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39. 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 in a program can lead to two problems. First, the meaning of the literal is often obscured or unclear from the context. Second, changing a frequently used literal requires searching the entire program source for that literal and distinguishing the uses that must be modified from those that should remain unmodified.

Avoid these problems by declaring class variables with meaningfully named constants, setting their values to the desired literals, and referencing the constants instead of the literals throughout the program. This approach clearly indicates the meaning or intended use of each literal. Furthermore, should the constant require modification, the change is limited to the declaration; searching the code is unnecessary.

Constants should be declared as static and final. However, constants should not be declared public and final if their values might change (see Guideline 31, “Do not apply public final to constants whose value might change in later releases,” for more details). For example,

private static final int SIZE = 25;

Although final can be used to specify immutable constants, there is a caveat when dealing with composite objects. See Guideline 73, “Never confuse the 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:

double area(double radius) {
  return 3.14 * 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 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:

double area(double radius) {
  return 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 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.

private static final double PI = 3.14;

double area(double radius) {
  return 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 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. The use of the literals 4.0, 3.0, and 2 does not violate this guideline, for reasons explained in the “Applicability” section of this guideline.

Compliant Solution (Predefined Constants)

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

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:

private static final int BUFSIZE = 512;

// ...

public void shiftBlock() {
  int nblocks = 1 + ((nbytes - 1) >> 9);  // BUFSIZE = 512 = 2^9
  // ...
}

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.

This code also fails to conform to The CERT® Oracle® Secure Coding Standard for Java [Long 2012], “NUM01-J. Do not perform bitwise and arithmetic operations on the same data.” Replacing a division operation with a right shift is considered a premature optimization. Normally, the compiler will do a better job of determining when this optimization should be performed.

Compliant Solution

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

private static final int BUFSIZE = 512;

// ...

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

Applicability

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

The use of symbolic constants should be restricted to cases in which they improve the readability and maintainability of the code. When the intent of the literal is obvious, or where the literal is not likely to change, using symbolic constants can impair code readability. The following code example obscures the meaning of the code by using too many symbolic constants.

private static final double FOUR = 4.0;
private static final double THREE = 3.0;

double volume(double radius) {
  return FOUR / THREE * Math.PI * radius * radius * radius;
}

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

[Core Java 2003]

[Long 2012]

NUM01-J. Do not perform bitwise and arithmetic operations on the same data

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