Java Util 类
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Java Util类
字符串
类型转化
//将其他类型转化为字符串类型
int a = 1234567;
String val = String.valueOf(a);
System.out.println(val);
String str = "1234567";
System.out.println(Integer.parseInt(str));
构造方法
String a = "abc";
String b = new String(“abc”).intern();
String c = new String("abc");
String d = new String(new char[]'a','b','c');
String e = new String(new byte[]21,123,45,-128,127);
System.out.println(e);
静态方法
格式化输出
//格式化字符输出
String name = "henry";
char gender = '男';
int age = 18;
double salary = 31415.926;
//String.format
System.out.println(String.format("%s,%c,%d,%.2f",name,gender,age,salary));
//Message.format效率最高（可复用）
System.out.println(MessageFormat.format("0,1,0,2,3",name,gender,age,salary));
//使用+进行字符串拼接
System.out.println(name + "," + gender + "," + age + "," + salary);
字符串拼接
//大量未知内容拼接字符串
//StringBuilder
String[] strs = "henry","pola","ariel","rose","jack","sherry","jerry","tom"
StringBuilder builder = new StringBuilder();
for(String str : strs)
builder.append(str);
builder.append(',')
builder.deleteCharAt(builder.length()-1);
System.out.println(builder);
//StringJoiner
StringJoiner sj = new StringJoiner(",","Hello","!");
for(String str : strs)
sj.add(str);
System.out.println(sj.toString());
String sj2 = String.join(",",strs);
System.out.println(sj2);
对象方法
字符串比较
//比较
String str1 = "I love you";
String str2 = "I like you";
String str3 = "i love you";
boolean equals = str1.equals(str2);
System.out.println(equals);
boolean equals2 = str1.equalsIgnoreCase(str3);
System.out.println(equals2);
包含
String email = "henry.chen@zbitedu.cn"
//String是否以prefix开始，或以suffix结尾
System.out.println(email.startWith("henry"));
System.out.println(email.endWith("cn"));
//contains
System.out.println(email.contains("chen"));
匹配
//matches
System.out.println(email.matches("(\\\\w3,|\\\\w2,\\\\.\\\\w2,)@[0-9a-zA-Z]2,\\\\.(com|cn|com\\\\.cn|net|org)"));
截取
//获取指定下标未知字符
char sign = email.charAt(3);
System.out.println(sign);
String name = email.substring(email.indexOf('@') + 1);
System.out.println(name);
String firstName = email.substring(email.indexOf('.')+1,email.indexOf('@'));
System.out.println(firstName);
查找
int i = email.indexOf('@');
Sysetm.out.println(i);
//指定起始位置
int i2 = email.indexOf('.',email.indexOf(".")+1);
System.out.println(i2);
分割
//多符号分割
String[] strs1 = email.split("\\\\.|@");
for(String s : strs2)
System.out.println(s);
替换
String s1 = email.replace('c','d');
System.out.println(s1);
String s2 = email.replace("(e|h)","A");
System.out.println(s2);
String s3 = email.replaceAll("(e|h)","#");
System.out.println(s3);
String s4 = email.replaceFirst("(e|h)","#");
System.out.println(s4);
其他
//大小写转换
System.out.println(email.toUpperCase());
System.out.println(email.toLowerCase());
//修剪
String str = " trim to size ";
str = str.trim();
System.out.println(str);
System.out.println(email.isEmpty());
System.out.println(email.length());
枚举类型
static enum Gender Student
private String name;
private Gender gender;
public Student(String name,Gender gender)
this.name = name;
this.gender = gender;
public static void main(String[] args) throws ParseException
Student stu = new Student("henry", Gender.MALE);
System.out.println(stu);
