What is an object state in java

What Is an Object?

Objects are key to understanding object-oriented technology. Look around right now and you’ll find many examples of real-world objects: your dog, your desk, your television set, your bicycle.

Real-world objects share two characteristics: They all have state and behavior. Dogs have state (name, color, breed, hungry) and behavior (barking, fetching, wagging tail). Bicycles also have state (current gear, current pedal cadence, current speed) and behavior (changing gear, changing pedal cadence, applying brakes). Identifying the state and behavior for real-world objects is a great way to begin thinking in terms of object-oriented programming.

Take a minute right now to observe the real-world objects that are in your immediate area. For each object that you see, ask yourself two questions: «What possible states can this object be in?» and «What possible behavior can this object perform?». Make sure to write down your observations. As you do, you’ll notice that real-world objects vary in complexity; your desktop lamp may have only two possible states (on and off) and two possible behaviors (turn on, turn off), but your desktop radio might have additional states (on, off, current volume, current station) and behavior (turn on, turn off, increase volume, decrease volume, seek, scan, and tune). You may also notice that some objects, in turn, will also contain other objects. These real-world observations all translate into the world of object-oriented programming.

Software objects are conceptually similar to real-world objects: they too consist of state and related behavior. An object stores its state in fields (variables in some programming languages) and exposes its behavior through methods (functions in some programming languages). Methods operate on an object’s internal state and serve as the primary mechanism for object-to-object communication. Hiding internal state and requiring all interaction to be performed through an object’s methods is known as data encapsulation — a fundamental principle of object-oriented programming.

Consider a bicycle, for example:

A bicycle modeled as a software object.

By attributing state (current speed, current pedal cadence, and current gear) and providing methods for changing that state, the object remains in control of how the outside world is allowed to use it. For example, if the bicycle only has 6 gears, a method to change gears could reject any value that is less than 1 or greater than 6.

Bundling code into individual software objects provides a number of benefits, including:

1. Modularity: The source code for an object can be written and maintained independently of the source code for other objects. Once created, an object can be easily passed around inside the system.
2. Information-hiding: By interacting only with an object’s methods, the details of its internal implementation remain hidden from the outside world.
3. Code re-use: If an object already exists (perhaps written by another software developer), you can use that object in your program. This allows specialists to implement/test/debug complex, task-specific objects, which you can then trust to run in your own code.
4. Pluggability and debugging ease: If a particular object turns out to be problematic, you can simply remove it from your application and plug in a different object as its replacement. This is analogous to fixing mechanical problems in the real world. If a bolt breaks, you replace it, not the entire machine.

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Java — Object and Classes

In this chapter, we will look into the concepts — Classes and Objects.

• Object − Objects have states and behaviors. Example: A dog has states — color, name, breed as well as behaviors – wagging the tail, barking, eating. An object is an instance of a class.
• Class − A class can be defined as a template/blueprint that describes the behavior/state that the object of its type support.

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 a behavior.

If we consider a dog, then its state is — name, breed, color, and the behavior is — barking, wagging the tail, running.

If you compare the software object with a real-world object, they have very similar characteristics.

Software objects also have a state and a 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 blueprint from which individual objects are created.

Following is a sample of a class.

Example

public class Dog < String breed; int age; String color; void barking() < >void hungry() < >void sleeping() < >>

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 initialized when the class is instantiated. Instance variables can be accessed from inside any method, constructor or blocks of that particular class.
• Class variables − Class variables are variables declared within a class, outside any method, with the static keyword.

A class can have any number of methods to access the value of various kinds of methods. In the above example, barking(), hungry() and sleeping() are methods.

Following are some of the important topics that need to be discussed when looking into classes of the Java Language.

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.

Following is an example of a constructor −

Example

public class Puppy < public Puppy() < >public Puppy(String name) < // This constructor has one parameter, name. > >

Java also supports Singleton Classes where you would be able to create only one instance of a class.

Note − We have two different types of constructors. We are going to discuss constructors in detail in the subsequent chapters.

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 keyword 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’ keyword is used to create the object.
• Initialization − The ‘new’ keyword is followed by a call to a constructor. This call initializes the new object.

