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Although final can be used to specify immutable constants, there is a caveat when dealing with composite objects. See OBJ50-J. Never confuse immutability of a reference with that of the referenced object for more details.

Noncompliant Code Example

This noncompliant code example calculates approximate dimensions of a sphere, given its radius:

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The methods use the seemingly arbitrary literals 3.14, 4.19, and 6.28 to represent various scaling factors used to calculate these dimensions. A developer or maintainer reading this code would have little idea about how they were generated or what they mean and consequently would not understand the function of this code.

Noncompliant Code Example

This noncompliant code example attempts to avoid the problem by explicitly calculating the required constants:

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The code uses the literal 3.14 to represent the value π. Although it removes some of the ambiguity from the literals, it complicates code maintenance. If the programmer were to decide that a more precise value of π is desired, all occurrences of 3.14 in the code would have to be found and replaced.

Compliant Solution (Constants)

In this compliant solution, a constant PI is declared and initialized to 3.14. Thereafter, it is referenced in the code whenever the value of π is needed.

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This technique reduces clutter and promotes maintainability. If a more precise approximation of the value of π is required, the programmer can simply redefine the constant.

Compliant Solution (Predefined Constants)

The class java.lang.Math defines a large group of numeric constants, including PI and the exponential constant E. Use predefined constants when they are available.

double area(double radius) {
  return Math.PI * radius * radius;
}

double volume(double radius) {
  return 4.0/3.0 * Math.PI * radius * radius * radius;
}

double greatCircleCircumference(double radius) {
  return 2 * Math.PI * radius;
}

Noncompliant Code Example

This noncompliant code example defines a constant BUFSIZE, but then defeats the purpose of defining BUFSIZE as a constant by assuming a specific value for BUFSIZE in the following expression:

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The programmer has assumed that BUFSIZE is 512, and right-shifting 9 bits is the same (for positive numbers) as dividing by 512. However, if BUFSIZE changes to 1024 in the future, modifications will be difficult and error prone.

Compliant Solution

This compliant solution uses the identifier assigned to the constant value in the expression:

private static final int BUFSIZE = 512;

// ...

public void exampleFunction(int nbytes) {
  int nblocks = 1 + (nbytes - 1) / BUFSIZE;
  // ...
}

Applicability

Using numeric literals makes code more difficult to read, understand, and edit.

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The values 4.0 and 3.0 in the volume calculation are clearly scaling factors used to calculate the sphere's volume and are not subject to change (unlike the approximate value for π), so they can be represented exactly. There is no reason to change them to increase precision because replacing them with symbolic constants actually impairs the readability of the code.

Bibliography

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