final (Java)

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In the Java programming language, the final keyword is used in several different contexts to define an entity which cannot later be changed.

Contents

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 Template:Javadoc:SE and Template:Javadoc:SE. All methods in a final class are implicitly final.

Example:

public final class MyFinalClass {...}

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.[1]

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.[2]

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;
 
    Sphere(double x, double y, double z, double r) {
         radius = r;
         xpos = x;
         ypos = y;
         zpos = z;
    }
 
    [...]
}

Any attempt to reassign radius, xpos, ypos, zpos 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 an compile error.

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

    public final Position pos;

where pos is an object with three properties pos.x, pos.y and pos.z. Then pos cannot be assigned to, but the three properties can, unless they are final themselves.

Like full immutability, finality of variables has great advantages, especially in optimization. For instance, Sphere will probably have a function returning its volume; knowing that its radius is constant allows us to memoize the computed volume. If we have relatively few Spheres and we need their volumes very often, the gain might be substantial. Making the radius of a Sphere final informs developers and compilers that this sort of optimization is possible in all code that uses Spheres.

References

External links

uk:Final (Java) pl:Final

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