Abstract Data Types II

Sufficient operations A set of operations on an ADT is sufficient if, together, 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

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

Necessary and sufficient operations Notice that “sufficient” applies to a set of operations, while “necessary” applies to individual operations in that set “Necessary” is relative to the set; an operation that is necessary in one set may not be necessary in another A necessary and sufficient set of operations is a set which is sufficient (meets all the requirements), but would not be sufficient if any one operation were omitted

Convenience operations An operation is a convenience operation if it could be accomplished by some overly complex combination of other operations 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?

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

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!

Types of operations A constructor creates a legal value of the ADT (typically 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 compute another value of the same ADT A mutative transformer changes the value of the ADT it is given An applicative transformer takes one value of an ADT and, without changing it, returns a new value of the same ADT

Requirements The constructors and transformers must together be able to create all legal values of the ADT A constructor or transformer should never create an illegal value It’s nice if the constructors alone can create all legal values, but sometimes this results in constructors with too many parameters for reasonable convenience The accessors must be able to extract any data needed by the application

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

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 The constructor might be called with illegal values of the parameters You may wish to create unique instances of objects A common solution to these problems is to embed the call or calls to a private constructor inside a static “factory” method Example: public static 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); }

Factory methods II With a normal constructor, public class Add { public Add( ) { } } each call to new Add() creates another Add object These objects are not equal unless you define your own equals(Object o) method But with a factory method, public class Add { static Add add = new Add(); // done once when class is loaded private Add( ) { } // hide this constructor from the world! public static Add getAdd( ) { return add; } } You can only ever have one Add object Hence, the default equals method (same as ==) always works

Example: String Constructors: Accessors: "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.

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

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

Mutable objects A StringBuffer is mutable: it can be changed The StringBuffer 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, doing the work, and converting back

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);

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

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

The End