Object:
Objects have states and behaviors. Example: A dog has states-color, name, breed as well as behaviors -wagging, barking, eating. An object is an instance of a class.
Objects in Java:
Let us now look deep into what are objects. If we consider the real-world we can find many objects around us, Cars, Dogs, Humans etc. All these objects have a state and behavior.
If we consider a dog then its state is . name, breed, color, and the behavior is . barking, wagging, running
If you compare the software object with a real world object, they have very similar characteristics.
Software objects also have a state and behavior. A software object's state is stored in fields and behavior is shown via methods.
So in software development methods operate on the internal state of an object and the object-to-object communication is done via methods.
Classes in Java:
A class is a blue print from which individual objects are created.
A sample of a class is given below:
public class Dog{
String breed;
int age;
String color;
void barking(){
}
void hungry(){
}
void sleeping(){
}
}
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A class can contain any of the following variable types.
· Local variables . variables defined inside methods, constructors or blocks are called local variables. The variable will be declared and initialized within the method and the variable will be destroyed when the method has completed.
· Instance variables . Instance variables are variables within a class but outside any method. These variables are instantiated when the class is loaded. Instance variables can be accessed from inside any method, constructor or blocks of that particular class.
· Class variables . Class variables are variables declared with in a class, outside any method, with the static keyword.
A class can have any number of methods to access the value of various kind of methods. In the above example, barking(), hungry() and sleeping() are variables.
Below mentioned are some of the important topics that need to be discussed when looking into classes of the Java Language.
Creating an Object:
As mentioned previously a class provides the blueprints for objects. So basically an object is created from a class. In java the new key word is used to create new objects.
There are three steps when creating an object from a class:
· Declaration . A variable declaration with a variable name with an object type.
· Instantiation . The 'new' key word is used to create the object.
· Initialization . The 'new' keyword is followed by a call o a constructor. This call initializes the new object.
Example of creating an object is given below:
class Puppy{
public Puppy(String name){
// This constructor has one parameter, name.
System.out.println("Passed Name is :" + name );
}
public static void main(String []args){
// Following statement would create an object myPuppy
Puppy myPuppy = new Puppy( "tommy" );
}
}
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If we compile and run the above program then it would produce following result:
Passed Name is :tommy
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Accessing Instance Variables and Methods:
Instance variables and methods are accessed via created objects. To access an instance variable the fully qualified path should be as follows:
/* First create an object */
ObjectReference = new Constructor();
/* Now call a variable as follows */
ObjectReference.variableName;
/* Now you can call a class method as follows */
ObjectReference.MethodName();
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Example:
This example explains how to access instance variables and methods of a class:
class Puppy{
int puppyAge;
public Puppy(String name){
// This constructor has one parameter, name.
System.out.println("Passed Name is :" + name );
}
public setAge( int age ){
puppyAge = age;
}
public getAge( ){
System.out.println("Puppy's age is :" + puppyAge );
return puppyAge;
}
public static void main(String []args){
/* Object creation */
Puppy myPuppy = new Puppy( "tommy" );
/* Call class method to set puppy's age */
myPuppy.setAge( 2 );
/* Call another class method to get puppy's age */
myPuppy.getAge( );
/* You can access instance variable as follows as well */
System.out.println("Variable Value :" + myPuppy.puppyAge );
}
}
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If we compile and run the above program then it would produce following result:
Passed Name is :tommy
Puppy's age is :2
Variable Value :2
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Constructors:
When discussing about classes one of the most important sub topic would be constructors. Every class has a constructor. If we do not explicitly write a constructor for a class the java compiler builds a default constructor for that class.
Each time a new object is created at least one constructor will be invoked. The main rule of constructors is that they should have the same name as the class. A class can have more than one constructor.
Example of a constructor is given below:
class Puppy{
public puppy(){
}
public puppy(String name){
// This constructor has one parameter, name.
}
}
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Java also supports Singleton Classes where you would be able to create only one instance of a class.
A Java method is a collection of statements that are grouped together to perform an operation. When you call the System.out.println method, for example, the system actually executes several statements in order to display a message on the console.
Now you will learn how to create your own methods with or without return values, invoke a method with or without parameters, overload methods using the same names, and apply method abstraction in the program design.
Creating a Method:
In general, a method has the following syntax:
modifier returnValueType methodName(list of parameters) {
// Method body;
}
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A method definition consists of a method header and a method body. Here are all the parts of a method:
· Modifiers: The modifier, which is optional, tells the compiler how to call the method. This defines the access type of the method.
· Return Type: A method may return a value. The returnValueType is the data type of the value the method returns. Some methods perform the desired operations without returning a value. In this case, the returnValueType is the keyword void.
· Method Name: This is the actual name of the method. The method name and the parameter list together constitute the method signature.
· Parameters: A parameter is like a placeholder. When a method is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument. The parameter list refers to the type, order, and number of the parameters of a method. Parameters are optional; that is, a method may contain no parameters.
· Method Body: The method body contains a collection of statements that define what the method does.
