Abstract Data Types II

2 Sufficient operations Operations on an ADT are sufficient if they meet all the requirements They must be able to create all the values and perform all the operations required by the application Remember that the application cannot directly access the internal values They should be able to create all the values and perform all the operations required by any application in a given class of applications

3 Necessary operations An operation on an ADT is necessary if omitting it would fail to meet the requirements If the application can implement an operation easily and efficiently by combining other operations, that operation is unnecessary It’s OK to have unnecessary operations if they add significantly to the convenience of using the ADT

4 Convenience operations An operation is a convenience operation if it could be accomplished by some overly complex combination of other operations People like things simple A class with 50 or more methods is just overwhelming Convenience operations should be justified Will it be used often? Does it really simplify the user’s task? Would the user expect this operation to be provided? Is it significantly more efficient?

5 Necessary and sufficient operations A class should define a necessary and sufficient set of operations Convenience operations should be justified Similarly, a class should have a necessary and sufficient data representation In general, a class should not contain data that can be easily computed from other data in the class

6 Example: Strings Necessary and sufficient operators: A constructor: public String(char[] chs) Ways to access data: public int length() public charAt(int index) Would you be happy with just these? If you invented the String class, could you justify operations such as equals and string concatenation? Convenience operators aren’t all bad!

7 Types of operations A constructor is creates a value of the ADT from input values An accessor uses a value of the ADT to compute a value of some other type A transformer uses a value of the ADT to computer another value of the ADT A mutative transformer changes the value of the ADT it is given An applicative transformer takes one ADT and, without changing it, returns a new ADT

8 Requirements The constructors and transformers must together be able to create all legal values of the ADT Or, at least, any needed by the application For a general use ADT, this means all legal values The accessors must be able to extract any data Or, at least, any needed by the application

9 Operations in Java Constructors can be implemented with Java constructors A constructor’s job is to construct an object of a class in a valid state That should be a constructor’s only job Accessors and transformers can be implemented with Java methods Mutative transformers are typically (but not always) implemented as void methods Sometimes they both modify an object and return it

10 Factory methods The problem with a constructor is that it will always construct an object of a given type This isn’t always what you want You may want to check the constructor’s parameters for validity You may want to return an existing object, if there is one For example, you really only need one EOL Token You may want to return objects of different types You can solve these problems by using a factory method Example: public Animal create(String voice) { if (voice.equals("woof")) return new Dog(); if (voice.equals("meow")) return new Cat(); if (voice.equals("moo")) return new Cow(); throw new IllegalArgumentException(voice); }

11 Another factory method class Token { Type t; Object v; Token eolToken = new Token(Type.EOL, "\n"); private Token(Type t, Object v) { // constructor this.t = t; this.v = v; } public Token create(Type t, Value v) { if (t == Type.EOL) return eolToken; else return new Token(t, v); } }

12 Example ADT: String Constructors: "This is syntactic sugar for a constructor" public String(char[] chs) Accessors: public int length() public char charAt() Transformers (applicative only): public String substring(int i, int j) public String concat(String that) (also +) Etc.

13 Immutable objects A String is immutable: it cannot be changed The String class has no mutative transformers Operations such as string concatenation create new Strings Advantages: Efficient (for most uses) Easy to use and simple to understand (changing a String in one object doesn’t change it in other objects) Disadvantage: Every call to a transformer creates a new String

14 Example: StringBuilder Constructors: public StringBuilder(String s) Accessors: public int length() public char charAt() Transformers (applicative): none Transformers (mutative): public StringBuilder append(Object obj) Etc.

15 Mutable objects A StringBuilder is mutable: it can be changed The StringBuilder class has both applicative and mutative transformers Advantage: Efficient (for doing a lot of string manipulation) Disadvantage: Can be confusing (example coming up shortly) Operations on Strings are done by converting to StringBuffers (like StringBuilders), doing the work, and converting back

16 Safe use of Strings public class Person { private String name; Person(String name) { this.name = name; } } String jan = "Jan"; Person doctor = new Person(jan); String dan = "D" + jan.substring(1, 2); Person secretary = new Person(dan);

17 Unsafe use of StringBuilders public class Person { private StringBuilder name; Person(StringBuilder name) { this.name = name; } } StringBuilder buffer = new StringBuilder("Jan"); Person doctor = new Person(buffer); buffer.setCharAt(0, 'D'); Person secretary = new Person(buffer);

18 Summary A class should define a necessary and sufficient set of operations Convenience operations should be justified Operations can be classified as: Constructors Accessors Transformers (applicative or mutative) Immutable objects are often preferable to mutable objects

19 The End