• A static method cannot refer to an instance variable of the class, and it cannot invoke a nonstatic method of the class

(1)

Chapter 5

Defining Classes II

Static Methods Static Methods

A i h d i h b d i h lli

• A static method is one that can be used without a calling object

• A static method still belongs to a class and its definition isA static method still belongs to a class, and its definition is given inside the class definition

• When a static method is defined, the keyword staticis placed in the method header

public static returnedType myMethod(parameters) { . . . }

{ . . . }

• Static methods are invoked using the class name in place of a calling object

returnedValue = MyClass.myMethod(arguments);

Pitfall: Invoking a Nonstatic Method Within a Static Method

• A static method cannot refer to an instance variable of the class, and it cannot invoke a nonstatic method of the class

– A static method has no this, so it cannot use an instance variable or method that has an implicit or explicit thisfor a calling object

A t ti th d i k th t ti th d

– A static method can invoke another static method, however

Tip: You Can Put a main in any Class Tip: You Can Put a main in any Class

Al h h h i h d i f b i lf i l

• Although the main method is often by itself in a class separate from the other classes of a program, it can also be contained within a regular class definition also be contained within a regular class definition

– In this way the class in which it is contained can be used to create objects in other classes, or it can be run as a

create objects in other classes, or it can be run as a program

– A main method so included in a regular class definition is especially useful when it contains diagnostic code for the class

(2)

Another Class with a main Added (Part 1 of 4)

Another Class with a main Added (Part 2 of 4)

Another Class with a main Added (Part 3 of 4)

Another Class with a main Added

(Part 4 of 4)

(3)

Static Variables Static Variables

A t ti i bl i i bl th t b l t th l

• A static variable is a variable that belongs to the class as a whole, and not just to one object

– There is only one copy of a static variable per class, unlike instance

i bl h h bj t h it

variables where each object has its own copy

• All objects of the class can read and change a static variable

• Although a static method cannot access an instance variable,Although a static method cannot access an instance variable, a static method can access a static variable

• A static variable is declared like an instance variable, with the addition of the modifierstatic

addition of the modifier static

private static int myStaticVariable;

Static Variables Static Variables

S i i bl b d l d d i i i li d h

• Static variables can be declared and initialized at the same time

private static int myStaticVariable = 0;

p y ;

• If not explicitly initialized, a static variable will be automatically initialized to a default value

– booleanstatic variables are initialized to false

– Other primitive types static variables are initialized to the zero of their typeyp

– Class type static variables are initialized to null

• It is always preferable to explicitly initialize static variables th th l th d f lt i iti li ti

rather than rely on the default initialization

Static Variables Static Variables

• A static variable should always be defined private, unless it is also a defined constant

The value of a static defined constant cannot be altered – The value of a static defined constant cannot be altered,

therefore it is safe to make it public

– In addition to static, the declaration for a static defined constant must include the modifierfinal which indicates constant must include the modifier final, which indicates that its value cannot be changed

public static final int BIRTH_YEAR = 1954;

• When referring to such a defined constant outside its class, use the name of its class in place of a calling object

int year = MyClass BIRTH YEAR;

int year = MyClass.BIRTH_YEAR;

The Math Class The Math Class

Th M th l id b f t d d

• The Math class provides a number of standard mathematical methods

– It is found in theIt is found in the java.langjava langpackage so it does notpackage, so it does not require an importstatement

– All of its methods and data are static, therefore they are invoked with the class nameM thinstead of a calling invoked with the class name Mathinstead of a calling object

– The Mathclass has two predefined constants, E(e, the base of the natural logarithm system) and PI(, 3.1415 . . .)

area = Math.PI * radius * radius;;

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Some Methods in the Class Math (Part 1 of 5)

Some Methods in the Class Math (Part 2 of 5)

Some Methods in the Class Math (Part 3 of 5)

Some Methods in the Class Math

(Part 4 of 5)

(5)

Some Methods in the Class Math (Part 5 of 5)

Random Numbers Random Numbers

• The Math class also provides a facility to p y generate pseudo‐random numbers

public static double random()

– A pseudo‐random number appears random but is reall enerated b a deterministi f n tion

public static double random()

really generated by a deterministic function

• There is also a more flexible class named Random

• Sample use:

