Post on 23-Jun-2015
description
Comparable/Comparator InterfacesTopics:
Comparable and Comparator interfaces in JCF“Java Collections Framework”
Function objects
1
Back to Java: Comparable / Comparator In computing, we often want to order a set of items
Find the max/best or min/worstSort them in orderNote how this is different than what is needed for
search (where we just need equals)Need a way to compare two items
Want a flexible way to “compare” using different criteria
In Java, let’s us use Collection(s) methods to meet our own special needs. (Powerful!)
2
Check out the Collections classClass Collections
utility methods for doing operations on Collections, Lists (that almost always contain homogenous elements)
Note very similar Arrays classSee MSD textbook, Section 9.5, pp. 666f
Methods (static, mostly for Lists)search a list for an item: binarySearch()sort(), max(), min() -- uses compareTo() or Comparator
objectreverse(), fill(), shuffle(), copy(), replaceAll()List list2 = Collections.unmodifiableList(list1); // p. 668
3
Comparable InterfaceFirst solution: Can we ask an object how it
compares to a second object?Sure: string1.compareTo(string2)Programming convention: Return value as follows:
zero if the samenegative value if first item strictly less than secondpositive value if first item strictly greater than second
Java provides a Comparable interfaceint compareTo(YourClass o)
Note the parameter is an object of the same type as the class in which you’re defining compareTo()
4
Hang on: What’s an Interface?When defining a class, state that it “implements” an
interface. E.g.public class Watch implements TimeKeeper {
What else is the meaning of the interface TimeKeeper?A set of methods that any implementing class must
includeTimeKeeper interface doesn’t define how these
methods are codedWatch (the implementing class) is promising to
include those in its definition5
ExampleA TimeKeeper Interface defined with its 2
methods: getTime() and set Time()Now a Watch class is declared to implement
TimeKeeperWatch is promising to have these 2 methods as
part of its definition (getTime() and setTime())Watch can implement those how ever it wantsIt means Watch, by implementing TimeKeeper,
can handle the TimeKeeper role
6
Interface gives an Object another TypeWith this definition:
public class Watch implements TimeKeeper {You can think of Watch in these ways:
You can treat a Watch object as a TimeKeeperA Watch object can do “TimeKeeper things”A Watch object can be used anywhere a TimeKeeper is legal to
useA Watch object has more than one type
It’s a Watch (defined by the class Watch)It’s a TimeKeeper (defined by the interface)
7
Interface gives an Object another Type
Interfaces are legal Java types. Therefore can be usedTo declare variablesAs a parameter type (passing a variable to a
method)As a return type
8
Example If you have a method that took a TimeKeeper as a
parameterA Watch could be used as a parameter to that
method, because a Watch plays the role of a TimeKeeper
If that method takes a TimeKeeper as a parameter, then we can pass it a Watch
You can think of the Watch object as having more than one type. (Every Watch also fills the role as Time Keeper)
9
What are we getting at?The method sort can take an ArrayList as an
argument (an ArrayList of something) Of what? It can take an ArrayList of anything that meets this interface called Comparable
The parameter has to be an ArrayList of any class that implements the comparable interface – that means it has the compareTo() method – which means inside of sort, it knows it can call get() to get 2 items from the ArrayList and use the compareTo() method – its guaranteed to have it because that class implements the Comparable interface
10
Writing compareTo for Your ClassesIf you ever want to put your own objects in
Collections, and use sort(), max(), min(),…1. Make your class implement Comparable2. Implement the compareTo() method in your class
How to write compareTo()?Think about state-variables that determine natural
orderCompare them and return proper-valueNote: For number values, you can subtract. For object values, call compareTo() on them.
11
Let’s Get PracticalYou can sort an ArrayList… BUT the ArrayList has
to have things inside it that implement the Comparable Interface
Can we sort an ArrayList of Strings?Go to API for class String (Google: “java api string”)On “All Implemented Interfaces” – one of them is
comparable!Scroll to “Method Summary” – has compareTo()
method!So you can definitely sort an ArrayList of Strings
12
Example: Writing compareTo()
Imagine something like an entry in a phonebookOrder by last name, first name, then number
int compareTo(PhoneBookEntry item2 ) {int retVal= last.compareTo(item2.last);if ( retVal != 0 ) return retVal;retVal = first.compareTo(item2.first);if ( retVal != 0 ) return retVal;retVal = phNum - item2.phNum;return retVal;
}
13
See Example done in classECLIPSE EXAMPLE
14
Under the Hood for Sorting How might a sort() or any other method use this? Imagine:
Its parameter is of the type List<Comparable> ArrayList is a type-of List in Java (more on this later)
Inside a loop, code might look like this:Comparable item1 = theList.get(i);Comparable item2 = theList.get(j);int cmpResult = item1.compareTo(item2);
Such code will work when the list stores any class that implements Comparable!
