CS110- Lecture 15 April 13, 2005

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CS110- Lecture 15April 13, 2005

• Announcements
• Project 3 is due tomorrow at 10.45 PM.
• No labs next week.
• Agenda
• Comparing objects
• Project 3 Questions
• Interfaces
• Method Overloading
• Sorting

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Comparing Strings

• public int compareTo(String s) // not boolean
• hello.compareTo(help!) // -4 (l - p)
  
• help!.compareTo(hello) // 4 ( p l)
• hello.compareTo(hell) // 1 ( 5 - 4)
• hello.compareTo(hello) // 0
• The idea is
• x.compareTo(y) 0 just when x.equals(y)
• x.compareTo(y) lt 0 when x lt y
• x.compareTo(y) gt 0 when x gt y
• compareTo is wonderful for sorting
  (alphabetizing)

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String compareTo pseudocode

• March through the character arrays looking for
  first char difference
  (be sure not to run off the end, since
  lengths may differ)
• if you find a char difference, return it
  (numerically)
• if no difference when you reach the end of the
  shorter string, return the difference in lengths
  (0 if the same)

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Comparing Objects

• The idea is same
• obj1.compareTo(obj2)
• if objects are equal, return 0
• if obj1 is greater than obj2, return ve.
• if obj2 is greater than obj1, return ve.
• But how we decide that when objects are equal,
  one object is less than the other or greater than
  the other.
• We choose our own criteria.

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Comparing Objects

• For e.g.
• A pixel is greater than other pixel if its
  luminance is more
• A student is less than the other if his/her
  age is less than other student
• or
• A student is less than other student if
  his/her name comes alphabetically before the
  other student.

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Interfaces

• We have used the term interface to refer to the
  set of public methods through which we can
  interact with one another.
• We now we are going to formalize this concept
  using a particular language construct in Java
• A java interface is a collection of constants and
  abstract methods.

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Abstract Method

• An abstract method is a method that does not have
  an implementation.
• There is no body of code defined for an abstract
  method.
• The header of the method, including its parameter
  list, is simply followed by a semicolon.
• An abstract method can be preceded by the
  reserved word abstract.
• An interface cannot be instantiated.

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Comparable Interface

• public interface Comparable
• public int compareTo(Object o)
• All the methods in interfaces are abstract
  and have public visibility by default.
• A class implements an interface by providing
  method implementations for each of the abstract
  methods defined in the interface.

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Interfaces

• When a class implements an interface
• the class definition becomes
• public class AClass implements AnInterface
• For e.g.
• public class String implements Comparable
• public class Pixel implements Comparable
• public class ImageBase implements ActionListener

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Comparator Interface

• public interface Comparator
• public int compare(Object o1, Object o2)
• public boolean equals(Object obj)
• Any class that will implement this Interface
  should ideally provide implementation for these
  method. But sometimes we dont provide
  implementation of equals method (use equals of
  Object class)

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Interfaces

• Cannot instantiate an interface
• Cannot do the following
• // cannot instantiate Comparable class
• // since it is not a concrete class
• Comparable c new Comparable()
• However we can do the following
• Comparable c new String()

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Method Overloading

• Often the method name is sufficient to indicate
  which method is being called by a specific
  invocation.
• But we can use the same method name with
  different parameters lists for multiple methods.
  This technique is known as method overloading.

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Method Overloading

• So a method name can be used again as long as the
  number of parameters, the type of those
  parameters, and/or the order of the types of
  parameters are different.
• A methods name along with the number, type and
  order of its parameters is called the methods
  signature.