CASE/RE-FACTORING
AND PROGRAM SLICING
COMP 319
© University of Liverpool
slide 1
CASE tool construction
• File level - programming environment
• Language level - program
representation, compiling, testing
• Work flow level - stages in the software
engineering process itself: specification,
design, development, verification,
validation, management.
COMP319
© University of Liverpool
slide 2
Slide 3
Program Language Level
Tool construction at the language level
exploits form – which is usually either:
• Grammar – to capture the notion of text
based instructions
• Graph – to deal with the concept of
sequence
COMP319 Software Engineering II
Refactoring
• Why CASE?
- not alter functionality (must be correct)
- cover all instances (i.e. be complete)
- Keep code tidy and to standard format
- Be quick
COMP319
© University of Liverpool
slide 4
Re-factor types
• Encapsulate field
public int getLength() {
return(length);
}
• Re-name method, field
• String pw
• -
• String password
COMP319
© University of Liverpool
slide 5
Generalisation of type
class Customer {
}

class Person {
}
class Customer extends Person {
}
COMP319
© University of Liverpool
slide 6
Code breaking up re-factors
• Extract method
void setLength(int length) {
if (length<0) {
throw (new BadArgumentException());
}
this.length=length;
}

void validateLength(int length) {
if (length<0) {
throw (new BadArgumentException());
}
}
void setLength(int length) {
validateLength(length);
this.length=length;
}
COMP319
© University of Liverpool
slide 7
Graphs
•
•
•
•
•
A diagram depicting a network
Points at the end of arcs
Nodes at the junction of arcs
Regions enclosed by arcs
Used for representing:
- Solid figures (vertices, edges, faces)
- Electrical circuits
- Relationships between entities
• Graph or network theory
COMP319
© University of Liverpool
slide 8
Dependence graphs
• All computing systems have dependencies
• Control dependence
- 1 method calling another
• Data dependence
- 1 expression effecting another
- A=B*2
- C=A*4
• Control/data dependence
- If (age<=18) { println(“Age invalid”);
COMP319
© University of Liverpool
slide 9
Program dependency graphs
• Term appears in a paper by Kuck,
Muraoka & Chen (1981) although the
idea is in Turing’s early description of
“algorithms” in 1936
• Captures sequence/time between
entities (compare connection/distance)
• Control Dependence
• Data Dependence
COMP319
© University of Liverpool
slide 10
Example Program and its Dependence Graph
COMP319
© University of Liverpool
slide 11
Example Program and its Dependence Graph
COMP319
© University of Liverpool
slide 12
Dependency graph usage
• Optimisation
- Multiple independent statements can
run in parallel
- Code that never runs can be removed
- Boolean skip=true;
- If (!skip) then
- Loop invariance
- For (k=1;k<max_items;k++) {
- sum=sum+a*b;
-}
COMP319
© University of Liverpool
slide 13
Program slicing
• Interactive method for
- Debugging
- Program understanding
- Program maintenance
• Program reduction technique
(highlighter)
• A demonstration of SDG allowing:
- Control flow analysis
- Data flow analysis
COMP319
© University of Liverpool
slide 14
How does it work
• Choose v a variable or set of variables
• Choose n a point of interest
• Using the dependence graph the slice v
at n is constructed
• The slice v at n can be compiled and
studied separately
• Slices may be forward or backward from
n
COMP319
© University of Liverpool
slide 15
Backward Slicing
Original program
x = 1;
y = 2;
z = y-2;
r = x;
z = x+y;
/* the slice point is the end of the program */.
Backward Slice
x = 1;
y = 2;
z = x+y;
Debugging with a slice
Pass = 0 ; Fail = 0 ; Count = 0 ;
while (!eof())
{ TotalMarks=0;
scanf("%d",Marks);
if (Marks >= 40)
Pass = Pass + 1;
if (Marks < 40)
Fail = Fail + 1;
Count = Count + 1;
TotalMarks = TotalMarks+Marks ;
}
printf("Out of %d, %d passed and %d failed\n",
Count, Pass, Fail) ;
average = TotalMarks/Count;
/* point of
interest */
printf("The average was %d\n",average) ;
PassRate = Pass/Count*100 ;
printf("This is a pass rate of %d\n",PassRate) ;
COMP319
© University of Liverpool
slide 17
Bug location with backward slicing
while (!eof())
{
TotalMarks = 0;
scanf("%d",Marks);
Count = Count + 1;
TotalMarks = TotalMarks+Marks;
}
average = TotalMarks/Count;
printf("The average was
%d\n",average) ;
COMP319
© University of Liverpool
slide 18
Forward Slicing
Original program
x = 1; /* considering changing this line */
y = 3;
p = x + y ;
z = y -2 ;
if(p==0)
r++ ;
Forward Slice
/* Change to first line will affect */
p = x + y ;
if(p==0)
r++ ;
COMP319
© University of Liverpool
slide 19
Maintenance - Example
n = 0; product = 1; sum = 1;
scanf("%d",&x) ;
while (x >= 0)
{
sum = sum + x;
product = product * x ;
n = n + 1;
scanf("%d",&x);
}
average = (sum - 1) / n ;
printf("The total is %d\n",sum) ;
printf("The product is %d\n",product) ;
printf("The average is %d\n",average) ;
COMP319
© University of Liverpool
slide 20
Maintenance – backward slice
sum = 1;
scanf("%d",&x) ;
while (x >= 0)
{
sum = sum + x;
scanf("%d",&x);
}
printf("The total is %d\n",sum) ;
COMP319
© University of Liverpool
slide 21
Maintenance – forward slice
n = 0; product = 1; sum = 0;
scanf("%d",&x) ;
while (x >= 0)
{ sum = sum + x; /* AFFECTED */
product = product * x ;
n = n + 1;
scanf("%d",&x);
}
Average = (sum - 1) / n ; /* AFFECTED */
printf("The total is %d\n",sum) ; /* AFFECTED */
printf("The product is %d\n",product) ;
printf("The average is %d\n",average) ; /* AFFECTED
*/
COMP319
© University of Liverpool
slide 22
Types of slicing
• Static – described above. Slices are
constructed at compile time
• Dynamic slicing where slices are
constructed once the input is known
• Conditional slicing done at breakpoints
during execution
• Inter-modular slicing - complex systems
COMP319
© University of Liverpool
slide 23
The Horowitz, Prins, & Rep
(HPR) algorithm (merging)
• Step 1. Determine changed and preserved
slices e.g. adding a diameter calculation
• Step 2. Form the merged graph. Using the
idea of ‘graph union’
• Step 3. Test for interference i.e the merged
graph preserves all the slices of all the
variants.
• Step 4. Construct source from the merged
graph
COMP319
© University of Liverpool
slide 24
• Why richer constructs?
- More useful slicing
• SDG and inter-module slicing
- SDG from parse trees
- Other methods of generating an SDG
- Calls and variable scope handling
• Pointers, aliases, classes
COMP319
© University of Liverpool
slide 25
JSlice
• Java slicing software developed by
National University of Singapore
• Performs dynamic slicing
• Uses a compressed trace which records
- Flow control instructions
- Data manipulation
• JVM
- Kaffe
- Clean room implementation of Java
COMP319
© University of Liverpool
slide 26