Following is an example of creating an object −

Example

public 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" ); >>

If we compile and run the above program, then it will produce the following result −

Output

Accessing Instance Variables and Methods

Instance variables and methods are accessed via created objects. To access an instance variable, following is the fully qualified path −

/* 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();

Example

This example explains how to access instance variables and methods of a class.

public class Puppy < int puppyAge; public Puppy(String name) < // This constructor has one parameter, name. System.out.println("Name chosen is :" + name ); > public void setAge( int age ) < puppyAge = age; >public int 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 ); >>

If we compile and run the above program, then it will produce the following result −

Output

Name chosen is :tommy Puppy's age is :2 Variable Value :2

Source File Declaration Rules

As the last part of this section, let’s now look into the source file declaration rules. These rules are essential when declaring classes, import statements and package statements in a source file.

• There can be only one public class per source file.
• A source file can have multiple non-public classes.
• The public class name should be the name of the source file as well which should be appended by .java at the end. For example: the class name is public class Employee<> then the source file should be as Employee.java.
• If the class is defined inside a package, then the package statement should be the first statement in the source file.
• If import statements are present, then they must be written between the package statement and the class declaration. If there are no package statements, then the import statement should be the first line in the source file.
• Import and package statements will imply to all the classes present in the source file. It is not possible to declare different import and/or package statements to different classes in the source file.

Classes have several access levels and there are different types of classes; abstract classes, final classes, etc. We will be explaining about all these in the access modifiers chapter.

Apart from the above mentioned types of classes, Java also has some special classes called Inner classes and Anonymous classes.

Java Package

In simple words, it is a way of categorizing the classes and interfaces. When developing applications in Java, hundreds of classes and interfaces will be written, therefore categorizing these classes is a must as well as makes life much easier.

Import Statements

In Java if a fully qualified name, which includes the package and the class name is given, then the compiler can easily locate the source code or classes. Import statement is a way of giving the proper location for the compiler to find that particular class.

For example, the following line would ask the compiler to load all the classes available in directory java_installation/java/io −

A Simple Case Study

For our case study, we will be creating two classes. They are Employee and EmployeeTest.

First open notepad and add the following code. Remember this is the Employee class and the class is a public class. Now, save this source file with the name Employee.java.

The Employee class has four instance variables — name, age, designation and salary. The class has one explicitly defined constructor, which takes a parameter.

Example

import java.io.*; public class Employee < String name; int age; String designation; double salary; // This is the constructor of the class Employee public Employee(String name) < this.name = name; >// Assign the age of the Employee to the variable age. public void empAge(int empAge) < age = empAge; >/* Assign the designation to the variable designation.*/ public void empDesignation(String empDesig) < designation = empDesig; >/* Assign the salary to the variable salary.*/ public void empSalary(double empSalary) < salary = empSalary; >/* Print the Employee details */ public void printEmployee() < System.out.println("Name:"+ name ); System.out.println("Age:" + age ); System.out.println("Designation:" + designation ); System.out.println("Salary:" + salary); >>

As mentioned previously in this tutorial, processing starts from the main method. Therefore, in order for us to run this Employee class there should be a main method and objects should be created. We will be creating a separate class for these tasks.

Following is the EmployeeTest class, which creates two instances of the class Employee and invokes the methods for each object to assign values for each variable.

Save the following code in EmployeeTest.java file.

import java.io.*; public class EmployeeTest < public static void main(String args[]) < /* Create two objects using constructor */ Employee empOne = new Employee("James Smith"); Employee empTwo = new Employee("Mary Anne"); // Invoking methods for each object created empOne.empAge(26); empOne.empDesignation("Senior Software Engineer"); empOne.empSalary(1000); empOne.printEmployee(); empTwo.empAge(21); empTwo.empDesignation("Software Engineer"); empTwo.empSalary(500); empTwo.printEmployee(); >>

Now, compile both the classes and then run EmployeeTest to see the result as follows −

Output

C:\> javac Employee.java C:\> javac EmployeeTest.java C:\> java EmployeeTest Name:James Smith Age:26 Designation:Senior Software Engineer Salary:1000.0 Name:Mary Anne Age:21 Designation:Software Engineer Salary:500.0

What is Next?

In the next session, we will discuss the basic data types in Java and how they can be used when developing Java applications.

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