Classes and Objects

A class–the basic building block of an object-oriented language such as Java–is a template that describes the data and behavior associated with instances of that class. When you instantiate a class you create an object that looks and feels like other instances of the same class. The data associated with a class or object is stored in variables; the behavior associated with a class or object is implemented with methods. Methods are similar to the functions or procedures in procedural languages such as C.

In the Java language, the simplest form of a class definition is

class name {
    . . .
}

The keyword class begins the class definition for a class named name. The variables and methods of the class are embraced by the curly brackets that begin and end the class definition block. The “Hello World” application has no variables and has a single method named main.

Access level of class:

Java class has mainly two type of access level:

Default: class objects are accessible only inside the package.

Public: class objects are accessible in code in any package.

What are members of Class?

When we create a class its totally incomplete without defining any member of this class same like we can understand one family is incomplete if they have no members.

Field: field is nothing but the property of the class or object which we are going to create .for example if we are creating a class called computer then they have property like model, mem_size, hd_size, os_type etc

Method: method is nothing but the operation that an object can perform it define the behavior of object how an object can interact with outside world .startMethod (), shutdownMethod ().

Access Level of members: Access level is nothing but where we can use that members of the class.

Each field and method has an access level:

• private: accessible only in this class
• package or default: accessible only in this package
• protected: accessible only in this package and in all subclasses of this class
• public: accessible everywhere this class is available

Declaring Classes

You’ve seen classes defined in the following way:

class MyClass {
    // field, constructor, and 
    // method declarations
}

This is a class declaration. The class body (the area between the braces) contains all the code that provides for the life cycle of the objects created from the class: constructors for initializing new objects, declarations for the fields that provide the state of the class and its objects, and methods to implement the behavior of the class and its objects.

The preceding class declaration is a minimal one. It contains only those components of a class declaration that are required. You can provide more information about the class, such as the name of its superclass, whether it implements any interfaces, and so on, at the start of the class declaration. For example,

class MyClass extends MySuperClass implements YourInterface {
    // field, constructor, and
    // method declarations
}

means that MyClass is a subclass of MySuperClass and that it implements the YourInterface interface.

You can also add modifiers like public or private at the very beginning—so you can see that the opening line of a class declaration can become quite complicated. The modifiers public andprivate, which determine what other classes can access MyClass, are discussed later in this lesson. The lesson on interfaces and inheritance will explain how and why you would use the extendsand implements keywords in a class declaration. For the moment you do not need to worry about these extra complications.

In general, class declarations can include these components, in order:

1. Modifiers such as public, private, and a number of others that you will encounter later.
2. The class name, with the initial letter capitalized by convention.
3. The name of the class’s parent (superclass), if any, preceded by the keyword extends. A class can only extend (subclass) one parent.
4. A comma-separated list of interfaces implemented by the class, if any, preceded by the keyword implements. A class can implement more than one interface.
5. The class body, surrounded by braces, {}.

Declaring Member Variables

There are several kinds of variables:

• Member variables in a class—these are called fields.
• Variables in a method or block of code—these are called local variables.
• Variables in method declarations—these are called parameters.

The Bicycle class uses the following lines of code to define its fields:

public int cadence;
public int gear;
public int speed;

Field declarations are composed of three components, in order:

1. Zero or more modifiers, such as public or private.
2. The field’s type.
3. The field’s name.

The fields of Bicycle are named cadence, gear, and speed and are all of data type integer (int). The public keyword identifies these fields as public members, accessible by any object that can access the class.

Access Modifiers

The first (left-most) modifier used lets you control what other classes have access to a member field. For the moment, consider only public and private. Other access modifiers will be discussed later.

• public modifier—the field is accessible from all classes.
• private modifier—the field is accessible only within its own class.

