Creating classes and objects in java

Summary of Creating and Using Classes and Objects

A class declaration names the class and encloses the class body between braces. The class name can be preceded by modifiers. The class body contains fields, methods, and constructors for the class. A class uses fields to contain state information and uses methods to implement behavior. Constructors that initialize a new instance of a class use the name of the class and look like methods without a return type.

You control access to classes and members in the same way: by using an access modifier such as public in their declaration.

You specify a class variable or a class method by using the static keyword in the member’s declaration. A member that is not declared as static is implicitly an instance member. Class variables are shared by all instances of a class and can be accessed through the class name as well as an instance reference. Instances of a class get their own copy of each instance variable, which must be accessed through an instance reference.

You create an object from a class by using the new operator and a constructor. The new operator returns a reference to the object that was created. You can assign the reference to a variable or use it directly.

Instance variables and methods that are accessible to code outside of the class that they are declared in can be referred to by using a qualified name. The qualified name of an instance variable looks like this:

objectReference.variableName 

The qualified name of a method looks like this:

objectReference.methodName(argumentList) 

objectReference.methodName() 

The garbage collector automatically cleans up unused objects. An object is unused if the program holds no more references to it. You can explicitly drop a reference by setting the variable holding the reference to null .

Источник

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.

Источник

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.

Источник