Type-Safe Wrapper

• Object-Oriented Design

Provide a type (and thus static type-checking) for values like IDs

IDs like customer id, contract id, etc. are mere integers or strings and get easily confused. This creates nasty bugs.

Create a wrapper around a string which holds the value. Then the compiler can do static type checking based on this wrapper class.

Have a wrapper class around a string, provide some conversion methods and override toString(), as well as equals() and hashCode().

• Don't allow null values to be wrapped. Either a variable is null or it has a value but having a wrapper around a null is confusing.

• Murphy's Law (ML): Static type checking makes certain bugs impossible

• More Is More Complex (MIMC): You have to write and maintain more code

• Value Object

public class CustomerId {
    private String value;
 
    public static CustomerId of(String value) {
        return new CustomerId(value);
    }
 
    public static CustomerId of(Integer value) {
        return new CustomerId(value.toString());
    }
 
    public CustomerId(String value) {
        if (value == null)
            throw new IllegalArgumentException("value may not be null");
 
        this.value = value;
    }
 
    public int asInt() throws NumberFormatException {
        return Integer.parseInt(value);
    }
 
    @Override
    public String toString() {
        return value;
    }
 
    @Override
    public boolean equals(Object that) {
        if (!that instanceof CustomerId)
            return false;
 
        return this.value.equals(((CustomerId) that).value);
    }
 
    @Override
    public int hashCode() {
        return value.hashCode();
    }
}

Stub

Discuss this wiki article and the pattern on the corresponding talk page.