上課講義 - 網路資料庫實驗室

Java 程式設計
基礎班 (7)
劉根豪
台大電機所網路資料庫研究室
Class 7 1
Java Data Structure
Email: kenliu@arbor.ee.ntu.edu.tw
動態資料結構
執行的時候可以動態變大或縮小
類型
Linked lists
Class 7
圖示
Class 7
Stacks
Queues
Binary trees
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3
5
回顧
Java I/O
Class 7
自我參考類別(self-referential
classes)
包含參考到相同類別物件的實體欄位
class Node {
private int data;
private Node nextNode; // reference to next linked node
}
欄位 nextNode 為連結
nextNode 連結到另一個Node object
Class 7
動態記憶體配置
在執行期要求更多的記憶體來儲存新的物
件
JVM的記憶體是一個可改變大小的heap
structure
可以用-Xms和-Xmx來跟JVM要求起始配置的
heap大小, e.g., java –Xmx1600mb myclass
WindowsXP+JRE1.4.0最大可以到16xxmb左
右
Class 7
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Linked Lists
Linked list
Class 7
Linear collection of self-referential classes
(nodes)
Connected by reference links
Nodes可以insert或delete,即使它不在list的開
頭或結尾
Last node is set to null to mark end of list
20.4 Linked Lists (cont.)
Class 7
(a)
(b)
firstNode
new Listnode
firstNode
new Listnode
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Graphical representation of operation insertAtFront.
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7 11
20.4 Linked Lists (cont.)
Class 7
(a)
(b)
removeItem
firstNode
firstNode
12
12
lastNode
12 7 11 5
lastNode
7 11 5
Graphical representation of operation removeFromFront.
7
9
11
Linked Lists
Class 7
Class 7
firstNode
lastNode
...
H D Q
20.4 Linked Lists (cont.)
(a)
firstNode
12
lastNode
12 7 11 5
new Listnode
(b) firstNode lastNode new Listnode
7 11 5
Graphical representation of operation insertAtBack.
20.4 Linked Lists (cont.)
Class 7
(a)
(b)
firstNode
12
lastNode
12 7 11 5
firstNode current
lastNode
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removeItem
Graphical representation of operation removeFromBack.
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Queues
Queue
先進先出
可以想像一個supermarket的購物checkout List
Nodes inserted only at tail (back)
Method enqueue
Nodes removed only from head (front)
Method dequeue
Class 7
Trees
Tree
Non-linear, two-dimensional data structure
(有別於 linked lists, stacks and queues)
每一個Node有兩個(或以上)的Links
Root node is the first node
Each link refers to a child
Left child is the first node in left subtree
Right child is the first node in right subtree
Children of a specific node are siblings
Nodes with no children are leaf nodes
Class 7
Binary Tree
Class 7
B
A D
Binary tree graphical representation.
C
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Stacks
先進後出
E.g., 停車
Class 7
Binary search tree
Special ordering of nodes
Values in left subtrees are less than values in right subtrees
Inorder traversal
Traverse left subtree, obtain node value, traverse right subtree
Preorder traversal
Obtain node value, traverse left subtree, traverse right subtree
Postorder traversal
Traverse left subtree, traverse right subtree, obtain node value
Class 7
Tree Traversals
見
http://www.student.seas.gwu.edu/~idsv/ids
v.html
Class 7
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Tree Travesals
InOrder7,11,17,25,31,43,44,47…
PreOrder47,25,11,7,17,43,31,44…
PostOrder7,17,11,31,44,43,25…
Class 7
47
25 77
11 43 65 93
7 17 31 44 68
Binary search tree containing 12 values.
Collections Overview
Collection
Data structure (object) that can hold references to
other objects
Collections framework
Interfaces declare operations for various collection
types
Belongs to package java.util
Class 7
Collections Overview
Class 7
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Java collections framework
Java collections framework
Provides reusable componentry
Common data structures
Example of code reuse
Class 7
Collections
Built-in support for collections
Similar to STL in C++
Collection type
Sequence/Set
Example ArrayList
Map type
Hashtable/dictionary
Example HashMap
Collections store pointers to objects!
Use inheritance and interfaces
Class 7
Collection Design
All classes implement a similar interface
add(), size(), iterator()…
Easy learning curve for using Collections
Possible to swap out the underlying implementation without
significant code change
Implemented as pointer to Object
Similar to using a void * in C
Require a cast back to the actual type
Example
String element = (String)arraylist.get(i)
Java checks all casts at run-time
Class 7
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Collection Messages
Basic messages
constructor()
Class 7
Creates a collection with no elements
size()
Number of elements in the collection
boolean add()
Add a new pointer/element at the end of the collection
Returns true is the collection is modified.
iterator()
Returns an Iterator Object
Iterators
Used to iterate through a collection
Abstracts away the underlying details of the
implementation
Iterating through an array is the same as a binary tree
Responds to
hasNext() - Returns true if more elements
next() - Returns the next element
remove() - removes element returned by previous
next() call.
Class 7
Lists
List
Ordered Collection that can contain
duplicate elements
Sometimes called a sequence
Implemented via interface List
ArrayList
Class 7
LinkedList
Vector
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Additional Collection Messages
Utilities
Additional useful methods
boolean isEmpty()
boolean contains(Object o)
Iterative search, uses equals()
boolean remove(Object o)
Iterative remove(), uses equals()
Boolean addAll(Collection c)
Class 7
Working with Iterators
Not valid to modify a collection directly
while an iterator is being used!
Should not call collection.add() or
collection.remove()
OK to modify the collection using the
iterator itself
iterator.remove()
Why?
Class 7
Vector
Class 7
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5

ArrayList
Most useful collection
Replaces the “Vector” class
Can grow over time
Methods
add()
int size()
Object get(int index)
Index is from 0 to size() -1
Must cast to appropriate type when used.
iterator()
We’ll see an ezample!
Class 7
ArrayList Demo: adding/size
Class 7
// add things...
for (int i= 0; i

ArrayList Demo: output
/* Output...
size:10
0
1
2
3
4
5
6
7
8
9
to string:[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
size:0
*/
Class 7
Sets
Set
Class 7
Collection that contains unique elements
HashSet
Stores elements in hash table
TreeSet
Stores elements in tree
Unmodifiable Collection
Unmodifiable Collection
將一個collection轉成不可被修改的collection
假設嘗試要modifiy它的話,系統會Throw
UnsorrtedOperationException
Class 7
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Algorithms
在Collections物件中提供了一些對collection的基本運作algorithms
Implemented as static methods
List algorithms
sort
binarySearch
reverse
shuffle
fill
copy
Collection algorithms
Min,max
在Arrays物件中提供了一些對array運作的函式
Class 7
Maps
Map
Associates keys to values
Cannot contain duplicate keys
Called one-to-one mapping
Class 7
要有hashcode()才行
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