One of the conceptual pillars supporting object-oriented software development is the abstract data type (ADT). David Parnas and others articulated this concept in the 1960s. For many years this concept has formed the basis for software construction, both object oriented and otherwise. All of the data structures to be presented in this book are formulated as abstract data types.

A data type is a program entity holding information that can be manipulated in a disciplined manner through a set of predefined operations. Predefined operations include commands that may be used to modify the value of the data type and queries that may be used to access the value of the data type. In the Java programming language an abstract data type is implemented using the class construct. The information structure (data structure) of the ADT is represented in the internal (usually private or protected) fields of the class. The commands are represented by methods that return type void. The queries are represented either by public fields or methods that return a nonvoid type representing field information.

Many software developers have found that ADTs aid in formulating clear and clean software architecture and promote greater understandability of the software and easier software maintenance. In structured programming languages such as C and Pascal the programmer must impose strict protocols in order to utilize ADTs. In the early 1980s two pre-object-oriented languages, Ada and Modula-2, were specifically designed to support and encourage the use of ADTs.