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有25匹馬，每匹馬都以一個固定不變的速度奔跑，每匹馬的速度都不一樣，如果讓你找出跑的最快的5匹馬，最少需要組織多少場比賽？注：每場比賽最多只能5匹馬參賽。

有25匹馬，每匹馬都以一個固定不變的速度奔跑，每匹馬的速度都不一樣，如果讓你找出跑的最快的5匹馬，最少需要組織多少場比賽？注：每場比賽最多只能5匹馬參賽。
re:悲觀10場，樂觀7場。

posted @ 2012-03-07 21:17 sevenduan 閱讀(833) | 評論 (1) | 編輯收藏

Eclipse:
ctrl+o
ctrl+space
ctrl+t
ctrl+k
ctrl+f8/f9/f10

alt+shift+x , t/j
alt+shift+d , t/j

Command:
cd -
tab
ctrl+a/e
ctrl+u/k/w

Vim
shift+g
m+'mark'
`+'mark'
. (repeat)

posted @ 2010-08-07 23:14 sevenduan 閱讀(247) | 評論 (0) | 編輯收藏

Java logging framework

Framework	Supported log levels	Standard appenders	Popularity	Cost / licence
Log4J	FATAL ERROR WARN INFO DEBUG TRACE	AsyncAppender, JDBCAppender, JMSAppender, LF5Appender, NTEventLogAppender, NullAppender, SMTPAppender, SocketAppender, SocketHubAppender, SyslogAppender, TelnetAppender, WriterAppender	Widely used in many project and platforms	Apache License, Version 2.0
Java Logging API	SEVERE WARNING INFO CONFIG FINE FINER FINEST	Depends on the underlying framework; Sun's default Java Virtual Machine (JVM) has the following: ConsoleHandler, FileHandler, SocketHandler, MemoryHandler	Not widely used^{[citation needed]}	Comes with the JRE
Apache Commons Logging	FATAL ERROR WARN INFO DEBUG TRACE	Depends on the underlying framework	Widely used, in conjunction with log4j	Apache License, Version 2.0
SLF4J	ERROR WARN INFO DEBUG TRACE	Depends on the underlying framework, which is pluggable	Probably small but growing	MIT License

posted @ 2010-07-18 22:15 sevenduan 閱讀(486) | 評論 (0) | 編輯收藏

java transaction summary

1 definition:

“A transaction is a complete unit of work. It may comprise many computational tasks,which may include user interface, data retrieval, and communications. A typicaltransaction modifies shared resources.”

2 transaction features:
ACID (atomicity, consistency, isolation, durability)

3 java spec
JTA, JTS

1

interface javax.transaction.TransactionManager
2

{
3

public abstract void begin();
4

public abstract void commit();
5

public abstract int getStatus();
6

public abstract Transaction getTransaction();
7

public void resume(Transaction tobj);
8

public abstract void rollback();
9

public abstract void setRollbackOnly();
10

public abstract void setTransactionTimeout(intseconds);
11

public abstract Transaction suspend() ;
12

}

4 Common XAResource
JDBC 2.0:
A JDBC driver that supports distributed transactions implements the javax.transaction.xa.XAResource interface, the javax.sql.XAConnectioninterface, and the javax.sql.XADataSource interface.

JMS 1.0:

a JMS provider javax.transaction.xa.XAResource interface, the implements the javax.jms.XAConnection and the javax.jms.XASession interface.

5 Common TransactionManager

5.1 EJB Transaction Options:
NotSupported
    If the method is called within a transaction, this transaction is suspended during the time of the method execution.
Required
    If the method is called within a transaction, the method is executed in the scope of this transaction; otherwise, a new transaction is started for the execution of the method and committed before the method result is sent to the caller.
RequiresNew
    The method will always be executed within the scope of a new transaction. The new transaction is started for the execution of the method, and committed before the method result is sent to the caller. If the method is called within a transaction, this transaction is suspended before the new one is started and resumed when the new transaction has completed.
Mandatory
    The method should always be called within the scope of a transaction, else the container will throw the TransactionRequired exception.
Supports
    The method is invoked within the caller transaction scope; if the caller does not have an associated transaction, the method is invoked without a transaction scope.
Never
    The client is required to call the bean without any transaction context; if it is not the case, a java.rmi.RemoteException is thrown by the container.