In the spirit of encapsulation, it is common to make fields private. This means that they can only be directly accessed from the Bicycle class. We still need access to these values, however. This can be done indirectly by adding public methods that obtain the field values for us:

public class Bicycle {
        
    private int cadence;
    private int gear;
    private int speed;
        
    public Bicycle(int startCadence, int startSpeed, int startGear) {
        gear = startGear;
        cadence = startCadence;
        speed = startSpeed;
    }
        
    public int getCadence() {
        return cadence;
    }
        
    public void setCadence(int newValue) {
        cadence = newValue;
    }
        
    public int getGear() {
        return gear;
    }
        
    public void setGear(int newValue) {
        gear = newValue;
    }
        
    public int getSpeed() {
        return speed;
    }
        
    public void applyBrake(int decrement) {
        speed -= decrement;
    }
        
    public void speedUp(int increment) {
        speed += increment;
    }
}

Types

All variables must have a type. You can use primitive types such as int, float, boolean, etc. Or you can use reference types, such as strings, arrays, or objects.

Variable Names

All variables, whether they are fields, local variables, or parameters, follow the same naming rules and conventions that were covered in the Language Basics lesson, Variables—Naming.

In this lesson, be aware that the same naming rules and conventions are used for method and class names, except that

• the first letter of a class name should be capitalized, and
• the first (or only) word in a method name should be a verb.

Real world example of Class in Java Programming:

In real world if we want to understand about the class everything of same quality  can be visualize as a class e.g. men,women,birds ,bicycles ,cars or  we can say vehicle .

The entire vehicle will make one class they have the property like no_of_wheels, color, model, brand etc.now we can think changeGear () and speedOfvehicle (), applyBreak () etc as a method on that class. Similarly all human being also can be one class now their member will be a men ,women ,child.,isAlive() ,isDeath() can be their method or behavior of that class .again we can make Men or women a separate class and define their property and method accordingly,

In short in java every problem we get solution can be think in terms of class and object.

One Java class example:

Class Stock {

Public commodity;

Public price;

Public void buy (int no_of commodity) {}

Public boolean sale () {}

}

In this example Stock is called Class and commodity, price are field and buy() and sale() are two methods defined inside class. To access elements of Class you need to create an instance of class Stock. You can create instance of Class using keyword new as shown below

Stock highBetaStock = new Stock();

For calling method of Stock just call by using instance.

highBetaStock.buy(1000);

highBetaStock.sell();

Defining Methods

Here is an example of a typical method declaration:

public double calculateAnswer(double wingSpan, int numberOfEngines,
                              double length, double grossTons) {
    //do the calculation here
}

The only required elements of a method declaration are the method’s return type, name, a pair of parentheses, (), and a body between braces, {}.

More generally, method declarations have six components, in order:

1. Modifiers—such as public, private, and others you will learn about later.
2. The return type—the data type of the value returned by the method, or void if the method does not return a value.
3. The method name—the rules for field names apply to method names as well, but the convention is a little different.
4. The parameter list in parenthesis—a comma-delimited list of input parameters, preceded by their data types, enclosed by parentheses, (). If there are no parameters, you must use empty parentheses.
5. An exception list—to be discussed later.
6. The method body, enclosed between braces—the method’s code, including the declaration of local variables, goes here.

Modifiers, return types, and parameters will be discussed later in this lesson. Exceptions are discussed in a later lesson.

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Definition: Two of the components of a method declaration comprise the method signature—the method’s name and the parameter types.

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The signature of the method declared above is:

calculateAnswer(double, int, double, double)

Naming a Method

Although a method name can be any legal identifier, code conventions restrict method names. By convention, method names should be a verb in lowercase or a multi-word name that begins with a verb in lowercase, followed by adjectives, nouns, etc. In multi-word names, the first letter of each of the second and following words should be capitalized. Here are some examples:

run
runFast
getBackground
getFinalData
compareTo
setX
isEmpty

Typically, a method has a unique name within its class. However, a method might have the same name as other methods due to method overloading.

Overloading Methods

The Java programming language supports overloading methods, and Java can distinguish between methods with different method signatures. This means that methods within a class can have the same name if they have different parameter lists (there are some qualifications to this that will be discussed in the lesson titled “Interfaces and Inheritance”).

