Open-closed Principle

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“Software entities (classes, modules, methods, etc.) should be open for extension, but closed for modification”

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Original author: Bertrand Meyer

PDF:

Model Format MagicDraw

The “Open/Closed Principle” (OCP) states “software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification”; that is, such entities should allow their behaviour to be extended without modifying their source code.

The “Open/Closed Principle” (OCP) was first advocated by Bertrand Meyer in the first edition (1988) of his book “Object-Oriented Software Construction” and is considered by many to be the cornerstone of object-oriented design because it underlays many other principles, patterns, and even methodologies.

Open–closed principle is one of the five SOLID principles of object-oriented design compiled by Robert C. Martin.

Code Examples

public class AccountClient {
	
	
	public boolean doSomething(Account a) {

	 switch (a.account_type) {
     case checkingAcc : foo(a); break;
     case savingsAcc : bar(a); break;
     case investmentAcc : qux(a); break;
     default : hmmm(a);
   }

/*
 * Problem: it is impossible to add a new account type without modifying the code.
*/

}

public class AccountClient {

public boolean doSomething(Account a) {

switch (a.account_type) {

case checkingAcc : foo(a); break;

case savingsAcc : bar(a); break;

case investmentAcc : qux(a); break;

default : hmmm(a);

}

* Problem: it is impossible to add a new account type without modifying the code.

}