Java Tutorial/Class Definition/Defining Class
Содержание
- 1 Checking whether the object referenced was of type String
- 2 Class declaration with a method that has a parameter
- 3 Class declaration with one method
- 4 Class that contains a String instance variable and methods to set and get its value
- 5 Class with a constructor to initialize instance variables
- 6 Creating Objects of a Class
- 7 Defining Classes: A class has fields and methods
- 8 Fields
- 9 Specifying initial values in a class definition
- 10 What Is a Java Class?
Checking whether the object referenced was of type String
<source lang="java">
class Animal {
public Animal(String aType) {
type = aType;
}
public String toString() {
return "This is a " + type;
}
private String type;
} public class MainClass {
public static void main(String[] a) {
Animal pet = new Animal("a");
if (pet.getClass() == Animal.class) {
System.out.println("it is an animal!");
}
}
}</source>
it is an animal!
Class declaration with a method that has a parameter
<source lang="java">
public class MainClass {
public static void main( String args[] )
{
GradeBook myGradeBook = new GradeBook();
String courseName = "Java ";
myGradeBook.displayMessage( courseName );
}
} class GradeBook {
public void displayMessage( String courseName )
{
System.out.printf( "Welcome to the grade book for\n%s!\n",
courseName );
}
}</source>
Welcome to the grade book for Java !
Class declaration with one method
<source lang="java">
public class MainClass {
public static void main( String args[] )
{
GradeBook myGradeBook = new GradeBook();
myGradeBook.displayMessage();
}
} class GradeBook {
public void displayMessage()
{
System.out.println( "Welcome to the Grade Book!" );
}
}</source>
Welcome to the Grade Book!
Class that contains a String instance variable and methods to set and get its value
<source lang="java">
public class MainClass {
public static void main( String args[] )
{
GradeBook myGradeBook = new GradeBook();
System.out.printf( "Initial course name is: %s\n\n",myGradeBook.getCourseName() );
String theName = "Java";
myGradeBook.setCourseName( theName ); // set the course name
myGradeBook.displayMessage();
}
} class GradeBook {
private String courseName; // course name for this GradeBook
// method to set the course name
public void setCourseName( String name )
{
courseName = name; // store the course name
}
// method to retrieve the course name
public String getCourseName()
{
return courseName;
}
// display a welcome message to the GradeBook user
public void displayMessage()
{
System.out.printf( "Welcome to the grade book for\n%s!\n",
getCourseName() );
}
}</source>
Initial course name is: null Welcome to the grade book for Java!
Class with a constructor to initialize instance variables
<source lang="java">
public class MainClass {
public static void main( String args[] )
{
Account account1 = new Account( 50.00 ); // create Account object
Account account2 = new Account( -7.53 ); // create Account object
System.out.printf( "account1 balance: $%.2f\n", account1.getBalance() );
System.out.printf( "account2 balance: $%.2f\n\n", account2.getBalance() );
double depositAmount; // deposit amount read from user
depositAmount = 10.10;
account1.credit( depositAmount ); // add to account1 balance
System.out.printf( "account1 balance: $%.2f\n", account1.getBalance() );
System.out.printf( "account2 balance: $%.2f\n\n", account2.getBalance() );
depositAmount = 12.12;
account2.credit( depositAmount ); // add to account2 balance
System.out.printf( "account1 balance: $%.2f\n", account1.getBalance() );
System.out.printf( "account2 balance: $%.2f\n", account2.getBalance() );
}
} class Account {
private double balance; // instance variable that stores the balance
// constructor
public Account( double initialBalance )
{
if ( initialBalance > 0.0 )
balance = initialBalance;
}
public void credit( double amount )
{
balance = balance + amount;
}
public double getBalance()
{
return balance;
}
}</source>
account1 balance: $50.00 account2 balance: $0.00 account1 balance: $60.10 account2 balance: $0.00 account1 balance: $60.10 account2 balance: $12.12
Creating Objects of a Class
<source lang="java">
class Sphere {
double radius; // Radius of a sphere
Sphere() {
}
// Class constructor
Sphere(double theRadius) {
radius = theRadius; // Set the radius
}
} public class MainClass {
public static void main(String[] arg){
Sphere sp = new Sphere();
}
}</source>
Defining Classes: A class has fields and methods
<source lang="java">
public class MainClass {
private int aField;
public void aMethod() {
}
}</source>
Fields
- Fields are variables.
- They can be primitives or references to objects.
For example, the Employee class has two fields, age and salary.
<source lang="java">
public class Employee{
int age; int salary
}</source>
- Field names should follow the camel naming convention.
- The initial of each word in the field, except for the first word, is written with a capital letter.
- For example: age, maxAge, address, validAddress, numberOfRows.
Specifying initial values in a class definition
<source lang="java">
class A {
A(int marker) {
System.out.println("Bowl(" + marker + ")");
}
void f(int marker) {
System.out.println("f(" + marker + ")");
}
} class B {
static A a = new A(1);
B() {
System.out.println("Table()");
staticA.f(1);
}
void f2(int marker) {
System.out.println("f2(" + marker + ")");
}
static A staticA = new A(2);
} class C {
A a = new A(3);
static A staticA = new A(4);
C() {
System.out.println("Cupboard()");
staticA.f(2);
}
void f3(int marker) {
System.out.println("f3(" + marker + ")");
}
static A staticA2 = new A(5);
} public class MainClass {
public static void main(String[] args) {
System.out.println("Creating new Cupboard() in main");
new C();
System.out.println("Creating new Cupboard() in main");
new C();
t2.f2(1);
t3.f3(1);
}
static B t2 = new B();
static C t3 = new C();
}</source>
Bowl(1) Bowl(2) Table() f(1) Bowl(4) Bowl(5) Bowl(3) Cupboard() f(2) Creating new Cupboard() in main Bowl(3) Cupboard() f(2) Creating new Cupboard() in main Bowl(3) Cupboard() f(2) f2(1) f3(1)
What Is a Java Class?
Classes are the fundamental building blocks of a Java program. You can define an Employee class as follows:
<source lang="java">
class Employee {
int age; double salary;
}</source>
- By convention, class names capitalize the initial of each word.
- For example: Employee, Boss, DateUtility, PostOffice, RegularRateCalculator.
- This type of naming convention is known as Pascal naming convention.
- The other convention, the camel naming convention, capitalize the initial of each word, except the first word.
- Method and field names use the camel naming convention.
