DCL03-J. Use meaningful symbolic constants to represent literal values in program logic

Java supports the use of various types of literals, such as integers (5, 2), floating point numbers (2.5, 6.022e+23), characters ('a', '\n'), booleans ('true', 'false'), and strings ("Hello\n"). Extensive use of literals within a program can lead to two problems: first, the meaning of the literal is often obscured or unclear from the context ("magic numbers"), and second, changing a frequently-used literal requires the entire program source code to be searched for occurrences of that literal, creating possible error sources if some of the occurrences are overlooked.

A solution to this problem is to declare meaningfully-named constants as class variables. Their values can be set to the desired literals, and these constants referenced inserting throughout the program rather than the literals themselves. The advantages to this approach are that the constant's name can clearly indicate its meaning or intended use, and should the constant need to be changed, its declaration can be modified without having to search the entire code for all its occurrences.

final

The final keyword in Java is used to declare constants. Its effect is to render the affected variable immutable. Attempting to change the value of a final-qualified variable results in a compile-time error. Because constants cannot be changed, it is desirable to define only one instance of them for the class; therefore constants should be declared with the static modifier as well. (DCL31-J. Qualify mathematical constants with the static and final modifiers)

The following code fragment demonstrates the use of static and final to create a constant:

private static final int SIZE=25;

This code declares the value SIZE to be of type int and to store the immutable value 25. This constant can subsequently be used wherever the value 25 is needed.

Although final is more often safe for creating compile time immutable constants, its use has a few caveats when dealing with mutable data. See OBJ03-J. Be careful about final reference for more details.

Noncompliant Code Example

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

double area(double radius) {
return 12.56*radius*radius; }

double volume(double radius) {
return 4.19*radius*radius*radius; }

double greatCircleCircumference(double radius) {
return 6.28*radius; }

The methods use the seemingly-random literals 12.56, 4.19, and 6.28 to represent various scaling factors used to calculate these dimensions. Someone reading this code would have no idea how they were generated or what they meant, and would therefore be unable to understand the function of this code.

Noncompliant Code Example

The following noncompliant code attempts to avoid the above issues by explicitly calculating the required constants

double area(double radius) {
return 4.0*3.14*radius*radius; }

double volume(double radius) {
return 4.0/3.0*3.14*radius*radius*radius; }

double greatCircleCircumference(double radius) {
return 2*3.14*radius; }

The code uses the literal "3.14" to represent the value pi. Although this removes some of the ambiguity from the literals, it complicates code maintenance. If the programmer were to decide that a more precise value of pi is desired, he would need to find all occurrences of "3.14" in the code and replace them.

Compliant Solution

In this compliant solution, a constant PI is first declared and initialized to 3.14, and is thereafter referenced in the code wherever the value of pi is needed.

private static final int PI = 3.14;

double area(double radius) {
return 4.0*PI*radius*radius; }

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

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

This reduces clutter and promotes maintainability, for if a different value for pi is required, the programmer can simply redefine the constant.

Compliant Solution: Predefined Constants

The class java.lang.Math defines a large set of numeric constants, such as PI and the exponential constant E. If such constants exists, it is preferable to use them rather than redefining their values.

double area(double radius) {
return 4.0*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; }

Exceptions

The use of symbolic constants should be restricted to cases where they improve the readability and maintainability of the code. Using them when the intent of the literal is obvious, or where the literal is not likely to change, can impair code readability. In the Compliant Solution above, the values 4.0 and 3.0 in the volume calculation are clearly scaling factors used to calculate the circle volume, and as such are not subject to change (unlike pi, they can be represented exactly; there is no reason to change them to increase precision). Hence, replacing them with symbolic constants would be inappropriate.

Risk Assessment

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

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Other Languages

This rule appears in the C++ Secure Coding Standard as DCL06-CPP. Use meaningful symbolic constants to represent literal values in program logic and in the C Secure Coding Standard as DCL06-C. Use meaningful symbolic constants to represent literal values in program logic.

References

[[API 06]]
[[Core Java 04]]