Jan 17 2007

Reopening Ruby Classes

In Ruby the implementation of a class is not closed. In Ruby, it is incredibly easy to add new methods to any existing class at runtime, even core system classes like String, Array, and Fixum. You just reopen a class and add new code. This language feature makes the language incredibly flexible. You might be wondering, why would you need this? Well, take a look at the API documentation of the Java String class. In most development organizations that I have been a part of we have had to write small string libraries that provide far more functionality that provided by the Java String class. The Apache Jakarta Commons Lang project provides a String helper class in StringUtils. StringUtils provides methods such as IsEmpty, Chomp/Chop, and LeftPad. StringUtils. StringUtils is a great class that has saved me from re-inventing the wheel several times, but it is another library to download, configure, learn, and import. Using the StringUtils class is more verbose calling a method on a string.

// Using StringUtils to trim a string.
str = StringUtils.trim(str);
// Calling a trim on a string.
str = str.trim();

Isn’t more concise and easier to read when you invoke a method on a string instead of using StringUtils? Unfortunately, even if an JCP is approved to add your custom string methods to the String class, the whole process is very time consuming. Each incremental release of Java takes years to develop. In Ruby, you can just reopen the String class to add the few methods you feel it lacks. To reopen the Ruby String class we can use the following code:

# reopen the class
String.class_eval do
  # define new method
  def get_size
    # method implementation

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