Practice with Composite and Interpreter Patterns

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Practice with Composite and Interpreter Patterns
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Another Composite Example Computing with Lists

• An IntList is either
• Empty(), the empty list, or
• NonEmpty(first,rest), a non-empty list, where
  first is an int and rest is an IntList.
• Some examples includeEmpty() NonEmpty(7,Empty()
  ) NonEmpty(12, NonEmpty(17, Empty()))

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IntList
This class is located in a library so it must be
imported

• import java.util.NoSuchElementException
• abstract class IntList
• abstract int getFirst()
• abstract IntList getRest()
• class Empty extends IntList
• int getFirst()
• throw new
• NoSuchElementException("getFirst applied to
  Empty()")
• int getRest()
• throw new
• NoSuchElementException("getRest applied to
  Empty()")
• public String toString() return "Empty()"

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Remarks on IntList

• The operations getFirst() and getRest() are
  included in the abstract class IntList because
  they are useful operations for clients of
  IntList.
• Invoking getFirst() or getRest() on the Empty
  list is a run-time error, which we implement by
  throwing an exception. The Java throw construct
  takes an object of type Exception (which is a
  built-in class). In the absence of a catch
  handler attached to a method on the call stack,
  throwing an exception aborts the computation and
  prints the String message embedded in the
  exception.

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Finger Exercise

• Open the class IntList.java in DrJava, compile
  it, and try evaluating
• new Empty().getFirst()
• il1 new NonEmpty(17, new Empty())
• il1
• Il1.getRest().getFirst()

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The Singleton Pattern

• Each execution of the expression new
  Empty()creates a new object. In principle,
  there is only one empty list, just like there is
  only one number 0. Hence, we would like to
  represent the empty list by a single library.
• The singleton pattern is the mechanism that we
  use to create a unique instance of a class. This
  pattern consists of two chunks of code
• a static final field in the class that holds the
  single instance of the class
• a private attribute on the class constructor, so
  no client can create another instance of the
  class.

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Singleton IntList

• import java.util.NoSuchElementException
• abstract class IntList
• abstract int getFirst()
• abstract IntList getRest()
• class Empty extends IntList
• static final Empty ONLY new Empty()
• private Empty()
• int getFirst()
• throw new
• NoSuchElementException("getFirst applied to
  Empty()")
• int getRest()
• throw new
• NoSuchElementException("getRest applied to
  Empty()")
• public String toString() return "Empty()"

Static member holding the unique instance
Private constructor