Objects initialization in java

Initializing Fields

As you have seen, you can often provide an initial value for a field in its declaration:

public class BedAndBreakfast < // initialize to 10 public static int capacity = 10; // initialize to false private boolean full = false; >

This works well when the initialization value is available and the initialization can be put on one line. However, this form of initialization has limitations because of its simplicity. If initialization requires some logic (for example, error handling or a for loop to fill a complex array), simple assignment is inadequate. Instance variables can be initialized in constructors, where error handling or other logic can be used. To provide the same capability for class variables, the Java programming language includes static initialization blocks.

Note: It is not necessary to declare fields at the beginning of the class definition, although this is the most common practice. It is only necessary that they be declared and initialized before they are used.

Static Initialization Blocks

A static initialization block is a normal block of code enclosed in braces, < >, and preceded by the static keyword. Here is an example:

A class can have any number of static initialization blocks, and they can appear anywhere in the class body. The runtime system guarantees that static initialization blocks are called in the order that they appear in the source code.

There is an alternative to static blocks — you can write a private static method:

The advantage of private static methods is that they can be reused later if you need to reinitialize the class variable.

Initializing Instance Members

Normally, you would put code to initialize an instance variable in a constructor. There are two alternatives to using a constructor to initialize instance variables: initializer blocks and final methods.

Initializer blocks for instance variables look just like static initializer blocks, but without the static keyword:

The Java compiler copies initializer blocks into every constructor. Therefore, this approach can be used to share a block of code between multiple constructors.

A final method cannot be overridden in a subclass. This is discussed in the lesson on interfaces and inheritance. Here is an example of using a final method for initializing an instance variable:

This is especially useful if subclasses might want to reuse the initialization method. The method is final because calling non-final methods during instance initialization can cause problems.

Previous page: Understanding Class Members
Next page: Summary of Creating and Using Classes and Objects

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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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Object Declaration and Initialization in Java

In this tutorial, we will learn object declaration and initialization in java with the help of examples.

We will also learn different ways to initialize value or data of the state of object inside a class. We will cover the following topics under this chapter.

• Object declaration in Java
• Object initialization in Java
• How to initialize state of an object in Java

So, let’s understand each topic one by one.

Object Declaration in Java

The process of defining a variable along with its data type and name is called the declaration of state of an object. It is also called declaration of variable.

For example, if we declare name and city as variables with data type String, these variables are called instance variables in Java.

Object Initialization in Java

The process of assigning a value of the variable is called initialization of state of an object. In other words, initialization is the process of storing data into an object.

In the below example, we have initialized variables name and city with “PIET” and “Nagpur” respectively.

How to Initialize State of Object in Java?

There are three ways by which we can initialize state of an object. In other words, we can initialize the value of variables in Java by using three ways. They are as follows:

Let’s see one by one with example programs step by step.

Object Initialization by using Constructor

A constructor in Java is a block of code within a class that is used to initialize objects of class. In other words, a constructor is used to initializing the value of variables.

The constructor should not have any return type even void also because if there is return type then JVM would consider as a method, not a constructor.

Let’s create a Java program in which we will store data into an object using constructor.

package objectPrograms; public class Student < // Step 1: Declaration of instance variables i.e state of objects. String name; int rollNo; int age; // Step 2: Declaration of a default constructor. The constructor name must be the same as the class name. Student() < // Step 3: Values initialization of state of objects i.e. values of variables. name = "Shubh"; rollNo = 05; age = 22; >// Step 4: Declare an instance method and print values of instance variables. void display() < System.out.println("Student's name:" +name); // Since this is an instance area. That's why, we can directly call the instance variables. System.out.println("Student's roll no: " +rollNo); System.out.println("Student's age:" +age); >// Step 5: Declare the main method. It is a static method. So, it is a static area. public static void main(String[] args) < // Step 6: Create an object of the class. Student st = new Student(); // It will call the default constructor. // Step 7: Now, call display method using object reference variable st because we cannot call non-static members directly in the static region. st.display(); >>

Output: Student's name: Shubh Student's roll no: 5 Student's age:22

Object Initialization by using Reference variable

We can also initialize the value of objects through the reference variable. So, let’s make a program where we will initialize value of variables using object reference variable.