• Returns a pseudo‐random number greater

double num = Math.random();

Returns a pseudo random number greater than or equal to 0.0 and less than 1.0

Wrapper Classes Wrapper Classes

W l id l t di

• Wrapper classes provide a class type corresponding to each of the primitive types

– This makes it possible to have class types that behaveThis makes it possible to have class types that behave somewhat like primitive types

– The wrapper classes for the primitive types byte, short, l fl t d bl and h are (in order)B t long,float,double, and charare (in order) Byte, Short,Long,Float,Double, and Character

• Wrapper classes also contain a number of useful pp predefined constants and static methods

Wrapper Classes Wrapper Classes

B i th f i f l f

• Boxing: the process of going from a value of a primitive type to an object of its wrapper class

– To convert a primitive value to an "equivalent" class typeTo convert a primitive value to an equivalent class type value, create an object of the corresponding wrapper class using the primitive value as an argument

The ne object ill contain an instance ariable that – The new object will contain an instance variable that

stores a copy of the primitive value

– Unlike most other classes, a wrapper class does not have a no‐argument constructor

Integer integerObject = new Integer(42);

(6)

Wrapper Classes Wrapper Classes

• Unboxing: the process of going from an object of a wrapper class to the corresponding value of a primitive type

a primitive type

– The methods for converting an object from the wrapper classes Byte,Short, Integer,Long,

d h i

Float,Double, and Characterto their corresponding primitive type are (in order) byteValue,shortValue,intValue,

longValue,floatValue,doubleValue, and charValue

– None of these methods take an argumentNone of these methods take an argument

int i = integerObject.intValue();

Automatic Boxing and Unboxing Automatic Boxing and Unboxing

St ti ith i 5 0 J t ti ll d b i

• Starting with version 5.0, Java can automatically do boxing and unboxing

• Instead of creating a wrapper class object using the newg pp j g operation (as shown before), it can be done as an automatic type cast:

Integer integerObject = 42;g g j ;

• Instead of having to invoke the appropriate method (such as intValue, doubleValue, charValue, etc.) in order to convert from an object of a wrapper class to a value of its convert from an object of a wrapper class to a value of its associated primitive type, the primitive value can be recovered automatically

i t i i t Obj t int i = integerObject;

Constants and Static Methods in Wrapper Classes

W l i l d f l t t th t

• Wrapper classes include useful constants that

provide the largest and smallest values for any of the primitive number types

primitive number types

– For example, Integer.MAX_VALUE,

Integer.MIN_VALUE,Double.MAX_VALUE, Double MIN VALUE etc

Double.MIN_VALUE, etc.

• The Boolean class has names for two constants of type Boolean

type Boolean

– Boolean.TRUEand Boolean.FALSEare the Boolean objects that correspond to the values trueand falseof the primitive typeboolean

the primitive type boolean

Constants and Static Methods in Wrapper Classes

• Wrapper classes have static methods that convert a correctly formed string representation of a number to the number of a given typeg yp

– The methods Integer.parseInt,Long.parseLong, Float.parseFloat, and Double.parseDoubledo this for the primitive types (in order) int,long,float, and d bl

double

• Wrapper classes also have static methods that convert from a numeric value to a string representation of the value

– For example, the expression

Double.toString(123.99);g( );

returns the string value "123.99"

• The Characterclass contains a number of static methods that are useful for string processing methods that are useful for string processing

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Some Methods in the Class Character (Part 1 of 3)

Some Methods in the Class Character (Part 2 of 3)

Some Methods in the Class Character (Part 3

of 3) Variables and Memory Variables and Memory

• A computer has two forms of memory

• Secondary memory is used to hold files for Secondary memory is used to hold files for

"permanent" storage

M i i d b h i

• Main memory is used by a computer when it is running a program

– Values stored in a program's variables are kept in

main memoryy

(8)

