java 基础
/** * 求a的n次方。 * @param a * @param n * @return */public double Factorial(double a,int n){ double v=1;//; for(int i=0;i<n;i++){// v*=a; } return v;}/** * 递归实现n的阶乘。 * @param n * @return */public long fac(int n){if(n==1){return 1;}else{return n*fac(n-1);}}/** * 1！+2！+ ...+n!(阶乘和) * @param a * @param m * @return */public static long sumFactorial(int n) throws Exception {long flag = 1;long sum = 0;for (int i = 1; i <= n; i++) {flag *= i;sum += flag;if (flag < 0) throw new Exception("溢出了！！！在第" + i + "个阶乘出现负数");if (sum < 0) throw new Exception("溢出了！！！在第" + i + "个阶乘之和出现负数");}return sum;}String.format:
System.out.println(String.format("%s","hi"));//hiSystem.out.println(String.format("输出结果：[%s][%s]","hello","world"));//输出结果：/** * 员工编号 */public void printStr(){ for(int i = 1;i <= 1000;i++){ System.out.println(String.format("A%09d", i));//很好 }}
/** * 分别是a,b,c,d四个球队，现在四个球队进行比赛,有多少种对决方案。 * @return */public String getVS(){String[] teams = {"a","b","c","d"};String result = "";for(int i=0;i<teams.length;i++){ for(int j=0;j<i;j++){ result += teams+" vs "+teams+"\n"; }}return result;}
/** *帮助理解for循环。 **/public class TestFor { static boolean foo(char c) { System.out.print(c); return true; } public static void main(String[] argv) { int i = 0; for (foo('A'); foo('B') && (i < 2); foo('C')) { i++; foo('D'); } }}
java 单态
public class Singleton {private static Singleton instance;//自己创建自己。/** * 生产该类对象的方法 * @return */ synchronized public static Singleton getSingletonInstance(){if(instance==null){instance=new Singleton();}return instance;}/** * 私有构造方法（不让外部new 实例） */private Singleton() {}}
java 获取控制台数据
public void printMac(){String result = new String();String os = System.getProperty("os.name");if (os != null && os.startsWith("Window")) {try {Process process = Runtime.getRuntime().exec("cmd.exe /c ipconfig /all");BufferedReader read = new BufferedReader(new InputStreamReader(process.getInputStream()));String s = read.readLine();while (s != null) {if (!s.equals("")) {result += "\n";result += s;}s = read.readLine();}System.out.println(result);} catch (Exception e) {throw new RuntimeException(e);}}}
/** * 字符串加1、补零 * @param str * @return */public static String getStrNum(String str){String result="";int src=Integer.parseInt(str);//转换成整型src++;result=Integer.toString(src);//加1后再转换成字符串while(result.length()<str.length()){//如果加1后的长度小于原来字符串，前面补零result="0"+result;}return result;}
如何将Hashmap线程同步
Map map=Collections.synchronizedMap(Map m);
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