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====== Type-Safe Wrapper ======

===== Alternative Names =====


===== Context =====

  * [[contexts:Object-Oriented Design]]


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

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

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

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

==== Dynamics ====

==== Implementation Hints ====
  * 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.

==== Variations ====


===== Origin =====


===== Advantages =====
/*reference principles here*/
  * [[principles:Murphy's Law]] (ML): Static type checking makes certain bugs impossible

===== Disadvantages =====
/*reference principles here*/
  * [[principles:More Is More Complex]] (MIMC): You have to write and maintain more code


===== Relations to Other Patterns =====

==== Generalizations ====
 * [[Value Object]]
==== Specializations ====

==== Alternative Patterns ====

==== Complementary Patterns ====

==== Pattern Collections ====


===== Examples =====

==== Example 1:  ====
<code java>
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();
    }
}
</code>

===== Description Status =====
/* Choose one of the following and comment out the rest: */
[[wiki:Stub]]
/*[[wiki:Incomplete]]*/
/*[[wiki:Complete]]*/


===== Further Reading =====


===== Discussion =====

Discuss this wiki article and the pattern on the corresponding [[talk:patterns:Type-safe Wrapper|talk page]].