Variables and Memory Variables and Memory

• Main memory consists of a long list of numbered locations called bytes

– Each byte contains eight bits: eight 0 or 1 digits

• The number that identifies a byte is called its address

– A data item can be stored in one (or more) of these bytes – The address of the byte is used to find the data item when y

needed

Variables and Memory Variables and Memory

V l f t d t t i th

• Values of most data types require more than one byte of storage

– Several adjacent bytes are then used to hold the data itemSeveral adjacent bytes are then used to hold the data item – The entire chunk of memory that holds the data is called

its memory location

– The address of the first byte of this memory location is used as the address for the data item

• A computer's main memory can be thought of as a A computer s main memory can be thought of as a long list of memory locations of varying sizes

Variables in Memory

Variables in Memory References References

• Every variable is implemented as a location in computer memory

• When the variable is a primitive type, the value of the variable is stored in the memory location assigned to the variable

– Each primitive type always require the same amount of memory to store its values

(9)

References References

Wh h i bl i l l h dd

• When the variable is a class type, only the memory address (or reference) where its object is located is stored in the memory location assigned to the variabley g

– The object named by the variable is stored in some other location in memory

Like primitives the value of a class variable is a fixed size – Like primitives, the value of a class variable is a fixed size

– Unlike primitives, the value of a class variable is a memory address or reference

h b h dd d h bl b f

– The object, whose address is stored in the variable, can be of any size

References References

T f i bl i h

• Two reference variables can contain the same reference, and therefore name the same object

Th i t t t th f (

– The assignment operator sets the reference (memory address) of one class type variable equal to that of another – Any change to the object named by one of theses variablesAny change to the object named by one of theses variables

will produce a change to the object named by the other variable, since they are the same object

variable2 = variable1;

Class Type Variables Store a Reference (Part 1 of 2)

Class Type Variables Store a Reference (Part 2 of

2)

(10)

Assignment Operator with Class Type Variables (Part 1 f 3)

1 of 3)

Assignment Operator with Class Type Variables (Part 2 f 3)

2 of 3)

Assignment Operator with Class Type Variables (Part 3 f 3)

3 of 3) Class Parameters Class Parameters

• All parameters in Java are call‐by‐value parameters

A parameter is a local variable that is set equal to the – A parameter is a local variable that is set equal to the

value of its argument

– Therefore, any change to the value of the parameter cannot change the value of its argument

• Class type parameters appear to behave differently from primitive type parameters differently from primitive type parameters

– They appear to behave in a way similar to parameters in languages that have the call‐by‐reference

parameter passing mechanism

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Class Parameters Class Parameters

• The value plugged into a class type parameter is a reference (memory address)

Therefore the parameter becomes another name for – Therefore, the parameter becomes another name for

the argument

– Any change made to the object named by the parameter (i.e., changes made to the values of its instance variables) will be made to the object named by the argument, because they are the same objecty g y j – Note that, because it still is a call‐by‐value parameter,

any change made to the class type parameter itself (i e its address) will not change its argument (the (i.e., its address) will not change its argument (the reference or memory address)

Parameters of a Class Type Parameters of a Class Type

Memory Picture for Display 5.14 (Part 1 of 3)

Memory Picture for Display 5.14

(Part 2 of 3)

(12)

Memory Picture for Display 5.14 (Part 3 of 3)

Differences Between Primitive and Class‐Type Parameters

• A method cannot change the value of a variable of a primitive type that is an p yp argument to the method

• In contrast a method can change the values of

• In contrast, a method can change the values of the instance variables of a class type that is an

h h

argument to the method

Comparing Parameters of a Class Type and a Primitive Type (Part 1 of 2)

Comparing Parameters of a Class Type and a

Primitive Type (Part 2 of 2)

(13)

A Toy Class to Use in Display 5.16 (Part 1 of 2)

A Toy Class to Use in Display 5.16 (Part 2 of 2)

Pitfall: Use of = and == with Variables of a Class Type

• Used with variables of a class type, the assignment operator (=) produces two variables that name the same objectj

– This is very different from how it behaves with primitive type variables

• The test for equality (==) also behaves differently forThe test for equality ( ) also behaves differently for class type variables

– The ==operator only checks that two class type variables have the same memory address

the same memory address

– Unlike the equalsmethod, it does not check that their instance variables have the same values

– Two objects in two different locations whose instance variablesTwo objects in two different locations whose instance variables have exactly the same values would still test as being "not

The Constant null The Constant null

lli i l t t th t b i d t i bl

• nullis a special constant that may be assigned to a variable of any class type

YourClass yourObject = null;

• It is used to indicate that the variable has no "real value"

– It is often used in constructors to initialize class type instance variables when there is no obvious object to usej

• nullis not an object: It is, rather, a kind of "placeholder" for a reference that does not name any memory location

– Because it is like a memory address use==or!=(instead ofequals) – Because it is like a memory address, use ==or !=(instead of equals)

to test if a class variable contains null

if (yourObject == null) . . .