package objectPrograms; public class Marks < // Declare instance variables. This is an instance area or region. String subject1; int sub1Marks; String subject2; int sub2Marks; // Declare main method. This is a static region. public static void main(String[] args) < // Create an object of the class. Marks mk = new Marks(); // Initialize values of objects through object reference variable. mk.subject1 = "Science"; mk.sub1Marks = 90; mk.subject2 = "Maths"; mk.sub2Marks = 99; // Now, we are adding total marks with data type int. int totalMarks = 90 + 99; // Now call marks using object reference variable and print values. System.out.println("Marks in Science:" +mk.sub1Marks); System.out.println("Marks in Maths:" +mk.sub2Marks); System.out.println("Total Marks: " +totalMarks); >>

Output: Marks in Science: 90 Marks in Maths: 99 Total Marks: 189

In the preceding example program, we have called variables using object reference variable “mk” and initialized them.

Object Initialization by using Method

A method in java is a set of code used to write the logic of application which performs some specific task or operation. When a method is called, it returns value to the caller.

It can also perform a task without returning any value. It can be called from anywhere. Therefore, we can initialize value of an object by using method.

Let’s create a Java program in which we will create two objects of the Rectangle class and initialize value to these objects by calling perValue() method.

Besides it, we will display state (data/value) of the objects by calling calculate() method using object reference variable because we cannot call directly non-static member into the static region.

package objectPrograms; public class Rectangle < int length; int breadth; // Declare an instance method and initialize parameters l and b with data type int. void perValue(int l, int b) < // Here, we are setting name of parameter differently from name of the variable // because we are not using this reference. length = l; breadth = b; >void calculate() < int perimeter = 2*(length + breadth); System.out.println("Perimeter of the Rectangle:" +perimeter); >public static void main(String[] args) < // Create the first object of the class. Rectangle rt = new Rectangle(); rt.perValue(20,30); // It will call perValue method and initialize values. // After that, it will be automatically initialized to the instance variables. rt.calculate(); // it will call calculate() method to display the output. // Create the second object of the class. Rectangle rt2 = new Rectangle(); rt2.perValue(50,50); rt2.calculate(); >>

Output: Perimeter of the Rectangle:100 Perimeter of the Rectangle:200

In this example program, we will calculate square and perimeter of a value 45 but we will initialize value of variable inside the constructor.

We will declare two methods calArea() and calPerimeter() where we will write logic for calculation of area and perimeter. Look at the following source code.

package squareProgram; public class Square < int side; Square() < side = 45; // Initialization of value of the instance variable. >void calArea() < int area = side * side; System.out.println("Area: " +area); >void calPerimeter() < int perimeter = 4 * side; System.out.println("Perimeter: " +perimeter); >public static void main(String[] args) < Square sq = new Square(); sq.calArea(); sq.calPerimeter(); >>

Output: Area: 2025 Perimeter: 180

In all the above example programs, we declared the main() method inside class. Now, let’s see another example in which we will define the main() method outside class.

It is a better approach than previous example because we create several classes in real-time project development and use it from another class. So, let’s understand the following source code.

package circleProgram; public class Circle < int radius; void area(int r) < radius = r; >void calArea() < double area = 3.14 * radius * radius; System.out.println(" Area of circle: " + area); >void calCircumference() < double circum = 2 * 3.14 * radius; System.out.println("Circumference of circle: " +circum); >>

package circleProgram; public class CircleTest < // Declaration of main method outside the class. public static void main(String[] args) < Circle cr = new Circle(); cr.area(30); cr.calArea(); cr.calCircumference(); >>

Output: Area of circle: 2826.0 Circumference of circle: 188.4

1. Declaration of a variable with data type and name is called declaration of state of an object (declaration of variable).

2. Assigning value to a variable is called initialization of state of an object. Initialization of variable means storing data into an object.

3. We can assign the value of variables in three ways: by using constructor, reference variable, and method.

In this tutorial, you learned object declaration and initialization in Java with various example programs. Hope that you have understood the basic concepts of object declaration and initialization.
Thanks for reading.
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