In all sample programs that you have seen, the main method is tagged with the static modifier. We are now ready to discuss the meaning of this modifier.
1. Static Fields
If you define a field as static, then there is only one such field per class. In contrast, each object has its own copy of nonstatic instance fields. For example, let’s suppose we want to assign a unique identification number to each employee. We add an instance field id and a static field nextId to the Employee class:
class Employee
{
private static int nextId = 1;
private int id;
…
}
Every Employee object now has its own id field, but there is only one nextId field that is shared among all instances of the class. Let’s put it another way. If there are 1,000 objects of the Employee class, then there are 1,000 instance fields id, one for each object. But there is a single static field nextId. Even if there are no Employee objects, the static field nextId is present. It belongs to the class, not to any individual object.
Let’s implement a simple method:
public void setId()
{
id = nextId;
nextId++;
}
Suppose you set the employee identification number for harry:
harry.setId();
Then, the id field of harry is set to the current value of the static field nextId, and the value of the static field is incremented:
harry.id = Employee.nextId;
Employee.nextId++;
2. Static Constants
Static variables are quite rare. However, static constants are more common. For example, the Math class defines a static constant:
public class Math
{
…
public static final double PI = 3.14159265358979323846;
…
}
You can access this constant in your programs as Math.PI.
If the keyword static had been omitted, then PI would have been an instance field of the Math class. That is, you would need an object of this class to access PI, and every Math object would have its own copy of PI.
Another static constant that you have used many times is System.out. It is declared in the System class as follows:
public class System
{
…
public static final PrintStream out = . . .;
…
}
As we mentioned several times, it is never a good idea to have public fields, because everyone can modify them. However, public constants (that is, final
fields) are fine. Since out has been declared as final, you cannot reassign another print stream to it:
System.out = new PrintStream(. . .); // ERROR–out is final
3. Static Methods
Static methods are methods that do not operate on objects. For example, the pow method of the Math class is a static method. The expression
Math.pow(x, a)
computes the power xa. It does not use any Math object to carry out its task. In other words, it has no implicit parameter.
You can think of static methods as methods that don’t have a this parameter. (In a nonstatic method, the this parameter refers to the implicit parameter of the method—see Section 4.3.7, “Implicit and Explicit Parameters,” on p. 150.)
A static method of the Employee class cannot access the id instance field because it does not operate on an object. However, a static method can access a static field. Here is an example of such a static method:
public static int getNextId()
{
return nextId; // returns static field
}
To call this method, you supply the name of the class:
int n = Employee.getNextId();
Could you have omitted the keyword static for this method? Yes, but then you would need to have an object reference of type Employee to invoke the method.
Use static methods in two situations:
- When a method doesn’t need to access the object state because all needed parameters are supplied as explicit parameters (example: Math.pow).
- When a method only needs to access static fields of the class (example: Employee.getNextId).
4. Factory Methods
Here is another common use for static methods. Classes such as LocalDate and NumberFormat use static factory methods that construct objects. You have already seen the factory methods LocalDate.now and LocalDate.of. Here is how the NumberFormat class yields formatter objects for various styles:
NumberFormat currencyFormatter = NumberFormat.getCurrencyInstance();
NumberFormat percentFormatter = NumberFormat.getPercentInstance();
double x = 0.1;
System.out.println(currencyFormatter.format(x)); // prints $0.10
System.out.println(percentFormatter.format(x)); // prints 10%
Why doesn’t the NumberFormat class use a constructor instead? There are two reasons:
- You can’t give names to constructors. The constructor name is always the same as the class name. But we want two different names to get the currency instance and the percent instance.
- When you use a constructor, you can’t vary the type of the constructed object. But the factory methods actually return objects of the class DecimatFormat, a subclass that inherits from NumberFormat. (See Chapter 5 for more on inheritance.)
5. The main Method
Note that you can call static methods without having any objects. For example, you never construct any objects of the Math class to call Math.pow.
For the same reason, the main method is a static method.
public class Application
{
public static void main(String[] args)
{
// construct objects here
…
}
}
The main method does not operate on any objects. In fact, when a program starts, there aren’t any objects yet. The static main method executes, and constructs the objects that the program needs.
The program in Listing 4.3 contains a simple version of the Employee class with a static field nextId and a static method getNextId. We fill an array with three Employee objects and then print the employee information. Finally, we print the next available identification number, to demonstrate the static method.
Note that the Employee class also has a static main method for unit testing. Try running both
java Employee
and
java StaticTest
to execute both main methods.
Source: Horstmann Cay S. (2019), Core Java. Volume I – Fundamentals, Pearson; 11th edition.