement all methods defined in the interfaceImplement all methods defined in the interface

Generic InterfacesGeneric Interfaces

Like any generic class, a generic interface Like any generic class, a generic interface depends on one or more unspecified classes depends on one or more unspecified classes (<E>).(<E>).

E.g. E.g. Iterator<E>

Warning!Warning!

Don’t change a list while an iterator is being Don’t change a list while an iterator is being usedused

ExamplesExamples

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