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Pitfall: Null Pointer Exception Pitfall: Null Pointer Exception

E th h l i bl b i iti li d t ll thi

• Even though a class variable can be initialized to null, this does not mean that nullis an object

– nullu is only a placeholder for an objects o y a p ace o de o a object

• A method cannot be invoked using a variable that is initialized to null

– The calling object that must invoke a method does not exist

• Any attempt to do this will result in a "Null Pointer Exception"

error message error message

– For example, if the class variable has not been initialized at all (and is not assigned to null), the results will be the same

The new Operator and Anonymous Objects The new Operator and Anonymous Objects

Th t i k t t hi h i iti li

• The newoperator invokes a constructor which initializes an object, and returns a reference to the location in memory of the object createdj

– This reference can be assigned to a variable of the object's class type

• Sometimes the object created is used as an argument to a

h d d d i

method, and never used again

– In this case, the object need not be assigned to a variable, i.e., given a name

• An object whose reference is not assigned to a variable is called an anonymous object

Another Approach to Keyboard Input Using

D bl D bl (P t 1 f 3)

Double.parseDouble (Part 1 of 3)

Another Approach to Keyboard Input Using

D bl D bl (P t 2 f 3)

Double.parseDouble (Part 2 of 3)

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Another Approach to Keyboard Input Using

D bl D bl (P t 3 f 3)

Double.parseDouble (Part 3 of 3)

Using and Misusing References Using and Misusing References

Wh iti it i i t t t

• When writing a program, it is very important to insure that private instance variables remain truly private

private

• For a primitive type instance variable, just adding the private modifier to its declaration should insure that there will be no privacy leaks

• For a class type instance variable, however, adding

th i t difi l i t ffi i t

the private modifier alone is not sufficient

Designing A Person Class: Instance Variables Designing A Person Class: Instance Variables

• A simple Personclass could contain instance variables representing a person's name, the date on which they were born, and the date on which they diedy

• These instance variables would all be class types: name of type String, and two dates of type Date

• As a first line of defense for privacy each of the instance

• As a first line of defense for privacy, each of the instance variables would be declared private

public class Person {

{

private String name;

private Date born;

private Date died; //null is still alive private Date died; //null is still alive

. . .

Designing a Person Class: Constructor Designing a Person Class: Constructor

• In order to exist, a person must have (at least) a name and a birth date

Therefore it would make no sense to have a no argument – Therefore, it would make no sense to have a no‐argument

Personclass constructor

• A person who is still alive does not yet have a date of death

– Therefore, the Personclass constructor will need to be able to deal with a nullvalue for date of death

• A person who has died must have had a birth date that preceded his or her date of death

– Therefore, when both dates are provided, they will need to be

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A Person Class Constructor A Person Class Constructor

public Person(String initialName, Date birthDate, Date deathDate) {

if ( i t t(bi thD t d thD t )) if (consistent(birthDate, deathDate)) { name = initialName;

born = new Date(birthDate);

if (deathDate null) if (deathDate == null)

died = null;

else

died = new Date(deathDate);

} else

{ System out println("Inconsistent dates ");

{ System.out.println( Inconsistent dates. );

System.exit(0);

} } }

Designing a Person Class: the Class Invariant Designing a Person Class: the Class Invariant

A t t t th t i l t f bj t f th l i

• A statement that is always true for every object of the class is called a class invariant

– A class invariant can help to define a class in a consistent and i d

organized way

• For the Personclass, the following should always be true:

– An object of the class Personj has a date of birth (which is not null), ( ) and if the object has a date of death, then the date of death is equal to or later than the date of birth

• Checking the Personclass confirms that this is true of every object created by a constructor, and all the other methods (e.g., the private method consistent) preserve the truth of this statement

Designing a Person Class: the Class Invariant Designing a Person Class: the Class Invariant

/** Class invariant: A Person always has a date of birth, and if the Person has a date of death, then the date of death is equal to or later than the date of birth.

