第03章-面向对象特征9.pptVIP

* 字符串操作类 字符串的连接运算(Concatenation) +与append方法比较 class MyTimer { private final long start; public MyTimer() { start = System.currentTimeMillis(); } public long getElapsed() { return System.currentTimeMillis() - start; } } public class AppDemo { static final int N = 47500; public static void main(String args[]) { MyTimer mt = new MyTimer(); String str1 = ; for (int i = 1; i = N; i++) str1 = str1 + *; System.out.println(#1’s time = + mt.getElapsed()); mt = new MyTimer(); StringBuffer sb = new StringBuffer(); for (int i = 1; i = N; i++) sb.append(*); String str2 = sb.toString(); System.out.println(#2’s time = + mt.getElapsed()); } } 运行结果： #1’s time = 138750 #2’s time = 20 效率和代码优化 * 字符串操作类 字符串的连接运算(Concatenation) +: 算术运算符 int a = 1, b = 2; String c = “ men”; String s = a + b + c; 3 men 隐含结合率 (a + b) + c String s = c + b + a; men 21 * 字符串操作类 java.util.StringTokenizer类 将一个字符串按特定的要求切分开 One String ? more Tokens 构造方法 public StringTokenizer(String?str) 按默认的分隔符将字符串分割– 四个字符“?\t\n\r\f” public StringTokenizer(String?str, String?delim) 按指定的分割符将字符串分割 方法 public boolean hasMoreTokens() 判断是否有token public String nextToken() 得到下一个token * 字符串操作类 java.util.StringTokenizer类 方法举例 String s = this is a test; StringTokenizer st = new StringTokenizer(s); while (st.hasMoreTokens()) { System.out.println(st.nextToken()); } 运行结果: this is a test String s = this:is:a:test”; StringTokenizer st = new StringTokenizer(s, “:”); while (st.hasMoreTokens()) { System.out.println(st.nextToken()); } 运行结果: this is a test * 执行外部程序 java.lang.Runtime类 public Process exec(String command) throws IOException Executes the specified string command in a separate process. public Process exec(String[] cmdarray) throws IOException Executes the specified command and arguments in a separate process. * 执行外部程序 java.lang.Runtime类 Window NT/2000/XP cmd.exe /C 命令 Windows 95/98 /C 命令 * 执行外部程序 java.lang.Runtime类 import java.io.IOException; class Test { public static v