A software specification indicates the task (or some aspect of the task) that is supposed to be performed when software executes. Types of Specifications Class Diagrams Object Diagrams Activity Diagrams (control flow diagrams) Incomplete specs Assertions (preconditions, postconditions, invariants) Others The Object of Data Abstraction and Structure, David D. Riley © Addison Wesley pub.

An assertion is a statement of fact that is presumed true relative to a code location(s). Example // assert: str is a String and str.length > 2 char firstChar, secondChar, bigChar; firstChar = str.charAt(0); secondChar = str.charAt(1); if (firstChar > secondChar) { bigChar = firstChar; } else { bigChar = secondChar; } /* assert: str.length > 2 and (str.charAt(0) ≥ str.charAt(1) implies bigChar == str.charAt(0)) and (str.charAt(0) ≤ str.charAt(1) implies bigChar == str.charAt(1)) */ The Object of Data Abstraction and Structure, David D. Riley © Addison Wesley pub.

Assertions are based on logic and certain program notations (i.e., variable references and possibly non-void method calls). Assertion Notation Assertions should NOT contain action verbs. SubAssertion1 and SubAssertion2 SubAssertion1 or SubAssertion2 SubAssertion1 implies SubAssertion2 Both subassertions must be true.One or both subassertion is true.When the first subassertion is true, the second must also be true. not SubAssertion1 The subassertion must be false. Logical Operators The Object of Data Abstraction and Structure, David D. Riley © Addison Wesley pub.

Another logical notation, known as quantification, permits expressing assertions about data structures. Assertion Notation forAll(Integer j : 0≤j≤3 | arr[j] > 0 ) forAll(type var : boundaryCondition | SubAssertion ) Form (universal quantification) Example meaning: arr[0] > 0 and arr[1] > 0 and arr[2] > 0 and arr[3] > 0 exists(Integer j : 0≤j≤3 | arr[j] == 7 ) exists(type var : boundaryCondition | SubAssertion ) Form (existential quantification) Example meaning: arr[0] ==7 or arr[1] == 7 or arr[2] == 7 or arr[3] == 7

Assume two arrays of double: arr1 and arr2 and arr1.length == arr2.length == 5 Quantification Examples forAll (Integer r : 0 ≤ r ≤ 3 | arr1[r] < arr1[r+1] ) forAll (w : 0 ≤ w ≤ 4 | arr1[w] == arr2[w] ) exists (k : 0 ≤ k ≤ 4 | arr1[k] == 33 and arr2[k] == 33 ) exists (k : 0 ≤ k ≤ 4 | ( arr1[k] < 0 and forAll (j : k < j ≤ 4 | arr2[k] == arr1[j]) ) ) forAll (j,k : 0 ≤ j,k ≤ 4 and j != k | arr1[j] != arr2[k] ) The Object of Data Abstraction and Structure, David D. Riley © Addison Wesley pub.

An assertion can be located anywhere within executable instructions. However, some locations have been found most effective: Where to place Assertions? Class Invariant Method Precondition Method Postcondition Loop Invariant The Object of Data Abstraction and Structure, David D. Riley © Addison Wesley pub.

Example /** class invariant distanceInMiles > 0 and timeInSeconds > 0 */ public class LapTime { private double distanceInMiles, timeInSeconds; /** pre: d > 0 and t > 0 post: distanceInMiles == d and timeInSeconds == t */ public LapTime(double d, double t) { distanceInMiles = d; timeInSeconds = t; } /** post: distanceInMiles == 60 and timeInSeconds == 3600 */ public void setTo60MPH() { distanceInMiles = 60; timeInSeconds = 3600; } // more methods on later slides } When is each assertion presumed to be true? The Object of Data Abstraction and Structure, David D. Riley © Addison Wesley pub.

Special Postcondition Notations // Within LapTime class /** post: result == distanceInMiles / (timeInSeconds*3600) */ public double milesPerHour() { double velocity; velocity = distanceInMiles/(timeInSeconds*60*60); return velocity } Return value (result) // Within LapTime class /** post: distanceInMiles == * 2 */ public void doubleTheMileage() { distanceInMiles = distanceInMiles * 2; } Previous value The Object of Data Abstraction and Structure, David D. Riley © Addison Wesley pub.

The Object of Data Abstraction and Structure, David D. Riley © Addison Wesley pub. Method Contract Method caller guarantees... precondition & class invariant (at time of method call) Method is required to ensure... postcondition & class invariant (at time of method return) Addendum: A modifies clause can stipulate what alterations are permitted

Using method “contracts,” fills in more design details. java.awt.Color - int redness - int blueness - int greenness - int opaqueness «alpha» «constructor» + Color(int r, int g, int b) + Color(float r, float g, float b, float a) «query» + int getRed() + Color darker() + Color brighter()... Example: the standard Color class. What does this class diagram tell you? What doesn’t it tell you? The Object of Data Abstraction and Structure, David D. Riley © Addison Wesley pub.

java.awt.Color Class Specifications Invariant (for every Color object, c) 0 ≤ redness ≤ 255 and 0 ≤ greenness ≤ 255 and 0 ≤ blueness ≤ 255 and 0 ≤ opaqueness ≤ 255 Constructor Methods public Color(int r, int g, int b) pre: 0 ≤ r ≤ 255 and 0 ≤ g ≤ 255 and 0 ≤ b ≤ 255 (throws IllegalArgumentException) modifies: redness, greenness, blueness, opaqueness post: redness == r and greenness == g and blueness == b and opaqueness == 255 public Color(float r, float g, float b, float a) pre: 0.0 ≤ r ≤ 1.0 and 0.0 ≤ g ≤ 1.0 and 0.0 ≤ b ≤ 1. 0 and 0.0 ≤ a ≤ 1.0 (throws IllegalArgumentException) post: redness == r*255 and greenness == g*255 and blueness == b*255 and opaqueness == a*255 The Object of Data Abstraction and Structure, David D. Riley © Addison Wesley pub.

java.awt.Color Class Specifications (continued) public Color darker() post: result.redness == redness * 0.7 and result. greenness == greenness * 0.7 and result. blueness == blueness * 0.7 and result. opaqueness == 255 Query Methods public int getRed() post: result == redness public Color brighter() post: (redness / 0.7) > 255 implies result.redness == 255 and (redness / 0.7) ≤ 255 implies result.redness == redness / 0.7 and (greenness / 0.7) > 255 implies result. greenness == 255 and (greenness / 0.7) ≤ 255 implies result. greenness == greenness / 0.7 and (blueness / 0.7) > 25 5 implies result. blueness == 255 and (blueness / 0.7) ≤ 255 implies result. blueness == blueness / 0.7 and result. opaqueness == The Object of Data Abstraction and Structure, David D. Riley © Addison Wesley pub.