5. Set
Set
Set
boolean add(E o) // add the specified element if it is not already present boolean remove(Object o) // remove the specified element if it is present boolean contains(Object o) // return true if it contains o // Set operations boolean addAll(Collection c) // Set union boolean retainAll(Collection c) // Set intersection
HashSet
public class Book {
private int id;
private String title;
// Constructor
public Book(int id, String title) {
this.id = id;
this.title = title;
}
@Override
public String toString() {
return id + ": " + title;
}
// Two book are equal if they have the same id
@Override
public boolean equals(Object o) {
if (!(o instanceof Book)) {
return false;
}
return this.id == ((Book)o).id;
}
// Consistent with equals(). Two objects which are equal have the same hash code.
@Override
public int hashCode() {
return id;
}
}
import java.util.HashSet;
import java.util.Set;
public class TestHashSet {
public static void main(String[] args) {
Book book1 = new Book(1, "Java for Dummies");
Book book1Dup = new Book(1, "Java for the Dummies"); // same id as above
Book book2 = new Book(2, "Java for more Dummies");
Book book3 = new Book(3, "more Java for more Dummies");
Set set1 = new HashSet();
set1.add(book1);
set1.add(book1Dup); // duplicate id, not added
set1.add(book1); // added twice, not added
set1.add(book3);
set1.add(null); // Set can contain a null
set1.add(null); // but no duplicate
set1.add(book2);
System.out.println(set1); // [null, 1: Java for Dummies,
// 2: Java for more Dummies, 3: more Java for more Dummies]
set1.remove(book1);
set1.remove(book3);
System.out.println(set1); // [null, 2: Java for more Dummies]
Set set2 = new HashSet();
set2.add(book3);
System.out.println(set2); // [3: more Java for more Dummies]
set2.addAll(set1); // "union" with set1
System.out.println(set2); // [null, 2: Java for more Dummies, 3: more Java for more Dummies]
set2.remove(null);
System.out.println(set2); // [2: Java for more Dummies, 3: more Java for more Dummies]
set2.retainAll(set1); // "intersection" with set1
System.out.println(set2); // [2: Java for more Dummies]
}
}
一個Set不能存放重復的元素，檢查元素的重復性就是通過重寫的equal()方法來檢查的。另外一個Set中只能存放一個null元素。addAll()是否是將兩個Set聯合，retainAll()是取兩個Set的交集。
需要注意的是Set中元素的排序是任意的，並不是插入的順序。
5.2 LinkedHashSet
import java.util.LinkedHashSet;
import java.util.Set;
public class TestLinkedHashSet {
public static void main(String[] args) {
Book book1 = new Book(1, "Java for Dummies");
Book book1Dup = new Book(1, "Java for the Dummies"); // same id as above
Book book2 = new Book(2, "Java for more Dummies");
Book book3 = new Book(3, "more Java for more Dummies");
Set set = new LinkedHashSet();
set.add(book1);
set.add(book1Dup); // duplicate id, not added
set.add(book1); // added twice, not added
set.add(book3);
set.add(null); // Set can contain a null
set.add(null); // but no duplicate
set.add(book2);
System.out.println(set); // [1: Java for Dummies, 3: more Java for more Dummies,
// null, 2: Java for more Dummies]
}
}
Iterator5.4 TreeSetdescendingIterator() // Returns an iterator over the elements in this set, // in descending order. Iterator iterator() // Returns an iterator over the elements in this set, in ascending order. // Per-element operation E floor(E e) // Returns the greatest element in this set less than or equal to the given element, // or null if there is no such element. E ceiling(E e) // Returns the least element in this set greater than or equal to the given element, // or null if there is no such element. E lower(E e) // Returns the greatest element in this set strictly less than the given element, // or null if there is no such element. E higher(E e) // Returns the least element in this set strictly greater than the given element, // or null if there is no such element. // Subset operation SortedSet headSet(E toElement) // Returns a view of the portion of this set // whose elements are strictly less than toElement. SortedSet tailSet(E fromElement) // Returns a view of the portion of this set // whose elements are greater than or equal to fromElement. SortedSet subSet(E fromElement, E toElement) // Returns a view of the portion of this set // whose elements range from fromElement, inclusive, to toElement, exclusive.
TreeSet是NavigableSet 和SortedSet的實現 .