Suppose that you have a class that can use calligraphy to draw various types of data (strings, integers, and so on) and that contains a method for drawing each data type. It is cumbersome to use a new name for each method—for example, drawString, drawInteger, drawFloat, and so on. In the Java programming language, you can use the same name for all the drawing methods but pass a different argument list to each method. Thus, the data drawing class might declare four methods named draw, each of which has a different parameter list.

public class DataArtist {
    ...
    public void draw(String s) {
        ...
    }
    public void draw(int i) {
        ...
    }
    public void draw(double f) {
        ...
    }
    public void draw(int i, double f) {
        ...
    }
}

Overloaded methods are differentiated by the number and the type of the arguments passed into the method. In the code sample, draw(String s) and draw(int i) are distinct and unique methods because they require different argument types.

You cannot declare more than one method with the same name and the same number and type of arguments, because the compiler cannot tell them apart.

The compiler does not consider return type when differentiating methods, so you cannot declare two methods with the same signature even if they have a different return type.

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:

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.

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 to a constructor. This call initializes the new object.

Example of creating an object is given below:

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 would produce the following result:

Passed Name is :tommy

Using the this Keyword

Within an instance method or a constructor, this is a reference to the current object — the object whose method or constructor is being called. You can refer to any member of the current object from within an instance method or a constructor by using this.

Using this with a Field

The most common reason for using the this keyword is because a field is shadowed by a method or constructor parameter.

For example, the Point class was written like this

public class Point {
    public int x = 0;
    public int y = 0;
        
    //constructor
    public Point(int a, int b) {
        x = a;
        y = b;
    }
}

but it could have been written like this:

public class Point {
    public int x = 0;
    public int y = 0;
        
    //constructor
    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
}

Each argument to the constructor shadows one of the object’s fields — inside the constructor x is a local copy of the constructor’s first argument. To refer to the Point field x, the constructor must use this.x.

Using this with a Constructor

From within a constructor, you can also use the this keyword to call another constructor in the same class. Doing so is called an explicit constructor invocation. Here’s another Rectangleclass, with a different implementation from the one in the Objects section.

public class Rectangle {
    private int x, y;
    private int width, height;
        
    public Rectangle() {
        this(0, 0, 1, 1);
    }
    public Rectangle(int width, int height) {
        this(0, 0, width, height);
    }
    public Rectangle(int x, int y, int width, int height) {
        this.x = x;
        this.y = y;
        this.width = width;
        this.height = height;
    }
    ...
}

This class contains a set of constructors. Each constructor initializes some or all of the rectangle’s member variables. The constructors provide a default value for any member variable whose initial value is not provided by an argument. For example, the no-argument constructor creates a 1×1 Rectangle at coordinates 0,0. The two-argument constructor calls the four-argument constructor, passing in the width and height but always using the 0,0 coordinates. As before, the compiler determines which constructor to call, based on the number and the type of arguments.

If present, the invocation of another constructor must be the first line in the constructor.

 More on Classes

This section covers more aspects of classes that depend on using object references and the dot operator that you learned about in the preceding section: returning values from methods, thethis keyword, class vs. instance members, and access control.

Controlling Access to Members of a Class

Access level modifiers determine whether other classes can use a particular field or invoke a particular method. There are two levels of access control:

• At the top level—public, or package-private (no explicit modifier).
• At the member level—public, private, protected, or package-private (no explicit modifier).

A class may be declared with the modifier public, in which case that class is visible to all classes everywhere. If a class has no modifier (the default, also known as package-private), it is visible only within its own package (packages are named groups of related classes — you will learn about them in a later lesson.)

At the member level, you can also use the public modifier or no modifier (package-private) just as with top-level classes, and with the same meaning. For members, there are two additional access modifiers: private and protected. The private modifier specifies that the member can only be accessed in its own class. The protected modifier specifies that the member can only be accessed within its own package (as with package-private) and, in addition, by a subclass of its class in another package.

The following table shows the access to members permitted by each modifier.

Access Levels

Modifier	Class	Package	Subclass	World
public	Y	Y	Y	Y
protected	Y	Y	Y	N
no modifier	Y	Y	N	N
private	Y	N	N	N

The first data column indicates whether the class itself has access to the member defined by the access level. As you can see, a class always has access to its own members. The second column indicates whether classes in the same package as the class (regardless of their parentage) have access to the member. The third column indicates whether subclasses of the class declared outside this package have access to the member. The fourth column indicates whether all classes have access to the member.

