Jan. 29 Tue. Class Notes

  Object & Class
  Instance & Class Variable
  Instance & Class Method
  public, private, final, abstract

Pass Message to a Program --- revisit
class Hello {
   public static void main(String[] args) { 
       String str = "Judy"; 
       if (args.length > 0) { 
          str = args[0]; 
       String msg = "Hello " + str ;
>java –cp . Hello Kathy Bob Dan
Hello Kathy

NOTE: Any single message passed through command line 
is a String object. args holds a collection of String objects
Reference Type vs Primitive Type

A reference type or an object can never be casted to a primitive type
A primitive type can never be casted to a reference type or an object type

String s = “123”;
int i = s; // illegal
int i2 = (int)s; //illegal

Use 8 wrapper classes to convert String object to primitive types.

String s = “123”;
byte b  =  Byte.parseByte(s);
short s  =  Short.parseShort(s);
int i   =  Integer.parseInt(s);
long lg   =  Long.parseLong(s);
float f  =  Float.parseFloat(s);
double d = Double.parseDouble(s);
void keyword is used to indicate that a method returns no value.

null can be assigned to any reference type to indicate an absence of reference
null is a special value that is a reference to nothing
null is printable
null is used to initialize an object type
new operator is used to allocate memory for object and arrays.
Object Creation with new
ObjectType obj = new ObjectType();
ObjectType obj2 = new ObjectType(parameter list);


public class First extends Object {
//default constructor 
   public First() {
   public static void main(String[] args) {
       System.out.println(“First Java Program”);
How to print out “First Java Program” with new operator
You have to write a method to do so
class First {
   public static void main(String[] args) {
       System.out.println(“First Java Program”);
   public static void print() {
     System.out.println(“First Java Program”); 

First.print(); //First Java Program
First f = new First();
f.print();  // First Java Program

What is the default constructor?

Default constructor is a constructor that takes no arguments
If you define no constructors at all in a class, then the 
compiler provides a default constructor. If you define even 
a single constructor, this default is not provided.

What is the construtor?

A constructor is a special method used to create an object with 
new operator
A constructor must have the same name as  the class
Constructors do not have a return type
Generally constructors are used to initialize the data fields
Constructors can be overloaded.
Use Default Constructor
First f = new First();
Hello h = new Hello();
Judy j = new Judy();
Tester t = new Tester();
DollarChanger dc = new DollarChanger();
Basic structure of a class

[modifier] class clsIdentifier [extends clsIdentifier                   implements   interfaceIdentifier,  ,  ] {

        Attributes or Properties block (fields)
How the attributes look like in a class?
class foo {
    //attributes block
    private int creditCardNo;
    public static final double PI  = 3.1415926;
    String name;
    protected char midName;
    public static format(double d, int I) {}
Attributes are represented by class fields and instance fields.
Members of a Class

1. Class fields
2. Instance fields 
3. Class methods
4. Instance methods

What is the class field?

1. A class field is declared with static modifier
2. A class field is associated with the class in which it is defined.

class Foo {
    public static final double PI = 3.1415926;
    public static String initial = “JX”;
    static double int counter;
What is the class method?

1. A class method is declared with the static modifier
2. A class method is associated with a class, rather      
   than with an object.

public static double format(double d, int i) {}
static String print() {}
public static int add(int a, int b) {}

What is the instance fields?

Any fields declared without the static modifier

class Foo {
   public int sum;
   public Calculator cal;
   final char midInitial = ‘X’;
What is the instance methods?

1. Any method not declared with the static modifier is an instance 
2. An instance method operates on an instance of a class(an object) 
instead of operating on the class itself
3. It requires an instantiated object of the class in order to be

How to call a class method?

1. className.staticMethod()
2. new className().staticMethod()

double d = Sara.format(3.4445, 2);// 3.44
Sara sr = new Sara();
double d2 = sr.format(3.4445,3);//3.445
int i = Calculator.add(5,5);  //10
int i2 = new Calculator().mul(5,5); //25

How to call an instance method?

Instantiate an object with new operator and then use the 
instance variable to call the instance method

class Foo {
   public void report() {…}
   long cal(long a, long b) {…}
   public  String getName() {…}
Foo  f = new Foo();
String name = f.getName();
private keyword

1. The least generous access modifier
2. Applies to member variables, methods, constructors or inner classes.
3. Visibility is inside the class and hidden everywhere else.

private String creditCardNo;
private int creditRanking(){}
final keyword

1. Applies to classes, methods, and variables.
2. A final variable(primitive types) may not be modified once it 
has been assigned a value
3. A final class may not be subclassed.
4. A final method may not be overridden.
5. A final variable(reference type) must stay the same, not the object.

abstract keyword

1. Applies to classes and methods
2. May not be instantiated(may not call its constructor)
3. Provide a way to defer implementation to subclasses
4. Abstract class must be subclassed

abstract class Student { }
abstract String getStatus();//abstract method, no method body
String getStatus() {} //not abstract

Create an Employee class


  Employee Number -- int
  Employee Last Name -- String
  Employee First Name -- String
  Employee Salary – double
  Company Number - int
  Employee Address –String
  Employee Telephone – String
  Employee Birthdate – Date
important points in the Employee class
    data hiding or info hiding
    member variables initialized by default values
public class Employee {
    private int num;
    private String lastName;
    private String firstName;
    private double salary;
    public static int companyID = 8888;
    public int getNum() {
        return num;
    public void setNum(int num) {
        this.num = num;
    public String getFirstName() {
        return firstName;
    public void setFirstName(String firstName) {
        this.firstName = firstName;
    public String getLastName() {
        return lastName;
    public void setLastName(String name) {
        lastName = name;
    public double getSalary() {
        return salary;
    public void setSalary(double salary) {
        this.salary = salary;
    public String toString() {
        String str = "Employee Number: " + num
                   +"\nFirst Name: " + firstName
                   + "\nLast Name: " + lastName
                   + "\nSalary:$" + salary;
        return str+"\n";
    public static void main(String[] args) {
        Employee e1 = new Employee();

        Employee yn = new Employee();
        System.out.println(yn.getLastName()); //yln
        String name = yn.getFirstName();
        System.out.println(name); //yfn
        double salary = yn.getSalary();
        System.out.println(salary); //dbl value
        System.out.println(e1.getSalary()); //200.00

        //See how the class field associated with class itself
        //Class variables are variables that are shared by every 
        //instantiation of a class

        System.out.println(Employee.companyID); //8888
        System.out.println(e1.companyID); //8888
        System.out.println(yn.companyID); //8888

Non-static method cannot be called in a static method.
Non static variable cannot be used by static methods

Illegal statements in main():
System.out.println(num); //illegal
String name = getFirstName(); //illegal
System.out.println(getFirstName()); //illegal
setFirstName(“Judy”); //illegal


Compile and run Employee class
Copy contents in main() of Employee class to the main() 
in YourName class and compile and run YourName class.
Practice with private keyword 
Add properties to YourName class
Practice with static field

Try to get a private field.
salary has private access in Employee