How we create objects in java

Java Classes and Objects

Everything in Java is associated with classes and objects, along with its attributes and methods. For example: in real life, a car is an object. The car has attributes, such as weight and color, and methods, such as drive and brake.

A Class is like an object constructor, or a «blueprint» for creating objects.

Create a Class

To create a class, use the keyword class :

Main.java

Create a class named » Main » with a variable x:

Remember from the Java Syntax chapter that a class should always start with an uppercase first letter, and that the name of the java file should match the class name.

Create an Object

In Java, an object is created from a class. We have already created the class named Main , so now we can use this to create objects.

To create an object of Main , specify the class name, followed by the object name, and use the keyword new :

Example

Create an object called » myObj » and print the value of x:

Multiple Objects

You can create multiple objects of one class:

Example

Create two objects of Main :

Using Multiple Classes

You can also create an object of a class and access it in another class. This is often used for better organization of classes (one class has all the attributes and methods, while the other class holds the main() method (code to be executed)).

Remember that the name of the java file should match the class name. In this example, we have created two files in the same directory/folder:

Main.java

Second.java

When both files have been compiled:

You will learn much more about classes and objects in the next chapters.

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How to Create Object in Java?

Class provides a blueprint for an object while object is the implementation of this class.

An object has three characteristics:

• State: Represents the value stored in the object. Eg. height, weight, etc.
• Behavior: Represents functionality performed by an object. Eg. running, sitting, etc.
• Identity: Each object has an unique ID and JVM uses this ID to identify an object.

This article focuses on how to create object in java.

Different ways to Create Objects in Java

There are six ways to create objects in Java:

1. new keyword
2. newInstance() method of Class class
3. newInstance() method of Constructor class
4. clone() method
5. Deserialization in Java
6. Factory Method Pattern

Create Object in Java Using new Keyword

This is the most common way regarding how to create object in java. The new keyword calls constructor of the class implicitly. The constructor can be both parameterized or non-parameterized . The new keyword allocates memory for the object and returns a reference to that object.

Explanation

• The object obj is created using the default no-arg constructor, setting the name attribute to «Ayush» by default.
• The object obj1 is created using the parameterized constructor, passing the argument «Priya» , which assigns the value «Priya» to the name attribute.

Create Object in Java Using newInstance() method

To use this object creation method, you need to know the class name and ensure that the class has a public default constructor.

• Use Class.forName() to load the Java class.
• To create an object of the loaded class, utilize the newInstance() method, which returns an object of the class.

Explanation:

• Class.forName(«ObjectCreate») loads the class «ObjectCreate» and assigns the reference to c . Then, (ObjectCreate) c.newInstance() creates a new object of the loaded class.
• We use a try-catch block to handle errors, such as when the class does not exist.

Create Object in Java Using newInstance() method of Constructor Class

The newInstance() method of Constructor class is similar to newInstance() method of class Class except that we can pass parameters for different parameterized constructor.

Explanation:

• In getConstructor() method, we specify the data types of the required arguments to find the desired constructor.
• In newInstance() method, we provide the necessary arguments.
• The newInstance() method returns a newly created object by invoking the appropriate constructor identified through the getConstructor() method.

Create Object in Java Using clone() Method

• The clone() method generates a new object with a distinct hash code and stores it in a separate memory location.
• clone() produces a replica of an existing object with identical attribute values.
• clone() generates an object of a class without invoking any class constructors.
• Modifying the values of one object does not affect the other.

Prerequisite: For utilizing this method, the class must implement the Cloneable interface and define its own clone() method.

Hence, the concept of cloning can also help in how to create object in java.

Create Object in Java Using Deserialization Method

• An object is created by JVM when we serialize and then deserialize objects.
• No constructor is used to create an object in deserialization .

Prerequisite: The class whose object is to be created must implement the Serializable interface.

Explanation:

• Initially, we serialize the serializeObject object, converting it into a byte sequence.
• The serialized object is then saved in a file named «file.ser» using the writeObject() method of the ObjectOutputStream class.
• The readObject() function is employed to read an object from the ObjectInputStream class, storing it in deserializeObj .
• Lastly, we print the data from the deserializeObj object.

Create Object in Java Using Factory Method Pattern

• To create an object using the Factory method pattern, we must define an abstract class or interface. Then define different classes which implement this interface.
• Factory Method Pattern is also known as Virtual Constructor .
• This is used when a class either does not know which subclasses will be required to create or the class wants its sub-classes specify the objects to be created.

Explanation:

• Using the interface «Calculator» and the function Calculate() , we constructed a basic calculator.
• The calculator is capable of performing operations such as addition, subtraction, multiplication, etc.
• The Calculate() method is defined by distinct concrete classes that implement the «Calculator» interface.
• To generate objects of the classes that implement the interface, relying on user input, we define the «CalculatorFactory» class.
• Ultimately, by providing arguments like «ADD,» «SUBS,» and «MULTIPLY,» we acquire instances of the concrete classes. The appropriate class instance is generated based on the argument provided.

