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final (Java) 120
final (Java)
In the Java programming language, the final keyword is used in several different contexts to define an entity which
cannot later be changed.
Final classes
A final class cannot be subclassed. This is done for reasons of security and efficiency. Accordingly, many of the
Java standard library classes are final, for example java.lang.System and java.lang.String. All methods in a final
class are implicitly final.
Example:
public final class MyFinalClass {...}
Restricted subclassers are often referred to as "soft final" classes. [1]
Final methods
A final method cannot be overridden by subclasses. This is used to prevent unexpected behavior from a subclass
altering a method that may be crucial to the function or consistency of the class. [2]
Example:
public class MyClass {
}
public final void myFinalMethod() {...}
A common misconception is that declaring a class or method final improves efficiency by allowing the compiler to
directly insert the method inline wherever it is called. This is not completely true; the compiler is unable to do this
because the classes loaded at runtime might not be the same versions of the ones that were just compiled. Further,
the runtime environment and JIT compiler have the information about exactly what classes have been loaded, and
are able to make better decisions about when to inline, whether or not the method is final. [3]
Final variables
A final variable can only be assigned once. This assignment does not grant the variable immutable status. If the
variable is a field of a class, it must be assigned in the constructor of its class. (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, if it was originally mutable.)
Unlike the value of a constant, the value of a final variable is not necessarily known at compile time.
Example:
public class Sphere {
public static final double PI = 3.141592653589793; // this is
essentially a constant
public final double radius;
public final double xpos;
public final double ypos;
public final double zpos;