Saturday, 7 July 2012

Item 30: Use enums instead of int constants

An enumerated type is a type whose legal values consist of a fixed set of constants. Before enum types were added to the language, a common pattern for representing enumerated types was to declare a group of named int constants, one for each member of the type:

// The int enum pattern - severely deficient!
public static final int APPLE_FUJI = 0;
public static final int APPLE_PIPPIN = 1;
public static final int APPLE_GRANNY_SMITH = 2;

public static final int ORANGE_NAVEL = 0;
public static final int ORANGE_TEMPLE = 1;
public static final int ORANGE_BLOOD = 2;

This technique, known as the int enum pattern, has many shortcomings. Note that the name of each apple constant is prefixed with APPLE_ and the name of each orange constant is prefixed with ORANGE_. This is because Java doesn’t provide namespaces for int enum groups. Prefixes prevent name clashes when two int enum groups have identically named constants.

Programs that use the int enum pattern are brittle. Because int enums are compile-time constants, they are compiled into the clients that use them.

There is no easy way to translate int enum constants into printable strings.

There is no reliable way to iterate over all the int enum constants in a group, or even to obtain the size of an int enum group.

You may encounter a variant of this pattern in which String constants are used in place of int constants. This variant, known as the String enum pattern, is even less desirable. While it does provide printable strings for its constants, it can lead to performance problems because it relies on string comparisons.

Luckily, as of release 1.5, the language provides an alternative that avoids the shortcomings of the int and string enum patterns and provides many added benefits. It is the (JLS, 8.9). Here’s how it looks in its simplest form:

public enum Apple { FUJI, PIPPIN, GRANNY_SMITH }
public enum Orange { NAVEL, TEMPLE, BLOOD }

The basic idea behind Java’s enum types is simple: they are classes that export one instance for each enumeration constant via a public static final field. Enum types are effectively final, by virtue of having no accessible constructors. Because clients can neither create instances of an enum type nor extend it, there can be no instances but the declared enum constants. In other words, enum types are instance-controlled. They are a generalization of singletons (Item 3), which are essentially single-element enums.

Enums provide compile-time type safety.

Enum types with identically named constants coexist peacefully because each type has its own namespace. You can add or reorder constants in an enum type without recompiling its clients. Finally, you can translate enums into printable strings by calling their toString method.

In addition to rectifying the deficiencies of int enums, enum types let you add arbitrary methods and fields and implement arbitrary interfaces. They provide high-quality implementations of all the Object methods, they implement Comparable (Item 12) and Serializable, and their serialized form is designed to withstand most changes to the enum type.

For a nice example of a rich enum type, consider the eight planets of our solar system. Each planet has a mass and a radius, and from these two attributes you can compute its surface gravity. This in turn lets you compute the weight of an object on the planet’s surface, given the mass of the object. Here’s how this enum looks. The numbers in parentheses after each enum constant are parameters that are passed to its constructor. In this case, they are the planet’s mass and radius:

// Enum type with data and behavior
public enum Planet {
MERCURY(3.302e+23, 2.439e6),
VENUS (4.869e+24, 6.052e6),
EARTH (5.975e+24, 6.378e6),
MARS (6.419e+23, 3.393e6),
JUPITER(1.899e+27, 7.149e7),
SATURN (5.685e+26, 6.027e7),
URANUS (8.683e+25, 2.556e7),
NEPTUNE(1.024e+26, 2.477e7);
private final double mass; // In kilograms
private final double radius; // In meters
private final double surfaceGravity; // In m / s^2
// Universal gravitational constant in m^3 / kg s^2
private static final double G = 6.67300E-11;
// Constructor
Planet(double mass, double radius) {
this.mass = mass;
this.radius = radius;
surfaceGravity = G * mass / (radius * radius);
public double mass() { return mass; }
public double radius() { return radius; }
public double surfaceGravity() { return surfaceGravity; }
public double surfaceWeight(double mass) {
return mass * surfaceGravity; // F = ma

It is easy to write a rich enum type such as Planet. To associate data with enum constants, declare instance fields and write a constructor that takes the data and stores it in the fields. Enums are by their nature immutable, so all fields should be final (Item 15). They can be public, but it is better to make them private and provide public accessors (Item 14).

