Bài giảng cấu trúc dữ liệu chương 4 nguyễn xuân vinh

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DATA STRUCTURES
[214331]

Iterator - Comparable - Comparator

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Nguyễn Xuân Vinh



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Java Collection Architecture

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The Differences!!!

 How to browse element?
 Elements of List are indexed.

List
index

0

1

2

3

4

5

6

7

8

9

value

3

8

9

7

5

12

0

0

0

0

Linked List

int value = list[0];



Elements of LinkedList, Sets and Maps can't be accessed

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by index

 How to remove element?

"the"
Set

"from"

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Map

3

"to"

"we"


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Examining sets and maps

 elements of Java Sets and Maps can't be accessed by index
 must use a "foreach" loop:

Set<Integer> scores = new HashSet<Integer>();
for (int score : scores) {
System.out.println("The score is " + score);

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}



Problem: foreach is read-only; cannot modify set while looping
for (int score : scores) {
if (score < 60) {
// throws a ConcurrentModificationException
scores.remove(score);
}

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}


 iterator: An object that allows a client to traverse the elements of any collection, regardless of its implementation.
 Remembers a position within a collection, and allows you to:
get the element at that position
advance to the next position

(possibly) remove or change the element at that position

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Iterators (11.1)



index

0

1

2

3

4

5

6

7

8

9

value

3

8

9

7

5

12

0

0

0

0

size

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list

Benefit: A common way to examine any collection's elements.

5

iterator

set

"to"

"we"

"from"

current element:
current index:

"the"

2

9

iterator

current element:

next element: "the"

"from"


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Iterator methods
hasNext()

returns true if there are more elements to examine

next()

returns the next element from the collection (throws a NoSuchElementException if there are
none left to examine)

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remove()

removes from the collection the last value returned by next() (throws IllegalStateException
if you have not called next() yet)

 Iterator interface in java.util
 every collection has an iterator() method that returns an iterator over its elements
Set<String> set = new HashSet<String>();
...
Iterator<String> itr = set.iterator();


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Iterator example
Set<Integer> scores = new HashSet<Integer>();
scores.add(38);
scores.add(94);
scores.add(87);
scores.add(43);
scores.add(62);

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...
Iterator<Integer> itr = scores.iterator();
while (itr.hasNext()) {
int score = itr.next();
System.out.println("The score is " + score);
// eliminate any failing grades
if (score < 60) {
itr.remove();
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System.out.println(scores);

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}

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}
// [62, 94, 87]


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Iterator example 2
Map<String, Integer> scores = new HashMap<String, Integer>();

scores.put("Kim", 38);
scores.put("Lisa", 94);
scores.put("Ryan", 87);
scores.put("Morgan", 43);
scores.put("Marisa", 62);

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...
Iterator<String> itr = scores.keySet().iterator();
while (itr.hasNext()) {
String name = itr.next();
int score = scores.get(name);
System.out.println(name + " got " + score);

// eliminate any failing students
if (score < 60) {

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itr.remove(); // removes name and score
}
}
System.out.println(scores);// {Marisa=62, Lisa=94, Ryan=87}


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Exercise

 Modify the Book Search program from last lecture to eliminate any words that are plural or all-uppercase from the
collection.


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Set/Map and ordering

 Some types have a notion of a natural ordering .
 TreeSet/Map store values sorted by their natural ordering.
Set<Integer> scores = new HashSet<Integer>();
scores.add(38);
scores.add(94);

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scores.add(87);
scores.add(43);

// unpredictable order

scores.add(62);
System.out.println(scores);

// [62, 94, 43, 87, 38]

Set<Integer> scores = new TreeSet<Integer>();
scores.add(38);
scores.add(94);

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System.out.println(scores);

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scores.add(43);

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scores.add(87);
// sorted natural order

scores.add(62);
// [38, 43, 62, 87, 94]


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Ordering our own types

 We cannot make a TreeSet or TreeMap of any arbitrary type, because Java doesn't know how to order the
elements.



The program compiles but crashes when we run it.

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Set<HtmlTag> tags = new TreeSet<HtmlTag>();
tags.add(new HtmlTag("body", true));
tags.add(new HtmlTag("b", false));
...

