Monday, December 21, 2009

Static in java

Static Members
Static members are those that belong to a class as a whole and not to a particular instance (object). A static variable is initialized when the class is loaded. Similarly, a class can have static methods. Static variables and static methods are collectively known as static members, and are declared with a keyword static. Static members in the class can be accessed either by using the class name or by using the object reference, but instance members can only be accessed via object references.

  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 the Math 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:
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 ok. Because out has been declared as final, you cannot reassign another print stream to it:
System.out = new PrintStream(. . .); // ERROR--out is final

If you look at the System class, you will notice a method setOut that lets you set System.out to a different stream. You may wonder how that method can change the value of a final variable. However, the setOut method is a native method, not implemented in the Java programming language. Native methods can bypass the access control mechanisms of the Java language. This is a very unusual workaround that you should not emulate in your own programs.

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 x^a. 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 non-static method, the this parameter refers to the implicit parameter of the method)

Because static methods don't operate on objects, you cannot access instance fields from a static method. But static methods can access the static fields in their class. 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.

It is legal to use an object to call a static method. For example, if harry is an Employee object, then you can call harry.getNextId() instead of Employee.getnextId(). However, we find that notation confusing. The getNextId method doesn't look at harry at all to compute the result. We recommend that you use class names, not objects, to invoke static methods.

You 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).

Also see difference between java and c++ static.
Points to note about static.

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