sets and maps

The Set interface
• A Set is unordered and has no duplicates
• Operations are exactly those for Collection
2
int size( );
boolean isEmpty( );
boolean contains(Object e);
boolean add(Object e);
boolean remove(Object e);
Iterator iterator( );
boolean containsAll(Collection c);
boolean addAll(Collection c);
boolean removeAll(Collection c);
boolean retainAll(Collection c);
void clear( );
Object[ ] toArray( );
Object[ ] toArray(Object a[ ]);

Iterators for sets
• A set has a method Iterator iterator( ) to create an iterator
over the set
• The iterator has the usual methods:
– boolean hasNext()
– Object next()
– void remove()
• Since sets have iterators, you can also use Java 5’s “enhanced
for loop”
• remove() allows you to remove elements as you iterate over
the set
• If you change the set in any other way during iteration, the
iterator will throw a ConcurrentModificationException
3

Iterating through a Set (Java 1.4)
• import java.util.*;
public class SetExample2 {
public static void main(String[] args) {
String[ ] words = { "When", "all", "is", "said", "and", "done",
"more", "has", "been", "said", "than", "done" };
Set mySet = new HashSet();
for (int i = 0; i < words.length; i++) {
mySet.add(words[i]);
}
for (Iterator iter = mySet.iterator(); iter.hasNext();) {
String word = (String) iter.next();
System.out.print(word + " ");
}
System.out.println();
}
}
• and has more When done all than said is been
4

Iterating through a Set (Java 5.0)
public class SetExample {
String[ ] words = { "When", "all", "is", "said", "and", "done",
"more", "has", "been", "said", "than", "done" };
Set<String> mySet = new HashSet<String>();
for (String word : words) {
mySet.add(word);
}
for (String word : mySet) {
System.out.print(word + " ");
}
System.out.println();
}
}
• and has more When done all than said is been
5

Set implementations
• Set is an interface; you can’t say new Set( )
• There are four implementations:
– HashSet is best for most purposes
– TreeSet guarantees that an iterator will return
elements in sorted order
– LinkedHashSet guarantees that guarantees that an
iterator will return elements in the order they were
inserted
– AbstractSet is a “helper” abstract class for new
implementations
• It’s poor style to expose the implementation, so:
• Good: Set s = new HashSet( );
Fair: HashSet s = new HashSet( );
6

Typical set operations
• Testing if s2 is a subset of s1
s1.containsAll(s2)
• Setting s1 to the union of s1 and s2
s1.addAll(s2)
• Setting s1 to the intersection of s1 and s2
s1.retainAll(s2)
• Setting s1 to the set difference of s1 and s2
s1.removeAll(s2)
7

Set equality
• Object.equals(Object), inherited by all
objects, really is an identity comparison
• Implementations of Set override equals so
that sets are equal if they contain the same
elements
• equals even works if two sets have different
implementations
• equals is a test on entire sets; you have to be
sure you have a working equals on individual
set elements
• hashCode has been extended similarly
– This is for sets, not elements of a collection!
8

Membership testing in HashSets
• When testing whether a HashSet contains a given object,
Java does this:
– Java computes the hash code for the given object
• Hash codes are discussed in a separate lecture
• Java compares the given object, using equals, only with elements in
the set that have the same hash code
• Hence, an object will be considered to be in the set only if
both:
– It has the same hash code as an element in the set, and
– The equals comparison returns true
• Moral: to use a HashSet properly, you must have a good
public boolean equals(Object) and a good public int
hashCode() defined for the elements of the set
9

The SortedSet interface
• A SortedSet is just like a Set, except that an
Iterator will go through it in ascending order
• SortedSet is implemented by TreeSet
10

Membership testing in TreeSets
• In a TreeSet, elements are kept in order
• That means Java must have some means of
comparing elements to decide which is
“larger” and which is “smaller”
• Java does this by using either:
– The int compareTo(Object) method of the
Comparable interface, or
– The int compare(Object, Object) method of the
Comparator interface
• Which method to use is determined when
the TreeSet is constructed
11

Comparisons for TreeSets
• new TreeSet()
– Uses the elements “natural order,” that is, it uses
compareTo(Object) from Comparable
– All elements added to this TreeSet must implement Comparable,
or you will get a ClassCastException
• new TreeSet(Comparator)
– Uses compare(Object, Object) from the given Comparator
– The Comparator specified in the constructor must be applicable to
all elements added to this TreeSet, or you will get a
ClassCastException
• Moral: to use a TreeSet properly, you must provide the
equals method and implement either Comparable or
Comparator for the elements of the set
12

How hard is it to use a Set?
• You must have a working equals(Object) and a
working hashCode() or comparison method
• If you don’t really care about iteration order,
every object inherits equals(Object) and
hashCode() from Object, and this is usually good
enough
– That is, assuming you are happy with the == test
• Strings do all this for you (they implement
equals, hashCode, and Comparable)
• Bottom line: If you don’t care about order, and
== is good enough, just use HashSet
13

Set tips
• add and remove return true if they modify the set
• Here's a trick to remove duplicates from a Collection
c:
– Collection noDups = new HashSet(c);
• A Set may not contain itself an an element
• Danger: The behavior of a set is undefined if you
change an element to be equal to another element
• Danger: A TreeSet may throw a
ConcurrentModificationException if you
change an element in the TreeSet
14

The Map interface
• A Map is an object that maps keys to values
• A map cannot contain duplicate keys
• Each key can map to at most one value
• Examples: dictionary, phone book, etc.
15

Map implementations
• Map is an interface; you can’t say new Map( )
• Here are two implementations:
– HashMap is the faster
– TreeMap guarantees the order of iteration
• It’s poor style to expose the implementation
unnecessarily, so:
• Good: Map map = new HashMap( );
Fair: HashMap map = new HashMap( );
16

Map: Basic operations
Object put(Object key, Object value);
Object get(Object key);
Object remove(Object key);
boolean containsKey(Object key);
boolean containsValue(Object value);
int size( );
boolean isEmpty( );
17

More about put
• If the map already contains a given key,
put(key, value) replaces the value associated
with that key
• This means Java has to do equality testing on
keys
• With a HashMap implementation, you need to
define equals and hashCode for all your keys
• With a TreeMap implementation, you need to
define equals and implement the
Comparable interface for all your keys 18

Map: Bulk operations
• void putAll(Map t);
– Copies one Map into another
– Example: newMap.putAll(oldMap);
• void clear();
– Example: oldMap.clear();
19

Map: Collection views
• public Set keySet( );
• public Collection values( );
• public Set entrySet( );
– returns a set of Map.Entry (key-value) pairs
• You can create iterators for the key set,
the value set, or the entry set (the set of
entries, that is, key-value pairs)
• The above views provide the only way to
iterate over a Map
20

Map example
public class MapExample {
Map<String, String> fruit = new HashMap<String, String>();
fruit.put("Apple", "red");
fruit.put("Pear", "yellow");
fruit.put("Plum", "purple");
fruit.put("Cherry", "red");
for (String key : fruit.keySet()) {
System.out.println(key + ": " + fruit.get(key));
}
}
}
• Plum: purple
Apple: red
Pear: yellow
Cherry: red
21

Map.Entry
Interface for entrySet elements
• public interface Entry { // Inner interface of Map
Object getKey( );
Object getValue( );
Object setValue(Object value);
}
• This is a small interface for working with the
Collection returned by entrySet( )
• Can get elements only from the Iterator, and
they are only valid during the iteration
22

sets and maps

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sets and maps