To be consistent, birthDate must not be null If there To be consistent, birthDate must not be null. If there is no date of death (deathDate == null), that is consistent with any birthDate. Otherwise, the birthDate must come before or be equal to the deathDate.

/

*/

private static boolean consistent(Date birthDate, Date deathDate) {

{

if (birthDate == null) return false;

else if (deathDate == null) return true;

else return (birthDate.precedes(deathDate ||

birthDate.equals(deathDate));

}

Designing a Person Class: the equals and datesMatch Methods

Th d fi iti f l f th l P

• The definition of equals for the class Person includes an invocation of equals for the class String , and an invocation of the method equals String , and an invocation of the method equals for the class Date

• Java determines which equals method is being invoked from the type of its calling object

• Also note that the died instance variables are

d i th d t M t h th d i t d

compared using the datesMatch method instead

of the equals method, since their values may be

null u

(17)

Designing a Person Class: the equals Method

public boolean equals(Person otherPerson) public boolean equals(Person otherPerson) {

if (otherPerson == null) return false;

else

return (name.equals(otherPerson.name) &&

born.equals(otherPerson.born) &&

datesMatch(died, otherPerson.died));

} }

Designing a Person Class: the matchDate Method

/** To match date1 and date2 must either be the same date or both be null.

*/

private static boolean datesMatch(Date date1, Date date2) {

{

if (date1 == null)

return (date2 == null);

else if (date2 == null) //&& date1 != null else if (date2 == null) //&& date1 != null

return false;

else // both dates are not null.

return(date1 equals(date2));

return(date1.equals(date2));

}

Designing a Person Class: the toString Method

• Like the equalsmethod, note that the Personclass toStringmethod includes invocations of the Date class toStringgmethod

public String toString( ) {

{

String diedString;

if (died == null)

diedString = ""; //Empty stringg ; // p y g else

diedString = died.toString( );

return (name + ", " + born + "-" + diedString);

Copy Constructors Copy Constructors

A t t i t t ith i l

• A copy constructor is a constructor with a single argument of the same type as the class

• The copy constructor should create an object that is

• The copy constructor should create an object that is a separate, independent object, but with the

instance variables set so that it is an exact copy of the argument object

• Note how, in the Date copy constructor, the values

f ll f th i iti t i t i t i bl

of all of the primitive type private instance variables

are merely copied

(18)

Copy Constructor for a Class with Primitive Type Instance Variables

public Date(Date aDate) public Date(Date aDate) {

if (aDate == null) //Not a real date.

{

System.out.println("Fatal Error.");

System.exit(0);

}

month = aDate.month;; day = aDate.day;

year = aDate.year;

} }

Copy Constructor for a Class with Class Type Instance Variables

U lik th D t l th P l t i

• Unlike the Date class, the Person class contains three class type instance variables

• If the born and died class type instance variables

• If the born and died class type instance variables for the new Person object were merely copied, then they would simply rename the born and died variables from the original Person object

born = original.born //dangerous died = original died //dangerous died = original.died //dangerous

– This would not create an independent copy of the original object

Copy Constructor for a Class with Class Type Instance Variables

Th t l t t f th P l i " f "

• The actual copy constructor for the Personclass is a "safe"

version that creates completely new and independent copies of bornand died, and therefore, a completely new and , , p y independent copy of the original Personobject

– For example:

b D t ( i i l b )

born = new Date(original.born);

• Note that in order to define a correct copy constructor for a class that has class type instance variables, copy constructors c ass t at as c ass type sta ce a ab es, copy co st ucto s must already be defined for the instance variables' classes

Copy Constructor for a Class with Class Type Instance Variables

public Person(Person original) public Person(Person original) {

if (original == null) {

{

System.out.println("Fatal error.");

System.exit(0);

} }

name = original.name;

born = new Date(original.born);

if (original.died == null)( g ) died = null;

else

died = new Date(original.died);g }

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Pitfall: Privacy Leaks Pitfall: Privacy Leaks