Access levels affect you in two ways. First, when you use classes that come from another source, such as the classes in the Java platform, access levels determine which members of those classes your own classes can use. Second, when you write a class, you need to decide what access level every member variable and every method in your class should have.

Let’s look at a collection of classes and see how access levels affect visibility. The following figure shows the four classes in this example and how they are related.

Classes and Packages of the Example Used to Illustrate Access Levels

The following table shows where the members of the Alpha class are visible for each of the access modifiers that can be applied to them.

Visibility

Modifier	Alpha	Beta	Alphasub	Gamma
public	Y	Y	Y	Y
protected	Y	Y	Y	N
no modifier	Y	Y	N	N
private	Y	N	N	N

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Tips on Choosing an Access Level:If other programmers use your class, you want to ensure that errors from misuse cannot happen. Access levels can help you do this.

• Use the most restrictive access level that makes sense for a particular member. Use private unless you have a good reason not to.
• Avoid public fields except for constants. (Many of the examples in the tutorial use public fields. This may help to illustrate some points concisely, but is not recommended for production code.) Public fields tend to link you to a particular implementation and limit your flexibility in changing your code.

Nested Classes

Static nested classes, inner classes, anonymous inner classes, local classes, and lambda expressions are covered. There is also a discussion on when to use which approach.

When to Use Nested Classes, Local Classes, Anonymous Classes, and Lambda Expressions

As mentioned in the section Nested Classes, nested classes enable you to logically group classes that are only used in one place, increase the use of encapsulation, and create more readable and maintainable code. Local classes, anonymous classes, and lambda expressions also impart these advantages; however, they are intended to be used for more specific situations:

• Local Class: Use it if you need to create more than one instance of a class, access its constructor, or introduce a new, named type (because, for example, you need to invoke additional methods later).
• Anonymous class: Use it if you need to declare fields or additional methods.
• Lambda expression:
  • Use it if you are encapsulating a single unit of behavior that you want to pass to other code. For example, you would use a lambda expression if you want a certain action performed on each element of a collection, when a process is completed, or when a process encounters an error.
  • Use it if you need a simple instance of a functional interface and none of the preceding criteria apply (for example, you do not need a constructor, a named type, fields, or additional methods).
• Nested class: Use it if your requirements are similar to those of a local class, you want to make the type more widely available, and you don’t require access to local variables or method parameters.
  • Use a non-static nested class (or inner class) if you require access to an enclosing instance’s non-public fields and methods. Use a static nested class if you don’t require this access.

Enum Types

This section covers enumerations, specialized classes that allow you to define and use sets of constants.

An enum type is a special data type that enables for a variable to be a set of predefined constants. The variable must be equal to one of the values that have been predefined for it. Common examples include compass directions (values of NORTH, SOUTH, EAST, and WEST) and the days of the week.

Because they are constants, the names of an enum type’s fields are in uppercase letters.

In the Java programming language, you define an enum type by using the enum keyword. For example, you would specify a days-of-the-week enum type as:

public enum Day {
    SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
    THURSDAY, FRIDAY, SATURDAY 
}

You should use enum types any time you need to represent a fixed set of constants. That includes natural enum types such as the planets in our solar system and data sets where you know all possible values at compile time—for example, the choices on a menu, command line flags, and so on.

Here is some code that shows you how to use the Day enum defined above:

public class EnumTest {
    Day day;
    
    public EnumTest(Day day) {
        this.day = day;
    }
    
    public void tellItLikeItIs() {
        switch (day) {
            case MONDAY:
                System.out.println("Mondays are bad.");
                break;
                    
            case FRIDAY:
                System.out.println("Fridays are better.");
                break;
                         
            case SATURDAY: case SUNDAY:
                System.out.println("Weekends are best.");
                break;
                        
            default:
                System.out.println("Midweek days are so-so.");
                break;
        }
    }
    
    public static void main(String[] args) {
        EnumTest firstDay = new EnumTest(Day.MONDAY);
        firstDay.tellItLikeItIs();
        EnumTest thirdDay = new EnumTest(Day.WEDNESDAY);
        thirdDay.tellItLikeItIs();
        EnumTest fifthDay = new EnumTest(Day.FRIDAY);
        fifthDay.tellItLikeItIs();
        EnumTest sixthDay = new EnumTest(Day.SATURDAY);
        sixthDay.tellItLikeItIs();
        EnumTest seventhDay = new EnumTest(Day.SUNDAY);
        seventhDay.tellItLikeItIs();
    }
}

