Object-Oriented Technology
Concepts

Data Abstraction

Programs traditionally are built using data types that represent data primitives such as numbers and character strings. However, gains in programming productivity and program understandability are limited when all programs must be constructed from such low-level data primitives.

Significant increases in productivity and understandability can result when data primitives are combined to create new, higher-abstraction data types. These higher-abstraction types add to the set of available types, and are then used syntactically in the same manner as the original primitive types.

For example, naval architecture applications could benefit from creation of new data types representing the hull characteristics and hull geometry of a ship. Such data types might be represented in pseudo-code as follows:

  
  type HullCharacteristics {
    float lbp;     // length between perpendiculars
    float lwl;     // waterline length
    float loa;     // overall length
    float b, h, d; // beam, draft, depth
    float cb;      // block coefficient
    float cp;      // prismatic coefficient
  }

  type HullGeometry {
    float     ss;        // station spacing
    float[]   stations;  // array of station numbers
    float[][] y_offsets; // offset beam coordinates
    float[][] z_offsets; // offset depth coordinates
  }

Note that "float [ ] [ ]" represents a double-subscripted array of floats.

Given such type definitions, variables of these data types may then be declared in a manner analogous to that of declaring variables of primitve types:


     Data Type         Variable     Variable
  HullCharacteristics hull_1_char, hull_2_char; 
  HullGeometry        hull_1_geom, hull_2_geom;

The individual variables, or "fields," comprising the abstract types may be referenced using "dot notation:"

  
  hull_1_char.lbp   // LBP for hull 1 
  hull_2_char.lwl   // LWL for hull 2
  hull_1_geom.ss    // station spacing for hull 1

Arrays of the higher-abstraction types may also be created in the same manner as arrays of primitive types:


     Array Data Type     Variable       Variable
  HullCharacteristics[] series60_char, taylor_series_char;
  HullGeometry[]        series60_geom, taylor_series_geom;

The above two declarations using arrays of the abstract types define variables representing all hull characteristics and geometry of both Series60 and Taylor series hulls, yet with no more effort than that required to declare four variables of primitive type. The productivity benefit of data abstraction is clear, as is the readability of the succinct code that accomplishes such a non-trivial task.

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Object-Oriented Technology Concepts

Data Abstraction

Object-Oriented Technology
Concepts