Java session05

Java Programming Language
Objectives

In this session, you will learn to:
Understand the use of casting objects
Describe overloading methods and methods with variable
arguments
Describe overloading constructors and invoking parent class
constructors
Understand Wrapper classes
Understand autoboxing of primitive types
Create static variables, methods, and initializers

Ver. 1.0 Session 5 Slide 1 of 22

Casting Objects

Casting objects is used where you have received a
reference to a parent class, and you want to access the full
functionality of the object of the subclass:
– Use instanceof to test the type of an object.
– Restore full functionality of an object by casting.
– Check for proper casting using the following guidelines:
Casts upward in the hierarchy are done implicitly.
Downward casts must be to a subclass and checked by the
compiler.
The object type is checked at runtime when runtime errors can
occur.


Overloading Methods

The methods which perform closely related tasks can be
given the same name by overloading them.
Overloading can be used as follows:
public void println(int i)
public void println(float f)
public void println(String s)
Argument list must differ across various methods.
Return types can be different.


Methods Using Variable Arguments

• The varargs or variable arguments is a feature provided
by J2SE 5.0.
• It helps to pass variable number of arguments, of the same
type, as parameters, to a method.
• It can be used when you have a number of overloaded
methods, which share the same functionality.


Methods Using Variable Arguments (Contd.)

• The following example demonstrates the usage of
varargs:
public class Statistics {
public float average(int... nums) {
int sum = 0;
for ( int x : nums ) { nums is an array
of type int[]
sum += x;
}
return ((float) sum) / nums.length;
}
}
We can invoke the average method by passing any number
of arguments as integers.


Overloading Constructor

As with methods, constructors can be overloaded:
An example is:
public Employee(String name, double
salary,Date doB)
public Employee(String name, double salary)
public Employee(String name, Date DoB)
Argument lists must differ.
You can use the this reference at the first line of a constructor
to call another constructor.


Overloading Constructor (Contd.)

Example of overloading constructors:
public class Employee {
private static final double BASE_SALARY =
15000.00;
private String name;
private double salary;
private Date birthDate;
salary, Date DoB) {
this.name = name;
Initializes all
this.salary = salary; instance
this.birthDate = DoB; variables

}


Overloading Constructor (Contd.)

salary) {
used as a forwarding call
this(name, salary, null); to the first constructor
}
public Employee(String name, Date DoB) {
this(name, BASE_SALARY, DoB);
}
// more Employee code...
calls the first constructor
}
passing in the class
constant BASE_SALARY
– The this keyword in a constructor must be the first line of
code in the constructor.


Constructors Are Not Inherited

A subclass inherits all methods and variables from the
superclass (parent class).
A subclass does not inherit the constructor from the
superclass.
Two ways to include a constructor are:
Use the default constructor
Write one or more explicit constructors


Invoking Parent Class Constructors

• To invoke a parent class constructor, you must place a call
to super() in the first line of the constructor.
• You can call a specific parent constructor by the arguments
that you use in the call to super().
• If the parent class defines constructors, but does not
provide a no-argument constructor, then a compiler error
message is issued.


Invoking Parent Class Constructors (Contd.)

An example of invoking parent class constructor:
public class Manager extends Employee {
private String department;
public Manager(String name, double salary,

String dept)
{
super(name, salary);
department = dept;
}
public Manager(String name, String dept)
{
super(name);
department = dept; }

Invoking Parent Class Constructors (Contd.)

public Manager(String dept)
{
// This code fails: no super()
department = dept;
}
//more Manager code...
}


The Object Class

• The Object class is the root of all classes in Java.
• A class declaration with no extends clause implies extends
Object. For example:
public class Employee
{
...
}
is equivalent to:
public class Employee extends Object
{
...
}
Two important methods of object class are:
equals()
toString()


The equals Method

• The == operator determines if two references are identical
to each other (that is, refer to the same object).
• The equals() method determines if objects are equal but
not necessarily identical.
• The Object implementation of the equals() method uses
the == operator.
• User classes can override the equals method to implement
a domain-specific test for equality.

Note: You should override the hashCode method if you
override the equals method.


The toString Method

• The toString() method has the following characteristics:
– This method converts an object to a String.
– Use this method during string concatenation.
– Override this method to provide information about a
user-defined object in readable format.
– Use the wrapper class’s toString() static method to
convert primitive types to a String.


Wrapper Classes

• The Java programming language provides wrapper
classes to manipulate primitive data elements as objects.
• Each Java primitive data type has a corresponding wrapper
class in the java.lang package.
• Example of Primitive Boxing using wrapper classes:
int pInt = 420;
Integer wInt = new Integer(pInt);
// this is called boxing
int p2 = wInt.intValue();
// this is called unboxing


Autoboxing of Primitive Types

The autoboxing feature enables you to assign and retrieve
primitive types without the need of the wrapper classes.
Example of Primitive Autoboxing:
int pInt = 420;
Integer wInt = pInt; // this is called autoboxing
int p2 = wInt; // this is called autounboxing
The J2SE 5.0 compiler will create the wrapper object
automatically when assigning a primitive to a variable of the
wrapper class type.
The compiler will also extract the primitive value when
assigning from a wrapper object to a primitive variable.


The static Keyword

• The static keyword is used as a modifier on variables,
methods, and nested classes.
• The static keyword declares the attribute or method is
associated with the class as a whole rather than any
particular instance of that class.
• Thus,static members are often called class members, such
as class attributes or class methods.


The static Keyword (Contd.)

Static Attribute:
A public static class attribute can be accessed from outside the
class without an instance of the class.
Static Method:
A static method can be invoked without creating the instance of
the class.
Static methods can not access instance variables.
Static Initializers:
– A class can contain code in a static block that does not exist
within a method body.
– Static block code executes once only, when the class is
loaded.
– Usually, a static block is used to initialize static (class)
attributes.


Demonstration

Let see how to declare static members in a class, including
both member variables and methods.


Summary

In this session, you learned that:
– Casting objects is used where you have received a reference
to a parent class, and you want to access the full functionality
of the object of the subclass.
– The methods which perform closely related tasks can be given
the same name by overloading them.
– The varargs feature helps us to write a generic code to pass
variable number of arguments, of the same type to a method.
– The super keyword is used to call the constructor of the
parent class.
– The object class is the root of all classes. The two important
methods of the object class are equals() method and
tostring() method.


Summary (Contd.)

– Wrapper classes are used to manipulate primitive data
elements as objects. Each Java primitive data type has a
corresponding wrapper class in the java.lang package.
– Autoboxing feature of J2SE 5.0 enables you to assign and
retrieve primitive types without the need of the wrapper
classes.
– The static keyword declares members (attributes, methods,
and nested classes) that are associated with the class rather
than the instances of the class.


Java session05

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