Chapter 11
Abstract Data Types
and Encapsulation
Concepts
ISBN 0-321-19362-8
Chapter 11 Topics
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•
•
•
•
•
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The Concept of Abstraction
Introduction to Data Abstraction
Design Issues for Abstract Data Types
Language Examples
Parameterized Abstract Data Types
Encapsulation Constructs
Naming Encapsulations
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The Concept of Abstraction
• The concept of abstraction is fundamental in
programming (and computer science)
• Nearly all programming languages support
process abstraction with subprograms
• Nearly all programming languages designed
since 1980 have supported data abstraction
with some kind of module
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Introduction to Data Abstraction
• Def: An abstract data type is a user-defined data
type that satisfies the following two conditions:
1. The representation of, and operations on, objects of the
type are defined in a single syntactic unit; also, other
units can create objects of the type.
2. The representation of objects of the type is hidden from
the program units that use these objects, so the only
operations possible are those provided in the type's
definition.
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Introduction to Data Abstraction
• Advantage of Restriction 1:
– Program organization, modifiability (everything
associated with a data structure is together), and
separate compilation
• Advantage of Restriction 2:
– Reliability--by hiding the data representations, user
code cannot directly access objects of the type. User
code cannot depend on the representation, allowing the
representation to be changed without affecting user
code.
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Introduction to Data Abstraction
• Built-in types are abstract data types
e.g. int type in Java
– The representation is hidden
– Operations are all built-in
– User programs can define objects of int type
• User-defined abstract data types must have the
same characteristics as built-in abstract data
types
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Design Issues
• Language Requirements to support abstract data
types:
1. A syntactic unit in which to encapsulate the type
definition.
2. A method of making type names and subprogram
headers visible to clients, while hiding actual
definitions.
3. Some primitive operations must be built into the
language processor (usually just assignment and
comparisons for equality and inequality)
• Some operations are commonly needed, but must be
defined by the type designer
• e.g., iterators, constructors, destructors
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Design Issues
• Language Design Issues:
1. Encapsulate a single type, or something more?
2. What types can be abstract?
3. Can abstract types be parameterized?
4. What access controls are provided?
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Language Examples
1. Simula 67
– Provided encapsulation, but no information hiding
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Language Examples
2. Ada
– The encapsulation construct is the package
– Packages usually have two parts:
1. Specification package (the interface)
2. Body package (implementation of the entities named in the
specification)
– Information Hiding
1. Hidden types are named in the spec package, as in:
type NODE_TYPE is private;
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Language Examples
2. Representation of an exported hidden type is
specified in a special invisible (to clients) part of
the spec package (the private clause), as in:
package … is
type NODE_TYPE is private;
…
private
type NODE_TYPE is
record
…
end record;
…
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Language Examples
• A spec package can also define unhidden
types simply by providing the representation
outside a private clause
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Language Examples
• The reasons for the two-part type definition
are:
1. The compiler must be able to see the
representation after seeing only the spec package
(the compiler can see the private clause)
2. Clients must see the type name, but not the
representation (clients cannot see the private
clause)
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Language Examples
• Private types have built-in operations for
assignment and comparison with = and /=
– Limited private types have no built-in operations
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Language Examples
3. C++
– Based on C struct type and Simula 67 classes
– The class is the encapsulation device
– All of the class instances of a class share a single
copy of the member functions
– Each instance of a class has its own copy of the
class data members
– Instances can be static, stack dynamic, or heap
dynamic
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Language Examples
• Information Hiding:
– Private clause for hidden entities
– Public clause for interface entities
– Protected clause - for inheritance (see Ch. 12)
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Language Examples
• Constructors:
– Functions to initialize the data members of instances
(they DO NOT create the objects)
– May also allocate storage if part of the object is heapdynamic
– Can include parameters to provide parameterization of
the objects
– Implicitly called when an instance is created
– Can be explicitly called
