Transcript Lecture 11 - University of Virginia, Department of Computer Science

Lecture 11:
Subtyping and
Inheritance
CS201j: Engineering Software
University of Virginia
Computer Science
David Evans
http://www.cs.virginia.edu/evans
Menu
• Quiz Results
• Subtyping
• Inheritance
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CS 201J Fall 2003
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Class Speed
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Way too fast
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Too Fast
Just about right
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Too Slow
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Problem Set 5 Teams
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Assigned
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Read PS3 Comments
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6 of these
also said the
class is going
too fast
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Yes
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Skimmed
Looked at a few
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No, but will
before Exam 1
No, just killing
trees
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Still in course if not required?
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No
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Yes
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Not Required
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Sections
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Worthwhile
Not Worthwhile
I rarely go
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More working through More working in groups
problems on the
chalkboard
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Working in lab on
problem sets
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Comments
“I’m still not convinced why we focus more time on
programming concepts (invariants, requires, etc…)
instead of code. I’d rather see problem sets with
more implementation of code and less
writing/methodical work. I just feel there is so much
to Java we could be implementing and learning.”
Why not focus more on learning Java?
- Some of you will be professional programmers
Will need to program in lots of other languages
Will need to be better than average programmers
- Some of you won’t want to program again after this class
Concepts apply to more than just programming: whatever you design
will benefit from abstraction, invariants, specifications,
notions of subtyping/inheritance, understanding concurrency, etc.
- As students at a liberal arts institution, you should be learning intellectually valuable
things, not temporary skills
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Comments
• Spend more time on Java syntax, need to
cover Java more, etc.
– Two weeks ago you were asked, “send any
questions you have about Java programming”
– Only 2 people sent any questions
• More TAs
– Only 2-3 people at each of the lab hours
Sunday and Monday!
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Subtyping
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Subtyping
Cell
ConwayLifeCell is a subtype of Cell
Cell is a supertype of ConwayLifeCell
ConwayLifeCell ≤ Cell
ConwayLifeCell
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Subtype Substitution
• If B is a subtype of A, everywhere the code
expects an A, a B can be used instead
• Examples:
Cell c = c1; c1 must be a subtype of Cell
(note A is a subtype of A)
Cell c = new ConwayLifeCell ();
ConwayLifeCell c = new Cell ();
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Subtype Examples
java.util.Vector:
public void addElement (Object obj);
public class StringSet {
Vector elements;
public void insert (String s) {
elements.addElement (s);
}
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Why we can use
a String where
an Object is
expected?
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Java’s Type Hierarchy
Object
Cell
String
ConwayLifeCell
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Object is the ultimate supertype of
every object type.
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Node
PathInterpolator
Interpolator
Selector
RotationPathInterpolator
Leaf
SceneGraphObject
Not at all uncommon to have
class hierarchies like this!
Java
3D Class
Hierarchy Diagram
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http://java.sun.com/products/java-media/3D/collateral/j3dclass.html
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Subtype Examples
java.util.Vector:
public void addElement (Object obj);
public class IntSet {
Vector elements;
Primitive types
public void insert (int x) {
are not subtypes
elements.addElement (x); of Object.
elements.addElement (new Integer (x));
}
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But Integer is…
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Inheritance
• To implement a subtype, it is often useful
to use the implementation of its supertype
• This is also called “subclassing”
• In Java:
class B extends A
B is a subtype of A
B inherits from A
class C implements F
C is a subtype of F
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CS 201J Fall 2003
both subtyping and inheritance
just subtyping
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Charge
• Subtyping
– Allow one type to be used where another type
is expected
• Inheritance
– Reuse implementation of the supertype to
implement a subtype
• Thursday:
– When is it safe to say B is a subtype of A?
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