Note: In certain other languages, methods are referred to as procedures and functions. A method with a nonvoid return value type is called a function; a method with a void return value type is called a procedure.
Example:
Here is the source code of the above defined method called max(). This method takes two parameters num1 and num2 and returns the maximum between the two:
/** Return the max between two numbers */
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
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Calling a Method:
In creating a method, you give a definition of what the method is to do. To use a method, you have to call or invoke it. There are two ways to call a method; the choice is based on whether the method returns a value or not.
When a program calls a method, program control is transferred to the called method. A called method returns control to the caller when its return statement is executed or when its method-ending closing brace is reached.
If the method returns a value, a call to the method is usually treated as a value. For example:
int larger = max(30, 40);
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If the method returns void, a call to the method must be a statement. For example, the method println returns void. The following call is a statement:
System.out.println("Welcome to Java!");
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Example:
Following is the example to demonstrate how to define a method and how to call it:
public class TestMax {
/** Main method */
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
System.out.println("The maximum between " + i +
" and " + j + " is " + k);
}
/** Return the max between two numbers */
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
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This would produce following result:
The maximum between 5 and 2 is 5
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This program contains the main method and the max method. The main method is just like any other method except that it is invoked by the JVM.
The main method's header is always the same, like the one in this example, with the modifiers public and static, return value type void, method name main, and a parameter of the String[] type. String[] indicates that the parameter is an array of String.
The void Keyword:
This section shows how to declare and invoke a void method. Following example gives a program that declares a method named printGrade and invokes it to print the grade for a given score.
Example:
public class TestVoidMethod {
public static void main(String[] args) {
printGrade(78.5);
}
public static void printGrade(double score) {
if (score >= 90.0) {
System.out.println('A');
}
else if (score >= 80.0) {
System.out.println('B');
}
else if (score >= 70.0) {
System.out.println('C');
}
else if (score >= 60.0) {
System.out.println('D');
}
else {
System.out.println('F');
}
}
}
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This would produce following result:
C
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Here the printGrade method is a void method. It does not return any value. A call to a void method must be a statement. So, it is invoked as a statement in line 3 in the main method. This statement is like any Java statement terminated with a semicolon.
Passing Parameters by Values:
When calling a method, you need to provide arguments, which must be given in the same order as their respective parameters in the method specification. This is known as parameter order association.
For example, the following method prints a message n times:
public static void nPrintln(String message, int n) {
for (int i = 0; i < n; i++)
System.out.println(message);
}
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Here, you can use nPrintln("Hello", 3) to print "Hello" three times. The nPrintln("Hello", 3) statement passes the actual string parameter, "Hello", to the parameter, message; passes 3 to n; and prints "Hello" three times. However, the statement nPrintln(3, "Hello") would be wrong.
When you invoke a method with a parameter, the value of the argument is passed to the parameter. This is referred to as pass-by-value. If the argument is a variable rather than a literal value, the value of the variable is passed to the parameter. The variable is not affected, regardless of the changes made to the parameter inside the method.
For simplicity, Java programmers often say passing an argument x to a parameter y, which actually means passing the value of x to y.
Example:
Following is a program that demonstrates the effect of passing by value. The program creates a method for swapping two variables. The swap method is invoked by passing two arguments. Interestingly, the values of the arguments are not changed after the method is invoked.
public class TestPassByValue {
public static void main(String[] args) {
int num1 = 1;
int num2 = 2;
System.out.println("Before swap method, num1 is " +
num1 + " and num2 is " + num2);
// Invoke the swap method
swap(num1, num2);
System.out.println("After swap method, num1 is " +
num1 + " and num2 is " + num2);
}
/** Method to swap two variables */
public static void swap(int n1, int n2) {
System.out.println("\tInside the swap method");
System.out.println("\t\tBefore swapping n1 is " + n1
+ " n2 is " + n2);
// Swap n1 with n2
int temp = n1;
n1 = n2;
n2 = temp;
System.out.println("\t\tAfter swapping n1 is " + n1
+ " n2 is " + n2);
}
}
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This would produce following result:
Before swap method, num1 is 1 and num2 is 2
Inside the swap method
Before swapping n1 is 1 n2 is 2
After swapping n1 is 2 n2 is 1
After swap method, num1 is 1 and num2 is 2
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The finalize( ) Method:
It is possible to define a method that will be called just before an object's final destruction by the garbage collector. This method is called finalize( ), and it can be used to ensure that an object terminates cleanly.
For example, you might use finalize( ) to make sure that an open file owned by that object is closed.
To add a finalizer to a class, you simply define the finalize( ) method. The Java runtime calls that method whenever it is about to recycle an object of that class.
Inside the finalize( ) method you will specify those actions that must be performed before an object is destroyed.
The finalize( ) method has this general form:
protected void finalize( )
{
// finalization code here
}
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Here, the keyword protected is a specifier that prevents access to finalize( ) by code defined outside its class.
This means that you cannot know when.or even if.finalize( ) will be executed. For example, if your program ends before garbage collection occurs, finalize( ) will not execute.