Enum Tricks: Featured Enum Instead of Switch

Problem and the Solution

Switch/case is the common control structure implemented in most imperative programming languages. Switch is considered more readable than a series of if/else.

Here is a simple example:

// Switch with int literal
switch(c) {
    case 1: one(); break;
    case 2: two(); break;
    case 3: three(); break;
    default: thrownewUnsupportedOperationException(String.format("Operation %d is not supported", c));
}

Here is the list of the main problems in this code:

1. The relationship between int literals (1, 2, 3) and the executed code is not obvious.
2. If one of the values (e.g. 2) is not supported anymore and this switch is not updated accordingly, it will forever contain the unused code.
3. If a new possible value of c (e.g. 4) is introduced and the switch is not updated accordingly, the code will probably throw the UnsupportedOperationException at runtime without any compile time notifications.
4. Such a switch structure tends to be duplicated several times in code that makes problems 2 and 3 even more complicated.

The simplest fix can be done by using int constants instead of literals. First, let's define constants:

private static int ONE = 1;
private static int TWO = 2;
private static int THREE = 3;

Now, the code will look like this:

switch(c) {
    case ONE: one(); break;
    case TWO: two(); break;
    case THREE: three(); break;
    default: thrownewUnsupportedOperationException(String.format("Operation %d is not supported", c));
}

Obviously, in real life, the names of the constants must be self-descriptive. This snippet is more readable but all other disadvantages are still relevant. The next attempt to improve the initial code snippet uses enums introduced to Java language in version 5 in 2004. Let's define the following enum:

enum Action {ONE, TWO, THREE}

Now, the switch snippet will be changed slightly:

Action a = ...
switch (a) {
    case ONE: one(); break;
    case TWO: two(); break;
    case THREE: three(); break;
    default: thrownewUnsupportedOperationException(String.format("Operation %s is not supported", a));
}

This code is a little bit better: it will produce compilation error if one of the elements is removed from enum Action. However, it will not cause a compilation error if an additional element is added to enum Action. Some IDEs or static code analysis tools may produce a warning in this case, but who is paying attention to warnings? Fortunately, an enum can declare an abstract method that has to be implemented by each element:

enum Action {
ONE { @Override public void action() { } },
TWO { @Override public void action() { } },
THREE { @Override public void action() { } },
public abstract void action();

Now, the switch statement can be replaced by single line:

Action a = ...
a.action();

This solution does not have any of disadvantages enumerated above:

1. It is readable. The method is "attached" to enum element; one can write as many javadoc as it is needed if method meaning is unclear. The code that calls method is trivial: what can be simpler than method invocation?
2. There is no way to remove an enum constant without removing the implementation, so no unused code will remain if some functionality is no longer relevant.
3. A new enum element cannot be added without the implementation of method action(). Code without implementation can't be compiled.
4. If several actions are required, they can all be implemented in the enum. As we already mentioned, the code that calls specific function is trivial, so now there is no code duplication.

Conclusion

Although switch/case structure is well known and widely used in various programming languages, its use may cause a lot of problems. The solution that uses Java enums and described above does not have these disadvantages. The next article from this series will show how to extend the functionality of an existing enum.