Final (Java): Difference between revisions

Revision as of 16:00, 15 May 2013

Revision as of 01:13, 30 May 2013

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A '''final [[Variable (programming)|variable]]''' can only be initialized once, either via an initializer or an assignment statement. It does not need to be initialized at the point of declaration: this is called a "blank final" variable. A blank final instance variable of a class must be definitely assigned at the end of every constructor of the class in which it is declared; similarly, a blank final static variable must be definitely assigned in a static initializer of the class in which it is declared; otherwise, a compile-time error occurs in both cases.<ref>Java Language Specification #8.3.1.2.</ref> (Note: If the variable is a reference, this means that the variable cannot be re-bound to reference another object. But the object that it references is still [[mutable object|mutable]], if it was originally mutable.)

Unlike the value of a [[constant (computer science)|constant]], the value of a final variable is not necessarily known at compile time. It is considered good practice to represent final [[primitive type]] variablesconstants in all uppercase, using underscore to separate words.<ref>http://geosoft.no/development/javastyle.html</ref>

Example:

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public static final double PI = 3.141592653589793;

public final double RADIUSradius;

public final double XPOSxPos;

public final double YPOSyPos;

public final double ZPOSzPos;

Sphere(double x, double y, double z, double r) {

RADIUSradius = r;

XPOSxPos = x;

YPOSyPos = y;

ZPOSzPos = z;

}

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</source>

Any attempt to reassign <code>RADIUSradius</code>, <code>XPOSxPos</code>, <code>YPOSyPos</code>, or <code>ZPOSzPos</code> will meet with a compile error. In fact, even if the constructor doesn't set a final variable, attempting to set it outside the constructor will result in a compilation error.

To illustrate that finality doesn't guarantee immutability: suppose we replace the three position variables with a single one: