Software reuse means to “borrow” existing code. How can you reuse an existing class to make a new one? Such reuse is really an adaptation -- using the.

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Presentation transcript:

Software reuse means to “borrow” existing code. How can you reuse an existing class to make a new one? Such reuse is really an adaptation -- using the adapter design pattern Example: Create a SetOf3Doublets class by adapting a SimpleStringOfSet class. The Object of Data Abstraction and Structure, David D. Riley © Addison Wesley pub.

Domain set of String Invariant (for every SetOfDoublets) this ->size() == 3 and exists( e : this-> includes(e) | e.length() == 2 ) Constructor public SetOfDoublets(String s1, String s2, String s3) pre: s1.length()==2 and s2.length()==2 and s3.length==2 and s1!=s2 and s1!=s3 and s2!=s3 post: this == set{ s1, s2, s3 } Query Methods public boolean isIn(String s) post: result == this-> includes( s ) public boolean equals(SetOf3Doublets s) post: result == ( this = s ) “ = “ denotes set equality Update Methods public void replace( String oldS String newS) pre: newS.length()==2 and isIn(oldS) (throws IllegalStateException) post: this == SetOf3Doublets ADT Specifications The Object of Data Abstraction and Structure, David D. Riley © Addison Wesley pub.

SetOf3Doublets «constructor» + SetOf3Doublets(String s1, String s2, String s3) «query» + boolean isIn(String s) + String equals(SetOf3Doublets s) «update» + void replace(String oldS, String newS) How would you reuse SimpleSetOfString to design SetOf3Doublets? SimpleSetOfString «constructor» + SimpleSetOfString() «query» + int size() + boolean isIn(String s) + String equals(SimpleSetOfString s) «update» + void add(String s) + void remove(String s) + void union(SimpleSetOfString ss) + void intersection(SimpleSetOfString ss) The Object of Data Abstraction and Structure, David D. Riley © Addison Wesley pub.

Solution 1: Use Inheritance SetOf3Doublets «constructor» + SetOf3Doublets(String s1, String s2, String s3) «query» + boolean isIn(String s) + String equals(SetOf3Doublets s) «update» + void replace(String oldS, String newS) SimpleSetOfString «constructor» + SimpleSetOfString() «query» + int size() + boolean isIn(String s) + String equals(SimpleSetOfString s) «update» + void add(String s) + void remove(String s) + void union(SimpleSetOfString ss) + void intersection(SimpleSetOfString ss) The Object of Data Abstraction and Structure, David D. Riley © Addison Wesley pub.

public class SetOf3Doublets extends SimpleSetOfString { public SetOf3Doublets(String s1, String s2, String s3) { super(); add(s1); add(s2); add(s3); } public void replace(String oldS, String newS) { remove(oldS); add(newS); } This is an example of the adapter design pattern, because SetOf3Doublets is adapting SimpleSetOfString via inheritance. Methods that must be coded: SetOf3Doublets replace Methods that are reused without change: isIn equals The Object of Data Abstraction and Structure, David D. Riley © Addison Wesley pub.

Potential Problem with the inheritance version of the adapter design pattern SetOf3Doublets «constructor» + SetOf3Doublets(String s1, String s2, String s3) «query» + boolean isIn(String s) + String equals(SetOf3Doublets s) «update» + void replace(String oldS, String newS) SimpleSetOfString «constructor» + SimpleSetOfString() «query» + int size() + boolean isIn(String s) + String equals(SimpleSetOfString s) «update» + void add(String s) + void remove(String s) + void union(SimpleSetOfString ss) + void intersection(SimpleSetOfString ss) Unwanted members are inherited: size add remove union intersection inheriting size not harmful use of add, remove, and union might violate invariant The Object of Data Abstraction and Structure, David D. Riley © Addison Wesley pub.

Solution 2: Proxy Design Pattern SetOf3Doublets - SimpleSetOfString theSet «constructor» + SetOf3Doublets(String s1, String s2, String s3) «query» + boolean isIn(String s) + String equals(SetOf3Doublets s) «update» + void replace(String oldS, String newS) SimpleSetOfString «constructor» + SimpleSetOfString() «query» + int size() + boolean isIn(String s) + String equals(SimpleSetOfString s) «update» + void add(String s) + void remove(String s) + void union(SimpleSetOfString ss) + void intersection(SimpleSetOfString ss) theSet is a “proxy” that allows SetOf3Doublets to access SimpleSetOfString members. The proxy design pattern uses aggregation to accomplish the adapter design pattern. The Object of Data Abstraction and Structure, David D. Riley © Addison Wesley pub.

public class SetOf3Doublets { private SimpleSetOfString theSet; public SetOf3Doublets(String s1, String s2, String s3) { theSet = new SimpleSetOfString(); theSet.add(s1); theSet.add(s2); theSet.add(s3); } public void replace(String oldS, String newS) { theSet.remove(oldS); theSet.add(newS); } public boolean isIn(String s) { return theSet.isIn(s); } public boolean equals(SetOf3Doublets ss) { return theSet.equals(ss); } Proxy disadvantages: 1) must “reimplement” methods (see isIn & equals ) 2) slightly messier notation “ theSet.” Proxy advantage: provides maximum control over class members