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. Java I/O. Class 7 1. Class 7

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1 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 1 回顧 Java I/O 2 1

2 Java Data Structure 動態資料結構 執行的時候可以動態變大或縮小 類型 Linked lists Stacks Queues Binary trees 3 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 4 2

3 圖示 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 6 3

4 Linked Lists Linked list 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 7 Linked Lists firstnode lastnode... H D Q 8 4

5 20.4 Linked Lists (cont.) (a) firstnode 7 11 new Listnode 12 (b) firstnode 7 11 new Listnode 12 Graphical representation of operation insertatfront Linked Lists (cont.) (a) firstnode lastnode new Listnode (b) firstnode lastnode new Listnode Graphical representation of operation insertatback. 10 5

6 20.4 Linked Lists (cont.) (a) firstnode lastnode (b) firstnode lastnode removeitem Graphical representation of operation removefromfront Linked Lists (cont.) (a) firstnode lastnode (b) firstnode current lastnode removeitem Graphical representation of operation removefromback. 12 6

7 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 13 Stacks 先進後出 E.g., 停車 14 7

8 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 15 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 16 8

9 Binary Tree B A D C Binary tree graphical representation. 17 Tree Traversals 見 v.html 18 9

10 Tree Travesals Binary search tree containing 12 values. InOrder 7,11,17,25,31,43,44,47 PreOrder 47,25,11,7,17,43,31,44 PostOrder 7,17,11,31,44,43,25 19 Java collections framework Java collections framework Provides reusable componentry Common data structures Example of code reuse 20 10

11 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 21 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 22 11

12 Collections Overview 23 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 24 12

13 Collection Messages Basic messages constructor() 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 25 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) 26 13

14 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. 27 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? 28 14

15 Lists List Ordered Collection that can contain duplicate elements Sometimes called a sequence Implemented via interface List ArrayList LinkedList Vector 29 Vector 30 15

16 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! 31 ArrayList Demo: constructor import java.util.*; /* The ArrayList is replaces the old Vector class. ArrayList implements the Collection interface, and also the more powerful List interface features as well. Main methods:add(), size(), get(i), iterator() See the "Collection" and "List" interfaces. */ public static void demoarraylist() { ArrayList strings = new ArrayList(); 32 16

17 ArrayList Demo: adding/size // add things... for (int i= 0; i<10; i++) { // Make a String object out of the int String numstring = Integer.toString(i); // add pointer to collection strings.add(numstring); } // access the length System.out.println("size:" + strings.size()); 33 ArrayList Demo: looping // ArrayList supports a for-loop access style... // (the more general Collection does not support this) for (int i=0; i<strings.size(); i++) { String string = (String) strings.get(i); // Note: cast the pointer to its actual class System.out.println(string); } 34 17

18 ArrayList: iterating // ArrayList also supports the "iterator" style... Iterator it = strings.iterator(); while (it.hasnext()) { String string = (String) it.next(); and cast pointer System.out.println(string); } // get // Calling tostring() System.out.println("to string:" + strings.tostring()); 35 ArrayList Demo: removing // Iterate through and remove elements // get a new iterator (at the beginning again) it = strings.iterator(); while (it.hasnext()) { it.next(); // get pointer to elem it.remove(); // remove the above elem } } System.out.println("size:" + strings.size()); 36 18

19 ArrayList Demo: output /* Output... size: to string:[0, 1, 2, 3, 4, 5, 6, 7, 8, 9] size:0 */ 37 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 38 19

20 Sets Set Collection that contains unique elements HashSet Stores elements in hash table TreeSet Stores elements in tree 39 Maps Map Associates keys to values Cannot contain duplicate keys Called one-to-one mapping 要有 hashcode() 才行 40 20

21 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 41 21