The output is:

Mondays are bad.
Midweek days are so-so.
Fridays are better.
Weekends are best.
Weekends are best.

Java programming language enum types are much more powerful than their counterparts in other languages. The enum declaration defines a class (called an enum type). The enum class body can include methods and other fields. The compiler automatically adds some special methods when it creates an enum. For example, they have a static values method that returns an array containing all of the values of the enum in the order they are declared. This method is commonly used in combination with the for-each construct to iterate over the values of an enum type. For example, this code from the Planet class example below iterates over all the planets in the solar system.

for (Planet p : Planet.values()) {
    System.out.printf("Your weight on %s is %f%n",
                      p, p.surfaceWeight(mass));
}

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Note: All enums implicitly extend java.lang.Enum. Because a class can only extend one parent (see Declaring Classes), the Java language does not support multiple inheritance of state (see Multiple Inheritance of State, Implementation, and Type), and therefore an enum cannot extend anything else.

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In the following example, Planet is an enum type that represents the planets in the solar system. They are defined with constant mass and radius properties.

Each enum constant is declared with values for the mass and radius parameters. These values are passed to the constructor when the constant is created. Java requires that the constants be defined first, prior to any fields or methods. Also, when there are fields and methods, the list of enum constants must end with a semicolon.

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Note: The constructor for an enum type must be package-private or private access. It automatically creates the constants that are defined at the beginning of the enum body. You cannot invoke an enum constructor yourself.

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In addition to its properties and constructor, Planet has methods that allow you to retrieve the surface gravity and weight of an object on each planet. Here is a sample program that takes your weight on earth (in any unit) and calculates and prints your weight on all of the planets (in the same unit):

public enum Planet {
    MERCURY (3.303e+23, 2.4397e6),
    VENUS   (4.869e+24, 6.0518e6),
    EARTH   (5.976e+24, 6.37814e6),
    MARS    (6.421e+23, 3.3972e6),
    JUPITER (1.9e+27,   7.1492e7),
    SATURN  (5.688e+26, 6.0268e7),
    URANUS  (8.686e+25, 2.5559e7),
    NEPTUNE (1.024e+26, 2.4746e7);

    private final double mass;   // in kilograms
    private final double radius; // in meters
    Planet(double mass, double radius) {
        this.mass = mass;
        this.radius = radius;
    }
    private double mass() { return mass; }
    private double radius() { return radius; }

    // universal gravitational constant  (m3 kg-1 s-2)
    public static final double G = 6.67300E-11;

    double surfaceGravity() {
        return G * mass / (radius * radius);
    }
    double surfaceWeight(double otherMass) {
        return otherMass * surfaceGravity();
    }
    public static void main(String[] args) {
        if (args.length != 1) {
            System.err.println("Usage: java Planet <earth_weight>");
            System.exit(-1);
        }
        double earthWeight = Double.parseDouble(args[0]);
        double mass = earthWeight/EARTH.surfaceGravity();
        for (Planet p : Planet.values())
           System.out.printf("Your weight on %s is %f%n",
                             p, p.surfaceWeight(mass));
    }
}

If you run Planet.class from the command line with an argument of 175, you get this output:

$ java Planet 175
Your weight on MERCURY is 66.107583
Your weight on VENUS is 158.374842
Your weight on EARTH is 175.000000
Your weight on MARS is 66.279007
Your weight on JUPITER is 442.847567
Your weight on SATURN is 186.552719
Your weight on URANUS is 158.397260
Your weight on NEPTUNE is 199.207413