String I/O Parse etc.

一、The differences between the String, StringBuilder, and StringBuffer classes
1.Strings are immutable

String s = "abcde";
s = s.concat("more stuff");

Now there are three String Objects
abcde、more stuff、 abcde more stuff

2.To make Java more memory efficient, the JVM sets aside a special area of memory called the "String constant pool."

When the compiler encounters a String literal, it checks the pool to see if an identical String already exists. If a match is found, the reference to the new literal is directed to the existing String, and no new String literal object is created.

3.Create new strings

   String s="abc"                //create one String Object(in pool)
   String s= new String("abc");  //create two String 

objects,one in "String pool",and another in normal memory

4.StringBuilder is not safe thread because its method is not synchronize , but run faster than StringBuffer
      public synchronized StringBuffer append(String s)

二.Calendar主要用于日期的操纵，它是一个抽象类，所以

Calendar c = new Calendar();   //非法
Calendar c = Calendar.getInstance()  //合法
//当日期为2001年10月，此时调用
c.roll(Calendar.MONTH,9);
//此时日期为2001年7月，它只会改变日期，但不进位(即不改变年份)

三.DateFormat的用法

import java.text.*;
import java.util.*;
class Dates3 {
public static void main(String[] args) {
Date d1 = new Date(1000000000000L);
DateFormat[] dfa = new DateFormat[6];
dfa[0] = DateFormat.getInstance();
dfa[1] = DateFormat.getDateInstance();
dfa[2] = DateFormat.getDateInstance(DateFormat.SHORT);
dfa[3] = DateFormat.getDateInstance(DateFormat.MEDIUM);
dfa[4] = DateFormat.getDateInstance(DateFormat.LONG);
dfa[5] = DateFormat.getDateInstance(DateFormat.FULL);
for(DateFormat df : dfa)
System.out.println(df.format(d1));
}
}
//which on our JVM produces
9/8/01 7:46 PM
Sep 8, 2001
9/8/01
Sep 8, 2001
September 8, 2001
Saturday, September 8, 2001

如果我们把日期转化为SHORT型，然后在parse为日期，此时会丢失精度

Date d1 = new Date(1000000000000L);
System.out.println("d1 = " + d1.toString());
DateFormat df = DateFormat.getDateInstance(
DateFormat.SHORT);
String s = df.format(d1);
System.out.println(s);
try {
Date d2 = df.parse(s);
System.out.println("parsed = " + d2.toString());
} catch (ParseException pe) {
System.out.println("parse exc"); }
//which on our JVM produces
d1 = Sat Sep 08 19:46:40 MDT 2001
9/8/01
parsed = Sat Sep 08 00:00:00 MDT 2001

四.正则表达式的使用

import java.util.regex.*;
class RegexSmall {
public static void main(String [] args) {
Pattern p = Pattern.compile("ab"); // the expression
Matcher m = p.matcher("abaaaba"); // the source
boolean b = false;
while(b = m.find()) {
System.out.print(m.start() + " "+m.group());
//调用group()方法获得匹配的结果
}
}
}

五、习题中出现的错误
1.Remember, that the equals() method for the integer wrappers
will only return true if the two primitive types and the two values are equal.

class TKO {
public static void main(String[] args) {
String s = "-";
Integer x = 343;
long L343 = 343L;
if(x.equals(L343)) s += ".e1 ";
if(x.equals(343)) s += ".e2 ";
Short s1 = (short)((new Short((short)343)) / (new Short((short)49)));
if(s1 == 7) s += "=s ";
if(s1 < new Integer(7+1)) s += "fly ";
System.out.println(s);
} }

2.The %b (boolean) conversion character returns true for any
non-null or non-boolean argument.

System.out.format("%b", 123);

the %f (floating-point) conversion character won't automatically promote an integer type

System.out.printf("%f", 123);  //illegal

3.Only String objects can be operated on using the overloaded "+" operator.

StringBuffer sb = new StringBuffer("hello");
		sb += "xyz";  //illegal

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New words in this chapter
1.zillion    n.    庞大的数字、无法计算的数字
2.budge      v.    移动
3.devilish   adj.  如恶魔般的，精力旺盛的
4.concordant adj.  协调的
5.cohesion   n     结合，凝聚，内聚
6.lenient    adj.  宽大的，仁慈的
7.newbie     n     新手
8.guru       n     领袖，头头
9.chunk (of) n.    大块，矮胖的人或物
10.maven     n.    内行，专家