publicclass 日期 {publicstaticvoidmain(String[] args)throws Exception{//获取当前毫秒long date =System.currentTimeMillis();
SimpleDateFormat simple =newSimpleDateFormat("yyyy-MM-dd HH:mm:ss SSS");//格式化时间
Date d =newDate();//date-->String
String temp =simple.format(d);
System.out.println(temp);//String -->date
String da ="2019-05-15 12:32:34 333";
Date dad =simple.parse(da);
System.out.println(dad);
Date m =newDate(System.currentTimeMillis()-1000*60*10);
System.out.println(simple.format(m));
System.out.println(simple.format(newDate()));
Calendar c =Calendar.getInstance();//查看当前日历int i =c.get(Calendar.DAY_OF_WEEK);//常量是类名.
System.out.println(i);//查看是第几天
String ste ="2008-08-08";//Date kk =new SimpleDateFormat("yyyy-MM-dd".parse(ste));//c.settime(ste)设置的是ste中的日历//DecimalForamt数字格式化//创建数字格式化对象//需求:加入千分位
DecimalFormat dec =newDecimalFormat("###,###");//开始格式化//Number-->String
System.out.println(dec.format(1234567));//1,234,567//需求加入千分位,保留两位小数
DecimalFormat dec1 =newDecimalFormat("###,###");
System.out.println(dec1.format(1234567.123));//需求:加入千分位,保留4位小数,不够补0
DecimalFormat dec2 =newDecimalFormat("###,###.0000");
System.out.println(dec2.format(1234567.123));}}
ハッシュ・クラス
package 方法;import java.sql.Date;import java.text.ParseException;import java.text.SimpleDateFormat;import java.util.HashSet;import java.util.Iterator;import java.util.Set;import java.util.SortedSet;import java.util.TreeSet;import java.util.logging.SimpleFormatter;publicclass 哈希 {publicstaticvoidmain(String[] args)throws Exception {
Set s =newHashSet();
gong g =newgong("100","jaf");
gong g1 =newgong("101","jaf");
gong g2 =newgong("200","jxa");
s.add(g);
s.add(g1);
s.add(g2);
System.out.println(g.hashCode());//hashCode
System.out.println(s.size());//SortedSet无序不可重复,可以将存入的元素自动排序
SortedSet ss =newTreeSet();
ss.add(10);
ss.add(3);
ss.add(7);
ss.add(72);
Iterator it =ss.iterator();while(it.hasNext()){
Object o =it.next();
System.out.println(o);
String str ="2008-09-11";
SimpleDateFormat d =newSimpleDateFormat("yyyy-MM-dd");
Date da =(Date) d.parse(str);
SortedSet times =newTreeSet();
times.add(da);
Iterator iti =times.iterator();while(it.hasNext()){
Object p =iti.next();if(o instanceofDate){
Date da1=(Date)p;
System.out.println(d.format(da));}}}}}classgong{
String no;
String name;gong(String no,String name){this.name=name;this.no =no;}publicbooleanequals(Object o){if(this==o){returntrue;}if(o instanceofgong){
gong e=(gong)o;if(e.no.equals(this.no)&&e.name.equals(name)){returntrue;}}returnfalse;}publicinthashCode(){return no.hashCode();}}
Stringクラス
publicstaticvoidmain(String[] args){//一共创建了三个对象//常量池中一个,堆中两个byte[] bytes ={97,98,99,100};
String a =newString(bytes);
String b =newString(bytes,0,2);//0是第0位上的字符,1是字符的长度char[] c ={'我','是','中','国','人'};
String c1 =newString(c);
String d =newString(c,2,2);
System.out.println(d);//boolean endsWith(String endstr)
System.out.println("helloworld,java".endsWith("java"));//equalsIgnoreCase比较两个字符串是否相等,忽略大小写//byte getBytes将字符串转化为byte值byte[] m ="abcd".getBytes();for(int i=0; i<m.length; i++){
System.out.print(m[i]);}
System.out.println();//indexOf返回字符串第一次出现的索引
System.out.println("ajvaejidjngh.ds".indexOf("ae",3));
System.out.println("jfincnx".lastIndexOf("in"));//将字符串用另一个字符串代替
System.out.println("cmieanij".replace("ea","yy"));//用split将字符串分割
String time ="yan,wei,shuai";
String [] my =time.split(",");for(int i=0; i<my.length; i++){
System.out.println(my[i]);}//boolean startWith 判断字符串的开头是否为j
System.out.println("jancaienkg".startsWith("j"));//substring截取字符串,将之后的截取出来
System.out.println("hjnca/ennc".substring(5));
System.out.println("hjnca/ennc".substring(1,4));//toCharArray将字符串转化为数组char[] m2 ="我是闫伟".toCharArray();for(int i=0; i<m2.length; i++){
System.out.println(m2[i]);}//转化为大写
System.out.println("javac".toUpperCase());//转化为小写
System.out.println("javac".toLowerCase());//忽略前导和后导空白,中间的去不了
System.out.println(" d ss gda ".trim());}