abstract Java Keyword

The abstract keyword may modify a class or a method.
An abstract class can be extended (subclassed) but cannot be instantiated directly.
An abstract method is not implemented in the class in which it is declared, but must be overridden in some
subclass.
Examples
public abstract class MyClass
{
}
public abstract String myMethod();

Comments

Generalization and Specialization

Generalization creates a concept with a broader scope. Specialization creates a concept with a narrower scope. Reusing the interface! Inheritance: get the interface from the general class. Objects related by inheritance are all of the same type.

Aggregation and Decomposition

Idea: make new objects by combining existing objects. Reusing the implementation! An aggregation consists of a number of (sub-)concepts which collectively is considered a new concept. A decomposition splits a single concept into a number of (sub-)concepts.

The Class Concept

A class is a collection of objects (or values) and a corresponding set of methods. A class encapsulates the data representation and makes data access possible at a higher level of abstraction. Example 1: A set of vehicles with operations for starting, stopping, driving, get km/liter, etc. Example 2: A time interval, start time, end time, duration, overlapping intervals, etc. Example 3: A string, upper case, compare, lower case, etc. str.equals(otherStr) – class/Java style strcmp(str, otherStr) – C style

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