• The previously illustrated examples from the Person class show how an incorrect definition of a constructor can result in a privacy leakp y

• A similar problem can occur with incorrectly defined mutator or accessor methods

– For example:

public Date getBirthDate() {

return born; //dangerous return born; //dangerous }

– Instead of:

public Date getBirthDate() public Date getBirthDate() {

return new Date(born); //correct }

}

Mutable and Immutable Classes Mutable and Immutable Classes

Th th d tN f th P l

• The accessor method getNamefrom the Personclass appears to contradict the rules for avoiding privacy leaks:

public String getName()

p g g ()

{

return name; //Isn't this dangerous?

} }

• Although it appears the same as some of the previous examples, it is not: The classStringcontains no mutator examples, it is not: The class Stringcontains no mutator methods that can change any of the data in a Stringobject

Mutable and Immutable Classes Mutable and Immutable Classes

A l h i h d ( h h

• A class that contains no methods (other than constructors) that change any of the data in an object of the class is called an immutable class object of the class is called an immutable class

– Objects of such a class are called immutable objects It is perfectly safe to return a reference to an immutable – It is perfectly safe to return a reference to an immutable

object because the object cannot be changed in any way – The Stringgclass is an immutable class

Mutable and Immutable Classes Mutable and Immutable Classes

A l h i bli h d

• A class that contains public mutator methods or other public methods that can change the data in its objects is called a mutable class and its objects are objects is called a mutable class, and its objects are called mutable objects

– Never write a method that returns a mutable object – Never write a method that returns a mutable object – Instead, use a copy constructor to return a reference to a

completely independent copy of the mutable objectp y p py j

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Deep Copy Versus Shallow Copy Deep Copy Versus Shallow Copy

• A deep copy of an object is a copy that, with one exception, has no references in common with the original

– Exception: References to immutable objects are allowed to be shared

• Any copy that is not a deep copy is called a shallow copy

– This type of copy can cause dangerous privacy leaks in a program

Packages and Import Statements

• Java uses packages to form libraries of classes

• A package is a group of classes that have been

l d i di t f ld d th t b

placed in a directory or folder, and that can be used in any program that includes an import statement that names the package

statement that names the package

– The import statement must be located at the beginning of the program file: Only blank lines, comments and package statements may precede it comments, and package statements may precede it – The program can be in a different directory from the

package

Import Statements Import Statements

• We have already used import statements to include some predefined packages in Java, such

S f th j til k

as Scanner from the java.util package

import java.util.Scanner;

I i ibl k ll h l i k

• It is possible to make all the classes in a package available instead of just one class:

i t j til *

import java.util.*;

– Note that there is no additional overhead for importing the entire package

importing the entire package

The package Statement The package Statement

• To make a package, group all the classes

together into a single directory (folder), and add the following package statement to the

the following package statement to the beginning of each class file:

package package name;

p g p g _

– Only the .classfiles must be in the directory or folder, the .javafiles are optional

Only blank lines and comments may precede the – Only blank lines and comments may precede the

package statement

– If there are both import and package statements, the package statement must precede any import

statements

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The Package java lang The Package java.lang

• The package java.lang contains the classes that are fundamental to Java programming p g g

– It is imported automatically, so no import statement is needed

statement is needed

– Classes made available by java.lang include Math String and the wrapper classes

Math, String, and the wrapper classes

Package Names and Directories Package Names and Directories

A k i th th f th di t

• A package name is the path name for the directory or subdirectories that contain the package classes

• Java needs two things to find the directory for a package: the g y p g name of the package and the value of the CLASSPATH variable

– The CLASSPATHenvironment variable is similar to the PATH variable, and is set in the same way for a given operating system – The CLASSPATHvariable is set equal to the list of directories

(including the current directory, ".") in which Java will look for packages on a particular computer

– Java searches this list of directories in order, and uses the first directory on the list in which the package is found

A Package Name

A Package Name Pitfall: Subdirectories Are Not Automatically Imported

• When a package is stored in a subdirectory of the directory containing

• When a package is stored in a subdirectory of the directory containing another package, importing the enclosing package does not import the subdirectory package