5.2 Spring transaction:
Transaction isolation: The degree of isolation this transaction has from the work of other transactions. For example, can this transaction see uncommitted writes from other transactions? avaliable options:
ISOLATION_DEFAULT
ISOLATION_READ_UNCOMMITTED
ISOLATION_READ_COMMITTED
ISOLATION_REPEATABLE_READ
ISOLATION_SERIALIZABLE

Transaction propagation: Normally all code executed within a transaction scope will run in that transaction. However, there are several options specifying behavior if a transactional method is executed when a transaction context already exists: For example, simply running in the existing transaction (the most common case); or suspending the existing transaction and creating a new transaction. Spring offers the transaction propagation options familiar from EJB CMT. avaliable options:
PROPAGATION_MANDATORY
PROPAGATION_NESTED
PROPAGATION_NEVER
PROPAGATION_NOT_SUPPORTED
PROPAGATION_REQUIRED
PROPAGATION_REQUIRES_NEW
PROPAGATION_SUPPORTS

Transaction timeout: How long this transaction may run before timing out (automatically being rolled back by the underlying transaction infrastructure).
Read-only status: A read-only transaction does not modify any data. Read-only transactions can be a useful optimization in some cases (such as when using Hibernate).

6 transaction for web service
Protocol specifications:
WS-Transaction
OASIS Business Transaction Protocol (BTP)
Java API
JAXTX (JSR-156)

posted @ 2010-04-25 16:44 sevenduan 閱讀(611) | 評論 (0) | 編輯收藏

java nest class and java inner class

JavaScript里有兩個容易讓初學者混淆的概念：scope chain and closure。
比如說，我想創建10個函數，每個函數依次返回0-9.

1 //wrong: all function refer to global variable i=10
2 var fn_list=[];
3 for(var i=0;i<10;i++){
4  var _tempFn =function(){
5         return i;
6  }
7  fn_list.push(_tempFn);
8 }
9 //right: every function refer to its closure scope variable a
10 var fn_list=[];
11 for(var i=0;i<10;i++){
12  var _tempFn =function(a){
13         return function(){
14          return a;
15         };
16  }
17  fn_list.push(_tempFn(i));
18 }
19

Java里也有兩個讓初學者容易混淆的概念：nest class and inner class。
nest class就是static inner class,
而inner class就是no-static inner class。沒有為什么，sun就是這么定義的。
還是上面得例子，創建10個對象，每個對象的getValue接口依次返回0-9.

1 public class Test {
2     private int noStaticValue;
3     private static int staticValue;
4
5     public Test(int noSV, int sv) {
6         this.noStaticValue = noSV;
7         this.staticValue = sv;
8     }
9
10     public Test(int noSV) {
11         this.noStaticValue = noSV;
12     }
13
14     interface valueHolder {
15         int getValue();
16     }
17
18     class innerClass implements valueHolder {
19         public int getValue() {
20             return noStaticValue;
21         }
22     }
23
24     static class nestClass implements valueHolder {
25         public nestClass(int i) {
26             staticValue = i;
27         }
28
29         public int getValue() {
30             return staticValue;
31         }
32     }
33
34     public static void main(String[] args) {
35         Test context1 = new Test(0, 0);
36         valueHolder[] list = new valueHolder[10];
37         for (int i = 0; i < 10; i++) {
38             list[i] = new Test.nestClass(i);
39         }
40         for (valueHolder obj : list) {
41             System.out.println(obj.getValue());// always print 9
42         }
43         for (int i = 0; i < 10; i++) {
44             list[i] = new Test(i).new innerClass();
45         }
46         for (valueHolder obj : list) {
47             System.out.println(obj.getValue());// print 0-9
48         }
49     }
50 }

可見用inner class可以模擬closure的特性，就是運行時定義class的某些狀態。
inner class和nest class之間的區別就是后者是靜態類。前者必須通過wrap class的實例來調用new,e.g. new Test().new innerClass。
因為nest class是靜態類，所以可以添加static member 或者static method,而inner class 不行。
匿名內部類是inner class的一種特殊形式，所以也不能添加static member 或者static method。

posted @ 2010-04-17 23:07 sevenduan 閱讀(2787) | 評論 (5) | 編輯收藏

String與byte轉化要小心失真

先看一段代碼：

byte [] b = new byte[]{1,-1,2,-2};
System.out.println(Arrays.toString(new String(b).getBytes()));

輸出：
[1, -17, -65, -67, 2, -17, -65, -67]
解釋：
byte decode to String,String encode to byte 默認用UTF-8 charset.
decode遇到不支持的字符輸出 char ? , encode ? 就是 -17, -65, -67.
實現細節可見ByteToCharUTF8.java

解決辦法：使用 ISO8859_1 charset。

教訓：注意charset的范圍。

posted @ 2010-04-14 23:14 sevenduan 閱讀(2034) | 評論 (0) | 編輯收藏

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