Why You Should Program To Interfaces Part I
Submitted by curtisrcooley on Sun, 03/22/2009 - 19:52
The Open Closed Principle (OCP)
The Open Closed Principle (OCP) states that a system should be open to extension and closed to modification. In other words, it should be easy to add new functionality by not changing existing code. If you program to interfaces, it is much easier to follow the OCP.
Calculator No OCP
Let's build a very simple calculator.
class Calculator {
int add(int x, int y) {
return x + y;
}
}
Ok, so we can add, but what about subtract? We can't add subtract without modifying Calculator. That violates OCP.
class Calculator {
int add(int x, int y) {
return x + y;
}
int subtract(int x, int y) {
return x - y
}
}
Calculator OCP
Let's try it again, but program to an interface
interface Function {
int calculate(int x, int y);
}
class Add {
int calculate(int x, int y) {
return x + y;
}
}
We can now add, but what about subtract? Well, since we programmed to an interface, we extend our calculator with the subtract ability:
class Subtract {
int calculate(int x, int y) {
return x - y;
}
}
We can add any sort of two argument function to our calculator and not have to modify existing code. That's OCP in action.
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