While the Planet enum is simple, it is surprisingly powerful. Here is a short program that takes the earth-weight of an object (in any unit) and prints a nice table of the object’s weight on all eight planets (in the same unit):

public class WeightTable {
public static void main(String[] args) {
double earthWeight = Double.parseDouble(args[0]);
double mass = earthWeight / Planet.EARTH.surfaceGravity();
for (Planet p : Planet.values())
System.out.printf("Weight on %s is %f%n",
p, p.surfaceWeight(mass));

constant-specific method implementations:

// Enum type with constant-specific method implementations
public enum Operation {
PLUS { double apply(double x, double y){return x + y;} },
MINUS { double apply(double x, double y){return x - y;} },
TIMES { double apply(double x, double y){return x * y;} },
DIVIDE { double apply(double x, double y){return x / y;} };
abstract double apply(double x, double y);

Enum types have an automatically generated valueOf(String) method that translates a constant’s name into the constant itself.

stringToEnum map from a static block that runs after the constants have been created.

A disadvantage of constant-specific method implementations is that they make it harder to share code among enum constants.

For example, consider an enum representing the days of the week in a payroll package. This enum has a method that calculates a worker’s pay for that day given the worker’s base salary
(per hour) and the number of hours worked on that day. On the five weekdays, any time worked in excess of a normal shift generates overtime pay; on the two weekend days, all work generates overtime pay.

// Enum that switches on its value to share code – questionable

enum PayrollDay {
private static final int HOURS_PER_SHIFT = 8;
double pay(double hoursWorked, double payRate) {
double basePay = hoursWorked * payRate;
double overtimePay; // Calculate overtime pay
switch(this) {
overtimePay = hoursWorked * payRate / 2;
default: // Weekdays
overtimePay = hoursWorked <= HOURS_PER_SHIFT ?
0 : (hoursWorked - HOURS_PER_SHIFT) * payRate / 2;
return basePay + overtimePay;

This code is undeniably concise, but it is dangerous from a maintenance perspective.

Luckily, there is a nice way to achieve this. The idea is to move the overtime pay computation into a private nested enum, and to pass an instance of this strategy enum to the constructor for the PayrollDay enum. The PayrollDay enum then delegates the overtime pay calculation to the strategy enum, eliminating the need for a switch statement or constant-specific method implementation in PayrollDay. While this pattern is less concise than the switch statement, it is safer and more flexible:

// The strategy enum pattern
enum PayrollDay {
private final PayType payType;
PayrollDay(PayType payType) { this.payType = payType; }
double pay(double hoursWorked, double payRate) {
return, payRate);

// The strategy enum type
private enum PayType {
double overtimePay(double hours, double payRate) {
return hours <= HOURS_PER_SHIFT ? 0 :
(hours - HOURS_PER_SHIFT) * payRate / 2;
double overtimePay(double hours, double payRate) {
return hours * payRate / 2;
private static final int HOURS_PER_SHIFT = 8;
abstract double overtimePay(double hrs, double payRate);
double pay(double hoursWorked, double payRate) {
double basePay = hoursWorked * payRate;
return basePay + overtimePay(hoursWorked, payRate);

Switches on enums are good for augmenting external enum types with constant-specific behavior.

Enums are, generally speaking, comparable in performance to int constants. A minor performance disadvantage of enums over int constants is that there is a space and time cost to load and initialize enum types. Except on resource-constrained devices, such as cell phones and toasters, this is unlikely to be noticeable in practice.

In summary, the advantages of enum types over int constants are compelling. Enums are far more readable, safer, and more powerful. Many enums require no explicit constructors or members, but many others benefit from associating data with each constant and providing methods whose behavior is affected by this data. Far fewer enums benefit from associating multiple behaviors with a single method. In this relatively rare case, prefer constant-specific methods to enums that switch on their own values. Consider the strategy enum pattern if multiple enum constants share common behaviors.

Reference: Effective Java 2nd Edition by Joshua Bloch