例子——實現Comparable的TreeSet
public class AddressBookEntry implements Comparable {
private String name, address, phone;
public AddressBookEntry(String name) {
this.name = name;
}
@Override
public String toString() {
return name;
}
@Override
public int compareTo(AddressBookEntry another) {
return this.name.compareToIgnoreCase(another.name);
}
@Override
public boolean equals(Object o) {
if (!(o instanceof AddressBookEntry)) {
return false;
}
return this.name.equalsIgnoreCase(((AddressBookEntry)o).name);
}
@Override
public int hashCode() {
return name.length();
}
}
AddressBookEntry實現了Comparable接口，為了在TreeSet中進行使用，它重寫了compareTo()方法去比較name，它同時也重新了equals()和hashCode()方法，主要為了保持跟compareTo()的一致性。
import java.util.TreeSet;
public class TestTreeSetComparable {
public static void main(String[] args) {
AddressBookEntry addr1 = new AddressBookEntry("peter");
AddressBookEntry addr2 = new AddressBookEntry("PAUL");
AddressBookEntry addr3 = new AddressBookEntry("Patrick");
TreeSet set = new TreeSet();
set.add(addr1);
set.add(addr2);
set.add(addr3);
System.out.println(set); // [Patrick, PAUL, peter]
System.out.println(set.floor(addr2)); // PAUL
System.out.println(set.lower(addr2)); // Patrick
System.out.println(set.headSet(addr2)); // [Patrick]
System.out.println(set.tailSet(addr2)); // [PAUL, peter]
}
}
可以看到AddressBookEntry對象在add()操作過程中進行了排序，使用的就是Comparable實現的方法。
例子——實現Comparator的TreeSet
public class PhoneBookEntry {
public String name, address, phone;
public PhoneBookEntry(String name) {
this.name = name;
}
@Override
public String toString() {
return name;
}
}
import java.util.Set;
import java.util.TreeSet;
import java.util.Comparator;
public class TestTreeSetComparator {
public static class PhoneBookComparator implements Comparator {
@Override
public int compare(PhoneBookEntry p1, PhoneBookEntry p2) {
return p2.name.compareToIgnoreCase(p1.name); // descending name
}
}
public static void main(String[] args) {
PhoneBookEntry addr1 = new PhoneBookEntry("peter");
PhoneBookEntry addr2 = new PhoneBookEntry("PAUL");
PhoneBookEntry addr3 = new PhoneBookEntry("Patrick");
Comparator comp = new PhoneBookComparator();
TreeSet set = new TreeSet(comp);
set.add(addr1);
set.add(addr2);
set.add(addr3);
System.out.println(set); // [peter, PAUL, Patrick]
Set newSet = set.descendingSet(); // Reverse the order
System.out.println(newSet); // [Patrick, PAUL, peter]
}
}
上面我們創建了一個帶有BookComparator實例的TreeSet，這樣上面Set中對象的比較使用的就是BookComparator的compare方法，在上面例子中，調用TreeSet的descendingSet()方法來是集合倒序。
6. Queue
在CollectionQueue也額外添加了插入、獲取、查看的操作方法，每個方法都提供了兩種形式：一個是如果操作失敗會拋出異常，另外一個如果操作是否會返回一個值（false或者null，據具體操作而定）。// Insertion at the end of the queue boolean add(E e) // throws IllegalStateException if no space is currently available boolean offer(E e) // returns true if the element was added to this queue, else false // Extract element at the head of the queue E remove() // throws NoSuchElementException if this queue is empty E poll() // returns the head of this queue, or null if this queue is empty // Inspection (retrieve the element at the head, but does not remove) E element() // throws NoSuchElementException if this queue is empty E peek() // returns the head of this queue, or null if this queue is empty
// Insertion void addFirst(E e) void addLast(E e) boolean offerFirst(E e) boolean offerLast(E e) // Retrieve and Remove E removeFirst() E removeLast() E pollFirst() E pollLast() // Retrieve but does not remove E getFirst() E getLast() E peekFirst() E peekLast()一個Deque可以看做是一個FIFO對象（通過
add(e),remove(),element(),offer(e),poll(),peek()方法），也可以看做是一個LIFO隊列（通過push(e),pop(),peek()方法）。
Queue和Deque的實現類如下：
PriorityQueue: 這個隊列可以使用一個指定的順序去進行排序，而不是FIFO的順序。ArrayDeque: 使用queue或者deque來實現的動態數組，類似於ArrayListLinkedList: 它在實現ListQueue和Deque接口，它是雙向鏈表結構。 
Map接口聲明了下面抽象方法。
V get(Object key) // Returns the value of the specified key V put(K key, V value) // Associate the specified value with the specified key boolean containsKey(Object key) // Is this map has specified key? boolean containsValue(Object value) // Views SetkeySet() // Returns a set view of the keys Collection values() // Returns a collection view of the values Set entrySet() // Returns a set view of the key-value
Map接口的實現類包括：
HashMap:HashMap實現了MapTreeMap:實現了SortedMapLinkedHashMap:帶有鏈表的哈希表，方便插入和刪除Hashtable:它是對Map
