Self-Referential Classes

Classes can be self-referential.

For example:

class Link
{
    SomeType value;
    Link next;
}

Here, the next field refers to another Link instance, or is null.

Self-referential classes are particularly useful to model graphs. For example, we can create a simple expression tree along the following lines:

enum UnaryOp { UnaryPlus, UnaryMinus, Not }
enum BinaryOp { Plus, Minus, Multiply, Divide, And, Or }

class Node {}

class UnaryOperator extends Node
{
    UnaryOp operator;
    Node operand;
}

class BinaryOperator extends Node
{
    BinaryOp op;
    Node operand1;
    Node operand2;
}

class Operand extends Node
{
    long val;
}

The expression tree consists of leaf nodes of type Operand, and interior nodes of type UnaryOperator and BinaryOperator, with one or two descendants, respectively. All three of these classes are derived from a common base class Node. Note that Node is an empty class. This is one of the few cases where an empty base class is justified.

If we write an operation that, for example, accepts a Node parameter, passing that parameter results in transmission of the entire tree to the server:

interface Evaluator
{
    long eval(Node expression); // Send entire tree for evaluation
}

Self-referential classes are not limited to acyclic graphs: a class graph can contain cycles.