SourceMaking

Several methods do similar things but with different values contained in the method body.

Create one method that uses a parameter for the different values.

Motivation

You may see a couple of methods that do similar things but vary depending on a few values. In this case you can simplify matters by replacing the separate methods with a single method that handles the variations by parameters. Such a change removes duplicate code and increases flexibility, because you can deal with other variations by adding parameters.

Mechanics

• Create a parameterized method that can be substituted for each repetitive method.
• Compile.
• Replace one old method with a call to the new method.
• Compile and test.
• Repeat for all the methods, testing after each one.

You may find that you cannot do this for the whole method, but you can for a fragment of a method. In this case first extract the fragment into a method, then parameterize that method.

Example

The simplest case is methods along the following lines:

		

  class Employee {

    void tenPercentRaise () {

        salary *= 1.1;

  }

 

    void fivePercentRaise () {

        salary *= 1.05;

  }

 

which can be replaced with

		

  void raise (double factor) {

    salary *= (1 + factor);

  }

 

Of course that is so simple that anyone would spot it.

A less obvious case is as follows:

		

    protected Dollars baseCharge() {

        double result = Math.min(lastUsage(),100) * 0.03;

        if (lastUsage() > 100) {

            result += (Math.min (lastUsage(),200) - 100) * 0.05;

        };

        if (lastUsage() > 200) {

            result += (lastUsage() - 200) * 0.07;

        };

        return new Dollars (result);

    }

 

this can be replaced with

		

    protected Dollars baseCharge() {

        double result = usageInRange(0, 100) * 0.03;

        result += usageInRange (100,200) * 0.05;

        result += usageInRange (200, Integer.MAX_VALUE) * 0.07;

        return new Dollars (result);

    }

 

    protected int usageInRange(int start, int end) {

        if (lastUsage() > start) return Math.min(lastUsage(),end) - start;

        else return 0;

    }

 

The trick is to spot code that is repetitive on the basis of a few values that can be passed in as parameters.