Exception in thread "main" java.lang.ClassCastException
at java.util.TreeMap.put(TreeMap.java:542)

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at java.util.TreeSet.add(TreeSet.java:238)
at MyProgram.main(MyProgram.java:24)


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Comparable (10.2)
public interface Comparable<E> {
public int compareTo(E other);
}

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 A class can implement the Comparable interface to define a natural ordering function for its objects.
 A call of a.compareTo(b) should return:
a value < 0

if a comes "before" b in the ordering,

a value > 0

if a comes "after" b in the ordering,

or

if a and b are considered "equal" in the ordering.

0


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Comparable example
public class Point implements Comparable<Point> {
private int x;
private int y;
...
// sort by x and break ties by y
public int compareTo(Point other) {

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if (x < other.x) {
return -1;
} else if (x > other.x) {
return 1;
} else if (y < other.y) {

return -1;

// same x, smaller y

} else if (y > other.y) {
return 1;

// same x, larger y

} else {
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return 0;
}

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}
}

// same x and same y


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compareTo tricks

 subtraction trick - Subtracting related numeric values produces the right result for what you want compareTo to

return:

// sort by x and break ties by y
public int compareTo(Point other) {
if (x != other.x) {
return x - other.x;

// different x

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} else {
return y - other.y;

// same x; compare y

}
}



The idea:

if x

if x
if x

> other.x,

then x - other.x > 0

< other.x,

then x - other.x < 0

== other.x,

then x - other.x == 0


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compareTo tricks 2

 delegation trick - If your object's fields are comparable (such as strings), use their compareTo results to help you:
// sort by employee name, e.g. "Jim" < "Susan"
public int compareTo(Employee other) {
return name.compareTo(other.getName());

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}

 toString trick - If your object's toString representation is related to the ordering, use that to help you:

// sort by date, e.g. "09/19" > "04/01"
public int compareTo(Date other) {
return toString().compareTo(other.toString());

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}


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Comparable and sorting

 The Arrays and Collections classes in java.util have a static method sort that sorts the elements of
an array/list

Point[] points = new Point[3];
points[0] = new Point(7, 6);
points[1] = new Point(10, 2)
points[2] = new Point(7, -1);

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points[3] = new Point(3, 11);
Arrays.sort(points);
System.out.println(Arrays.toString(points));

// (3, 11), (7, -1), (7, 6), (10, 2)

List<Point> points = new ArrayList<Point>();
points.add(new Point(7, 6));
...

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Collections.sort(points);
System.out.println(points);
// (3, 11), (7, -1), (7, 6), (10, 2)


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Arrays class

Method name

Description

asList(value1, ..., valueN)

returns a List containing the given values as its elements

binarySearch(array, value)

returns the index of the given value in a sorted array (< 0 if not

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found)

copyOf(array)

returns a new array with same elements

equals(array1, array2)

returns true if the two arrays contain the same elements in the
same order

fill(array, value)

sets every element to have given value

sort(array)

arranges elements into ascending order

toString(array)

returns a string representing the array, such as "[10, 30, 17]"


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Collections class

Method name
binarySearch(list, value)

Description
returns the index of the given value in a sorted list (< 0 if not

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found)
copy(listTo, listFrom)

copies listFrom's elements to listTo

emptyList(), emptyMap(),

returns a read-only collection of the given type that has no

emptySet()

elements

fill(list, value)

sets every element in the list to have the given value

max(collection), min(collection)

returns largest/smallest element

replaceAll(list, old, new)

replaces an element value with another

reverse(list)

reverses the order of a list's elements

shuffle(list)

arranges elements into a random order

sort(list)

arranges elements into ascending order

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Comparable basics

 Ordering is done through the compareTo() method.


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System-Defined Comparable classes


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Understanding Comparable
public int compareTo(Object obj)

 Elements must be mutually comparable:
 You can only compare like elements.
 Or ClassCastException will be thrown.
 In most cases, the two objects must be of the same type (or a subtype)


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Understanding Comparable

 The return value states the relative position to the natural ordering:
 compareTo() method can return one of three values:
Negative number: current object > object compared to.
Positive number: current object < object compared to.
Zero: two objects are equal.


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Understanding Comparable

 The natural ordering should be consistent with equals():
 Two elements are equal (equals()): compareTo() should return zero.

 Your class implements Comparable and inconsistent with equals(): won't work properly within a SortedSet or

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SortedMap.


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Understanding Comparable

 Never call the method directly:
 Collections Framework call the compareTo() method for you when necessary.
 Not your responsibility to call the method.


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Comparator

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