Comparable/Comparator InterfacesTopics:

Comparable and Comparator interfaces in JCF“Java Collections Framework”

Function objects

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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!)

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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

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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()

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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

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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)

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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

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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)

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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

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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.

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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

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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;

}

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See Example done in classECLIPSE EXAMPLE

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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!

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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

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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

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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

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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!

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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

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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() );}

}

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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);

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End of Comparable/Comparator topics

Keep these techniques in mind as we move forward

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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

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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

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