一.获取UUID

UUID uuid = UUID.randomUUID();
String str = uuid.toString();
// 去掉“-”
String s = str.replaceAll("-", "");

二.获取固定格式的时间字符串

//获取当前时间对象
Date d = new Date(); 
//创建日期格式化类对象,”yyyy/MM/dd HH:mm:ss”是我们希望的日期出现格式
SimpleDateFormat sdf = new SimpleDateFormat(“yyyy-MM-dd HH:mm:ss”);
System.out.println(sdf.format(d));

三.获取[0, 100)之间的int整数

double d = Math.random();
int i = (int)(d*100);

四.把 Java util.Date 转成 sql.Date

java.util.Date utilDate = new java.util.Date(); 
java.sql.Date sqlDate = new java.sql.Date(utilDate.getTime());

五.Java中将String转化为Int

String str = "123";
try {
    int a = Integer.parseInt(str);
} catch (NumberFormatException e) {
    e.printStackTrace();
}

六.Java中将Int转String

    int num=100;
	//1
	String s1=""+num;
	System.out.println(s1);
	//2
	String s2 =String.valueOf(num);
	System.out.println(s2);
    //3
    Integer i =new Integer(num);
	String s3 =i.toString();
	System.out.println(s3);
	//4
	String s4 =Integer.toString(i);
	System.out.println(s4);

七.== 和 equals的使用

String s1,s2,s3 = "abc", s4 ="abc" ;
s1 = new String("abc");
s2 = new String("abc");
s1==s2   是 false      //两个变量的内存地址不一样，也就是说它们指向的对象不 一样，
s1.equals(s2) 是 true    //两个变量的所包含的内容是abc，故相等。

八.判断左边的对象是否是右边的类的实例

double obj=1;
if(obj instanceof Double){
System.out.println("true");
}

九.Strng中常用方法

// 1.获取字符串的长度（字符的个数）-------length()
String str = "adsfaxsdfas沙发上案发地方";
System.out.println(str.length());                // 18

// 2.获取指定索引处的字符----charAt(int index)
String str = "adsfaxsdfas沙发上案发地方";	
System.out.println(str.charAt(12));              // 发

// 3.获取str在字符串对象中第一次出现的索引-----String.indexOf(String str)
String str1="012345";
String str2="23";
System.out.println( str1.indexOf(str2) );        // 2

//4.从start开始截取字符串 ---substring(int start)
String str = "adsfaxsdfas沙发上案发地方";
System.out.println(str.substring(1));           // dsfaxsdfas沙发上发地方

//5.从start开始，到end结束截取字符串。包括start，不包括end ---- String substring(int start,int end)
String str = "adsfaxsdfas沙发上案发地方";
System.out.println(str.substring(1, 12));       // dsfaxsdfas沙

//6.判断字符串对象是否以指定的字符开头，参数toffset为指定从哪个下标开始 ----startsWith(String prefix,int toffset)
String str = "adsfaxsdfas沙发上案发地方";
System.out.println(str.startsWith("f", 3)); //true
System.out.println(str.startsWith("f", 4));  // false

//7.判断字符串对象是否以指定的字符结尾 ----endsWith(String str)
String str = "adsfaxsdfas沙发上案发地方";
System.out.println(str.endsWith("x")); // false
System.out.println(str.endsWith("方")); // true

//8.判断指定字符串是否为空 ----isEmpty()

//9.去除字符串两端空格 ----trim()
String str3 = "    a  c  e x u a n x u a n    ";
System.out.println(str3.trim());
System.out.println(str3);

//10.去除字符串中指定的的字符，然后返回一个新的字符串 ----split()
public static void main(String[] args) {
		
		String str = "adsfaxsdfas沙发上发地方";
		String array[] = str.split("a");
		printString(array);
}
public static void printString(String a[]) {
		for(int i=0;i<a.length;i++) {
			System.out.print(a[i]);
		}
}  // dsfxsdfs沙发上发地方


//11.截取字符串中指定位置的字符组成一个新的字符串 ----subSequence(int beginIndex,int endIndex )
String str = "adsfaxsdfas沙发上发地方";
System.out.println(str.subSequence(1, 10));   // dsfaxsdfa

//12.将指定字符替换成另一个指定的字符   ----replace(char oldChar, char newChar)
String str = "adsfaxsdfas沙发上发地方";
System.out.println(str.replace('a', 's'));   // sdsfsxsdfss沙发上发地方

//13.用新的内容替换全部旧内容 ----replaceAll(String regex,String replasement)
String str4 = "Hello,world!";
System.out.println(str4.replaceAll("l", "&"));   // He&&o,wor&d!

//14.返回指定字符出现的最后一次的下标 ----lastIndexOf(String str)

String str4 = "Hello,world!";
System.out.println(str4.lastIndexOf("l"));  // 9

//15.查看字符串中是都含有指定字符   ----contains(CharSequence s)
String str4 = "Hello,world!";
System.out.println(str4.contains("l")); // true

//16. 在原有的字符串的基础上加上指定字符串  ----concat(String str)
String str5 = "dr";
System.out.println(str5.concat("eam"));  // dream

十.单实例Singleton 示例

public class SimpleSingleton {
  private static SimpleSingleton singleInstance = new SimpleSingleton(); 
  //Marking default constructor private
  //to avoid direct instantiation.
  private SimpleSingleton() {
  }
  //Get instance for class SimpleSingleton
  public static SimpleSingleton getInstance() {
   
    return singleInstance;
  }
}

十一.得到当前方法的名字

String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();

十二.列出文件和目录

File dir = new File("directoryName"); 
 String[] children = dir.list(); 
 if (children == null) { 
   // Either dir does not exist or is not a directory 
 } else { 
   for (int i=0; i < children.length; i++) { 
     // Get filename of file or directory 
     String filename = children[i]; 
   } 
 } 
   
 // It is also possible to filter the list of returned files. 
 // This example does not return any files that start with `.'. 
 FilenameFilter filter = new FilenameFilter() { 
   public boolean accept(File dir, String name) { 
     return !name.startsWith("."); 
   } 
 }; 
 children = dir.list(filter); 
   
 // The list of files can also be retrieved as File objects 
 File[] files = dir.listFiles(); 
   
 // This filter only returns directories 
 FileFilter fileFilter = new FileFilter() { 
   public boolean accept(File file) { 
     return file.isDirectory(); 
   } 
 }; 
 files = dir.listFiles(fileFilter);