– Name is the same as the class name
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Language Examples
• Destructors
– Functions to cleanup after an instance is destroyed;
usually just to reclaim heap storage
– Implicitly called when the object’s lifetime ends
– Can be explicitly called
– Name is the class name, preceded by a tilde (~)
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Language Examples
• Friend functions or classes - to provide access
to private members to some unrelated units or
functions (NECESSARY in C++)
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Language Examples
• Evaluation of C++ Support for Abstract Data
Types
– Classes are similar to Ada packages for providing
abstract data types
– Difference: packages are encapsulations, whereas
classes are types
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Language Examples
• Java
– Similar to C++, except:
• All user-defined types are classes
• All objects are allocated from the heap and accessed
through reference variables
• Individual entities in classes have access control modifiers
(private or public), rather than clauses
• Java has a second scoping mechanism, package scope,
which can be used in place of friends
– All entities in all classes in a package that do not have access
control modifiers are visible throughout the package
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Language Examples
• C#
– Based on C++ and Java
– Adds two access modifiers, internal and protected
internal
– All class instances are heap dynamic
– Default constructors are available for all classes
– Garbage collection is used for most heap objects, so
destructors are rarely used
– structs are lightweight classes that do not support
inheritance
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Language Examples
• C# (continued)
– Common solution to need for access to data members:
accessor methods (getter and setter)
– C# provides properties as a way of implementing
getters and setters without requiring explicit method
calls
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C# Property Example
public class Weather {
public int DegreeDays { //** DegreeDays is a property
get {
return degreeDays;
}
set {
degreeDays = value;
}
}
private int degreeDays;
...
}
...
Weather w = new Weather();
int degreeDaysToday, oldDegreeDays;
...
w.DegreeDays = degreeDaysToday;
...
oldDegreeDays = w.DegreeDays;
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Parameterized Abstract Data Types
1. Ada Generic Packages
– Make the stack type more flexible by making the
element type and the size of the stack generic
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Example Program
package INT_STACK is new
GENERIC_STACK(100, INTEGER);
package FLOAT_STACK is new
GENERIC_STACK(500, FLOAT);
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Parameterized Abstract Data
Types
2. C++ Templated Classes
– Classes can be somewhat generic by writing
parameterized constructor functions, e.g.
stack (int size)
{
stk_ptr = new int [size];
max_len = size - 1;
top = -1;
}
stack stk(100);
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Parameterized Abstract Data
Types
• The stack element type can be parameterized
by making the class a templated class
• Java does not support generic abstract data
types
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Encapsulation Constructs
• Original motivation:
– Large programs have two special needs:
1. Some means of organization, other than simply
division into subprograms
2. Some means of partial compilation (compilation
units that are smaller than the whole program)
• Obvious solution: a grouping of subprograms
that are logically related into a unit that can be
separately compiled (compilation units)
– These are called encapsulations
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Encapsulation Constructs
•
•
Nested subprograms in Ada and Fortran 95
Encapsulation in C
–
Files containing one or more subprograms can be
independently compiled
– The interface is placed in a header file
– Problem: the linker does not check types between
a header and associated implementation
•
Encapsulation in C++
– Similar to C
– Addition of friend functions that have access to
private members of the friend class
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Encapsulation Constructs
•
Ada Package
–
Can include any number of data and subprogram
delcarations
– Two parts: specification and body
– Can be compiled separately
•
C# Assembly
–
–
Collection of files that appears to be a single
dynamic link library or executable
Larger construct than class; used by all .NET
programming languages
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Naming Encapsulations
• Large programs define many global names;
need a way to divide into logical groupings
• A naming encapsulation is used to create a
new scope for names
• C++ Namespaces
– Can place each library in its own namespace and
qualify names used outside with the namespace
– C# also includes namespaces
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Naming Encapsulations
• Java Packages
– Packages can contain more than one class
definition; classes in a package are partial friends
– Clients of a package can use fully qualified name
or use the import declaration
• Ada Packages
– Packages are defined in hierarchies which
correspond to file hierarchies
– Visibility from a program unit is gained with the
with clause
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