But, what happens if list-elements are of different classes (still Comparable, but different)? compareTo() fails!
15
Flexible Design using ComparatorsSolution #1: Make classes Comparable
Disadvantage: just one way to compare is possible, because there’s just one compareTo method per class
Possible solutions:Separate functions: sortByName(), sortByNum(),…
We can’t predict in advance how you’ll want to sort!Pass a parameter to indicate control:
sort(theList, “byName”) or sort(theList, “byNum”);Ugh. Same problem as beforeAnd the internals of sort() will grow to become very ugly
16
Function ObjectsWe need to somehow pass “how to execute”
information as a parameter to sort()We pass objects as parametersCan we pass a method/operation as an object?
Many languages support this, but in different ways:C and C++ – pointers to functionsC# – delegates Java – “function objects” that
implement a specified interface, and the one method in that interface does the needed work
17
Function Objects in Java Idea: encapsulate a
function inside a classNote: not our usual idea of
a classState? (None.)Identity? (Just need one
instance.)Represents an entity?
(Nope! Just a place to stash a function so it can be passed as a parameter.)
Warning / caveat!This idea is contrary to
many OO principles, but…Useful if done in limited
circumstancesUse it when the libraries
make it availableNot often part of your own
class-design But use it in libraries when
it’s part of the framework
18
Example: Comparator objects
We want to pass a function-object to a method:Collections.sort(someList, function-object-goes-here);
But what type should this object be?Use an Interface:
Interface name can be used as a type in the parameter list
Interface defines the method name itself!
19
Example: Comparator objects
Java’s Comparator interface: int compare( Object o1, Object o2);Notes: not compareTo()! Takes two parameters!
Define a class for each kind of comparison you want. E.g.Classes: CmpStudentByGpa,
CmpStudentByGpaDescClasses: CmpDogByName, CmpDogByBreed
20
Writing a Comparator Class
Example like one from MSD text, p. 647We have a Dog class with name, breed and gender
Compare two doggies by breed and then name
public class CmpDogByBreedAndName implements Comparator<Dog> {
public int compare(Dog d1, Dog d2) { int retVal = d1.getBreed().compareTo(d2.getBreed()); if ( retVal != 0 ) return retVal; return d1.getName().compareTo( d2.getName() );}
}
21
Use of Comparator methodsHow to use with Collections.sort()
ArrayList dogList = …;Collections.sort( dogList, new CmpDogByName() );Collections.sort( dogList, new CmpDogByBreed() );
(Do you understand what new does here?) Inside sort(), code looks something like this:
sort ( List theList, Comparator cmpObj ) {
// in some loop Object item1 = list.get(i); Object item2 = list.get(j); cmpResult = cmpObj.compare(item1,item2);
22
End of Comparable/Comparator topics
Keep these techniques in mind as we move forward
23
Java Aside: Anonymous ClassesThere’s a Java technique called anonymous classes
One of several types of nested class definitionYou’ll very often see it in GUI programming (Swing)
and with threadsSituation:
Sometimes Java’s design encourages us to create some thing that might be used just once
That thing needs to be wrapped up in a class, say because we need a function object
24
Creating and Using an Anonymous ClassExample: sort a list of Strings by their length
Collections.sort ( stringList, new Comparator() {public int compare( Object o1, Object o2 ) {
return ((String) o1).length() – ((String) o2).length();
}} ) ;
We’ve created a new Comparator “on the fly”new creates a new instance, but what kind?
Some object that implements ComparatorObject not named, and its “true” class not named!
What must a Comparator have? compare()We defined it right here, where it’s used!25
Anonymous Classes: Comments Anonymous classes are
unlike other classesThey have no nameTypically only implement
methods in their interface or superclass. No new methods!
Since they have no name, can only define and use them at one point in your code!
Hard to understand at first? Sure!Naming an abstraction is
important for human understanding!
Sorting, a Collection, Comparing
AdviceKeep them very short (and
simple)!Be ready to understand them
when you see them in Swing and with threads
26