In this section, you will learn about the basics of reference types. You learn to create variables that work with objects. > You will learn more about classes and objects later in the book.
Java programs are built with classes. You can use classes to create objects.
A reference type is a type which references an object.
The class can be any class. It can either be a part of the Java API or a class that you create.
When you create an object, you allocate memory to store the object. Here’s an example of how you can create an object.
new Object()
When you store the object in a variable, you assign a reference to the object. You don’t store the object itself.
A reference is the location of an object in memory. References are similar to pointers in C or C++. Unlike pointers, references are simple and easy to use.
For example, think that you’re writing a program which involves cars.
You have a class named Car which defines the behavior of a car.
You can declare a variable that stores a reference to a Car object like
this.
Car car;
Here, the variable car is of type Car. Remember that Java is case sensitive.
Which means Car with an uppercase C and car with a lowercase c are different.
To create a new instance of a class, you can use the new keyword. You have
to tell new the class whose instance you want.
When you create an instance of a class, you actually call a constructor. The constructor initializes the new object.
You don’t have to understand these concepts right away. You will understand them when you learn about them in detail.
Here’s an example where we create an instance of the class Car.
new Car()
You can assign a reference to the new object.
Car car;
car = new Car();
You can combine both these statements like this.
Car car = new Car();
Remember that a reference variable doesn’t actually store an object. It stores only the reference to an object in memory. As a result, two or more variables can refer to the same object.
Here’s an example where two variables refer to the same Car object.
Car carX = new Car();
Car carY = carX;
Here, we’ve declared two Car variables, named carX and carY. But we’ve
created only one Car object.
We first created a Car object and assigned its reference to carX. Then
the same reference is assigned to carY. This way, both carX and carY
refer to the same object.