• The import statement:The import statement:

import utilities.numericstuff.*;

imports the utilities.numericstuffpackage only

• The import statements:

import utilities.numericstuff.*;

import utilities.numericstuff.statistical.*;

import both theutilities numericstuffand import both the utilities.numericstuffand utilities.numericstuff.statisticalpackages

(22)

The Default Package The Default Package

• All the classes in the current directory belong to an unnamed package called the default p g f package

• As long as the current directory ( ) is part of

• As long as the current directory ( . ) is part of the CLASSPATH variable, all the classes in

h f l k ll

the default package are automatically available to a program

Pitfall: Not Including the Current Directory in Your Class Path

• If the CLASSPATH variable is set, the current

directory must be included as one of the alternatives

– Otherwise, Java may not even be able to find the .class files for the program itself

• If the CLASSPATH variable is not set, then all the class files for a program must be put in the current directory

Specifying a Class Path When You Compile Specifying a Class Path When You Compile

• The class path can be manually specified when a class is compiled p

– Just add –classpath followed by the desired class path

class path

– This will compile the class, overriding any previous CLASSPATH setting

CLASSPATH setting

• You should use the –classpath option again when the class is run

Name Clashes Name Clashes

I dditi t k i l lib i i d k

• In addition to keeping class libraries organized, packages provide a way to deal with name clashes: a situation in which two classes have the same name

– Different programmers writing different packages may use the same name for one or more of their classes

– This ambiguity can be resolved by using the fully qualified name (i.e.,

d th l b it k ) t di ti i h b t

precede the class name by its package name) to distinguish between each class

package_name.ClassName

If th f ll lifi d i d it i l t i t

– If the fully qualified name is used, it is no longer necessary to import the class (because it includes the package name already)

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Introduction to javadoc Introduction to javadoc

U lik l h C++ J l b th th i t f

• Unlike a language such as C++, Java places both the interface and the implementation of a class in the same file

• However, Java has a program called javadocp g j that

automatically extracts the interface from a class definition and produces documentation

– This information is presented in HTML format, and can be viewed with p , a Web browser

– If a class is correctly commented, a programmer need only refer to this API (Application Programming Interface) documentation in order to use the class

– javadoccan obtain documentation for anything from a single class to an entire package

Commenting Classes for javadoc Commenting Classes for javadoc

• The javadocprogram extracts class headings, the headings for some comments, and headings for all public methods instance variables and static variables public methods, instance variables, and static variables

– In the normal default mode, no method bodies or private items are extracted

• To extract a comment, the following must be true:

1. The comment must immediately precede a public class or method definition, or some other public item, p

2. The comment must be a block comment, and the opening /*

must contain an extra *( /** . . . */)

– Note: Extra options would have to be set in order to extract – Note: Extra options would have to be set in order to extract

line comments ( //) and private items

Commenting Classes for javadoc Commenting Classes for javadoc

• In addition to any general information, the comment preceding a public method definition should include descriptions of parameters, any value returned, and p p y any exceptions that might be thrown

– This type of information is preceded by the @symbol and is called an @ tag

– @ tags come after any general comment, and each one is on a line by itself

/**

General Comments about the method . . .

@param aParameter Description of aParameter

@return What is returned

@return What is returned . . .

@ Tags

• @ tags should be placed in the order found below

• If there are multiple parameters, each should have its own @param on a separate line, and each should be listed according to its left‐to‐p , g right order on the parameter list

• If there are multiple authors, each should have its own @author on a separate linep

@param Parameter_Name Parameter_Description

@return Description_Of_Value_Returned

@th E ti T E l ti

@throws Exception_Type Explanation

@deprecated

@see Package Name.Class Nameg _ _

@author Author

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Running javadoc Running javadoc

• To run javadocon a package, give the following command:

javadoc –d Documentation Directory Package Name javadoc –d Documentation_Directory Package_Name

– The HTML documents produced will be placed in the Documentation_Directory

f h d i d

– If the –dand Documentation_Directoryare omitted, javadocwill create suitable directories for the

documentation

• To run javadocon a single class, give the following command from the directory containing the class file:

javadoc ClassName.java ja adoc C ass a e.ja a

• To run javadoc on all the classes in a directory, give the following command instead: javadoc *.java