Conclusion

We explored different ways regarding how to create object in java:

• The new keyword is the most common way to create an object. new keyword can call both parameterized or non-parameterized constructors.
• The newInstance() method of class Class is used when the class name is known. Using this method, only no-arg constructor can be called to create objects.
• The newInstance() method of Constructor class is similar to newInstance() method of Class class execept it works with parameterized constructor too.
• The clone() method creates a new object by cloning the properties of an existing instance.
• JVM creates an object when we serialize and then deserialize an object.
• clone() method and deserialization do not use any constructor to create an object.
• Using factory methods, we can create objects of different classes based on certain parameters. In factory method pattern, a number of subclasses implement an interface or a abstract class.

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Creating Objects

As you know, a class provides the blueprint for objects; you create an object from a class. Each of the following statements taken from the CreateObjectDemo program creates an object and assigns it to a variable:

Point originOne = new Point(23, 94); Rectangle rectOne = new Rectangle(originOne, 100, 200); Rectangle rectTwo = new Rectangle(50, 100);

The first line creates an object of the Point class, and the second and third lines each create an object of the Rectangle class.

Each of these statements has three parts (discussed in detail below):

1. Declaration: The code set in bold are all variable declarations that associate a variable name with an object type.
2. Instantiation: The new keyword is a Java operator that creates the object.
3. Initialization: The new operator is followed by a call to a constructor, which initializes the new object.

Declaring a Variable to Refer to an Object

Previously, you learned that to declare a variable, you write:

This notifies the compiler that you will use name to refer to data whose type is type. With a primitive variable, this declaration also reserves the proper amount of memory for the variable.

You can also declare a reference variable on its own line. For example:

If you declare originOne like this, its value will be undetermined until an object is actually created and assigned to it. Simply declaring a reference variable does not create an object. For that, you need to use the new operator, as described in the next section. You must assign an object to originOne before you use it in your code. Otherwise, you will get a compiler error.

A variable in this state, which currently references no object, can be illustrated as follows (the variable name, originOne , plus a reference pointing to nothing):

Instantiating a Class

The new operator instantiates a class by allocating memory for a new object and returning a reference to that memory. The new operator also invokes the object constructor.

Note: The phrase «instantiating a class» means the same thing as «creating an object.» When you create an object, you are creating an «instance» of a class, therefore «instantiating» a class.

The new operator requires a single, postfix argument: a call to a constructor. The name of the constructor provides the name of the class to instantiate.

The new operator returns a reference to the object it created. This reference is usually assigned to a variable of the appropriate type, like:

Point originOne = new Point(23, 94); 

The reference returned by the new operator does not have to be assigned to a variable. It can also be used directly in an expression. For example:

int height = new Rectangle().height;

This statement will be discussed in the next section.

Initializing an Object

Here’s the code for the Point class:

This class contains a single constructor. You can recognize a constructor because its declaration uses the same name as the class and it has no return type. The constructor in the Point class takes two integer arguments, as declared by the code (int a, int b). The following statement provides 23 and 94 as values for those arguments:

Point originOne = new Point(23, 94);

The result of executing this statement can be illustrated in the next figure:

Here’s the code for the Rectangle class, which contains four constructors:

public class Rectangle < public int width = 0; public int height = 0; public Point origin; // four constructors public Rectangle() < origin = new Point(0, 0); >public Rectangle(Point p) < origin = p; >public Rectangle(int w, int h) < origin = new Point(0, 0); width = w; height = h; >public Rectangle(Point p, int w, int h) < origin = p; width = w; height = h; >// a method for moving the rectangle public void move(int x, int y) < origin.x = x; origin.y = y; >// a method for computing the area of the rectangle public int getArea() < return width * height; >>

Each constructor lets you provide initial values for the rectangle’s origin, width, and height, using both primitive and reference types. If a class has multiple constructors, they must have different signatures. The Java compiler differentiates the constructors based on the number and the type of the arguments. When the Java compiler encounters the following code, it knows to call the constructor in the Rectangle class that requires a Point argument followed by two integer arguments:

Rectangle rectOne = new Rectangle(originOne, 100, 200);

This calls one of Rectangle ‘s constructors that initializes origin to originOne . Also, the constructor sets width to 100 and height to 200. Now there are two references to the same Point object—an object can have multiple references to it, as shown in the next figure:

The following line of code calls the Rectangle constructor that requires two integer arguments, which provide the initial values for width and height. If you inspect the code within the constructor, you will see that it creates a new Point object whose x and y values are initialized to 0:

Rectangle rectTwo = new Rectangle(50, 100);

The Rectangle constructor used in the following statement doesn’t take any arguments, so it’s called a no-argument constructor:

Rectangle rect = new Rectangle();

All classes have at least one constructor. If a class does not explicitly declare any, the Java compiler automatically provides a no-argument constructor, called the default constructor. This default constructor calls the class parent’s no-argument constructor, or the Object constructor if the class has no other parent. If the parent has no constructor ( Object does have one), the compiler will reject the program.

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