Types All interfaces and classes are types. All variabes in Java must have a type.

The type of a variable determines what methods are visible. This is the visibility principle.

When a method call is communicated via a variable, the "pointee" is responsible for executing the method. This is the delegation principle, so-called because the job of executing the code is delegated to the pointee.

Types in Java are organized into a hierarchy. All classes automatically inherit from the root class java.lang.Object.

This is why any class you create automaticaly has the methods toString() and equals.

final Again

The final keyword can appear in two contexts in inheritance.

• If a class is marked final, it cannot be extended. The class String and the wrapper classes are all final classes.
• If a method in a class is marked final, it cannot be overrriden by a descendant class

Both of these stipuations are enforced at compile time. This article supplies a security rationale for making the String class final.

Abstract Classes

Functional Interfaces