These days I red some documents about the principle when doing software design. There are “SOLID” principle. Like the meaning the the English work “solid” to make the software strong enough.

What is SOLID?
SOLID is consist of first character of the following 4 design principles.

A class should have one single reponsibility.

Software entities should open for extension, close for modification.

Objects in a program should be replacable with the intances of their subtypes without alerting the correctness of that program.

Many client-specific interfaces are better than one general purpose interface.

Dependent upon abstractions, do not dependent on concretions.

Some words for the SOLID
Design principles were proposed by many skilled software engineer based on many experiences of development. It is difficult to understand all the concepts, but as a software engineer should remember them.

Some relevant links:
Object-oriented Design