As you saw in the text about the Java main program, the execution of a Java program starts in the
main() method of a class. But classes are more than just the execution starting point of a Java program.
Java classes are a mechanism used to group data (variables) and Java code (methods) together into coherent "modules". A class typically contains:
- Fields
- Constructors
- Methods
Fields are variables (data) that are local to the class, or instances (object) of that class. I will get back to instances later.
Constructors are methods that initialize an instance of the class. Typically a constructor sets the values of fields in the given instance.
Methods are operations that the class or instances of that class can perform. For instance, a method may perform an operation on input parameters, or change the value of fields kept internally in the object etc.
Not all classes have both fields, constructors and methods. Sometimes you have classes that only contain fields (data), and sometimes you have classes that only contain methods (operations). It depends on what the class is supposed to do.
Defining a Class
All it takes to define a class in Java is this:
public class MyClass {
}
This defines a public class called
MyClass. The class has no fields, constructors or methods.
The next example shows a class which is to model a car. Therefore the class has named
Car and has three fields.public class Car {
public String brand = null;
public String model = null;
public String color = null;
}
This code defines a class called Car. Thus a Car object has three fields. The class has no methods. Only field declarations. Fields are described in more detail in the text on fields.
Let's add a method to the
Car class.public class Car {
public String brand = null;
public String model = null;
public String color = null;
public void setColor(String newColor) {
this.color = newColor;
}
}
In the class definition above I have added a
setColor() method. When called, this method sets the internal color variable to a new value. Methods are described in more detail in the text onmethods.Classes and Objects
A Java class is a template for how objects of that class looks. In other words, the
Car class in the previous section is a template for how Car objects look.
To create objects of a certain class, you use the
new keyword. Here is an example:Car car1 = new Car();
Car car2 = new Car();
Car car3 = new Car();
car1.setColor("red");
car2.setColor("green");
car3.setColor("blue");
This example creates 3
Car variables, and assign a new instance of Car to each variable. Each variable now references a Car object.
After creating the 3
Car objects, the setColor() method is called on each object. Now the color (represented as a text) is set individually for each Car object.
Creating an object of a certain class is also called "instantiating" an object. The object is thus also called an "instance" of the given class. For instance, each of the
Car objects above are also called an instance of the Car class, or simply "Car instances".
