Basic OOP in Java Visit also http java sun com docs books tutorial java index html Objects and Classes in Java Program structure with user defined classes The basic structure of Java programs with user defined classes public class ClassName public static void main String args program statements user defined methods user defined classes Class Definitions The basic structure of Java classes class NameOfClass constructors methods fields Objects and Object Variables Java constructors construct and initialize objects Constructors have the same name as the class Objects are constructed by an application of a constructor and the operator new Object variables store the reference to the object not the object itself Example Date D new Date System out println D An object of a Date class is constructed and its reference is stored inside variable D Next we print out the object Class Fields and Methods Class fields define the state of an object Constructors define and initialize class fields Accessor methods access class fields of an object Mutator methods modify class fields Example inside main method We construct an object of Person class Person P new Person Harry Hacker Beatty Rd Sprinfield NJ We call mutator functions setAge and setSex P setAge 22 P setSex male We call accessor functions getFirstName getLastName and we print out the content of the corresponding class fields System out println P getFirstName System out println P getLastName Fields of Person class private String firstname private String lastname private String street private String city private String state private String sex private int age 0 Parameter Passing Two kinds of method parameters Primitive Types variables storing numbers characters boolean values Object References variables referring to class objects In both cases the content of variables is passes by value In the first case the method parameter contains the primitive type In the second case the parameter contains the reference to the object not the object itself Examples Passing Primitive Types inside main method int n 2 square n System out println n square method public static void square int i i i i System out println i Passing Object References inside main int n 2 int numbers new int n for int i 0 i numbers length i numbers i i prints 2 prints 4 for int i 0 i numbers length i System out println numbers i prints 0 first and 1 next swap numbers 0 1 for int i 0 i numbers length i System out println numbers i prints 1 first and 0 next swap method public static void swap int v int i int j int temp temp v i v i v j v j temp Person Class The code is contained in the directory java person class on dunx1 All methods of Person class are public They may be accessed by any method in any class All instance fields are private Only methods of Person class may access them public class PersonTest public static void main String args Person P new Person Harry Hacker Beatty Rd Sprinfield NJ P setAge 22 P setSex male System out println P getFirstName System out println P getLastName System out println P getStreet System out println P getCity System out println P getState System out println P getSex System out println P getAge class Person constructor public Person String aFirst String aLast String aStreet String aCity String aState firstname aFirst lastname aLast street aStreet city aCity state aState methods public String getFirstName return firstname public String getLastName return lastname public String getStreet return street public String getCity return city public String getState return state public String getSex return sex public int getAge return age public void setAge int aAge age aAge public void setSex String aSex sex aSex fields private String firstname private String lastname private String street private String city private String state private String sex private int age 0 An Example of Inheritance We define Manager class which extends Person class The code is available in the directory java manager class on dunx1 public class ManagerTest public static void main String args Manager P new Manager Harry Hacker Beatty Rd Sprinfield NJ P setAge 22 P setSex male P setSalary 75000 System out println P getFirstName System out println P getLastName System out println P getStreet System out println P getCity System out println P getState System out println P getSex System out println P getAge System out println P getSalary class Person constructor public Person String aFirst String aLast String aStreet String aCity String aState firstname aFirst lastname aLast street aStreet city aCity state aState methods public String getFirstName return firstname public void setSex String aSex sex aSex fields private String firstname private String lastname private String street private String city private String state private String sex private int age 0 class Manager extends Person construtor public Manager String aFirst String aLast String aStreet String aCity String aState keyword super allows us to use the constructor of Person class super aFirst aLast aStreet aCity aState salary 0 methods public int getSalary return salary public void setSalary int aSalary salary aSalary fields private int salary
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