System.out.println(Gender.MALE);
System.out.println(Gender.MAILE.ordinal());
日期
日期类的本质：long 纪元至此刻的毫秒数
//Instant 时间戳类
Instant now = Instant.now();
System.out.println(now.getEpochSecond());
System.out.println(now.toEpochMilli());
//给时间戳Instant关联上一个时区就可以创建出ZoneDateTime
Instant ins = Instant.ofEpochSecond(1568568760);
ZonedDateTime zdt = ins.atZone(ZoneId.systemDefault());
System.out.println(zdt);
//Date 日期类
Date date = new Date();
System.out.println(date);
System.out.println(date.getTime()); //获取指定日期对象的时间戳
Date time = new Date(1);
System.out.println("time: " + time); //根据指定的时间戳生成日期
int rst = time.compareTo(date); //时间比较
System.out.println(rst);
System.out.println(date.equals(time));
System.out.println(date.after(time));
System.out.println(date.before(time));
//将日期对象格式化为指定格式的字符串
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss SSS");
System.out.println(sdf.format(date));
//将【符合指定格式】的字符串解析为日期对象
String time = "2020-10-13 10:11:12 13";
Date _date = sdf.parse(time);
System.out.println(_date);
System.out.println(_date.getTime());
日历类
Calendar c = Calendar.getInstance();
c.set(2018,5,18,0,0,0);
c.setTimeInMillis(114456787654L);
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss SSS");
String _time = "2020-10-13 10:11:12 13";
Date _date = sdf.parse(_time);
c.setTime(_date);
System.out.println(c);
Date time = c.getTime();
System.out.println(time);
c.set(Calendar.YEAR, 2021);
c.set(Calendar.MONTH, 3);
c.set(Calendar.DATE, 27);
c.set(Calendar.HOUR, 11);
c.set(Calendar.MINUTE, 13);
c.set(Calendar.SECOND, 22);
int month = c.get(Calendar.MONTH) + 1;
System.out.println("Month：" + month);
//季度
int quarter = (month - 1 )/3+1;
System.out.println("quarter: " + quarter);
//年周
int weekYear = c.get(Calendar.WEEK_OF_YEAR);
System.out.println("weekYear: " + weekYear);
//月周
int weekMonth = c.get(Calendar.WEEK_OF_MONTH);
System.out.println("weekMonth: " + weekMonth);
//周几 周日~周六 1~7
int weekDay = c.get(Calendar.DAY_OF_WEEK);
System.out.println("weekDay: " + weekDay);
c.roll(Calendar.YEAR, -3);
c.add(Calendar.YEAR, 2);
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss SSS");
System.out.println(sdf.format(c.getTime()));
时区
TimeZone tzDefault = TimeZone.getDefault();
TimeZone tzGMT9 = TimeZone.getTimeZone("GMT+9:00");
TimeZone tzNY = TimeZone.getTimeZone("America/New_York");
System.out.println(tzDefault.getID());
System.out.println(tzGMT9.getID());
System.out.println(tzNY.getID());
利用Calendar进行时区转换
Calendar c = Calendar.getInstance();
c.clear();
c.set(2019, 10*//* 11月 *//*, 20, 8, 15, 0);
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
sdf.setTimeZone(TimeZone.getTimeZone("America/New_York"));
System.out.println(sdf.format(c.getTime()));
从java8开始新加入的API
LocalDateTime
LocalDate d = LocalDate.now();
LocalTime t = LocalTime.now();
LocalDateTime dt = LocalDateTime.now();
System.out.println(d);
System.out.println(t);
System.out.println(dt);
//转换
LocalDateTime dt = LocalDateTime.now();
LocalDate d = dt.toLocalDate();
LocalTime t = dt.toLocalTime();
//指定日期和时间
LocalDate d2 = LocalDate.of(2019,11,30);
LocalTime t2 = LocalTime.of(15,16,17);
LocalDateTime dt2 = LocalDateTime.of(2019,11,30,15,16,17);
LocalDateTime dt3 = LocalDateTime.of(d2, t2);
//字符串传入
LocalDateTime dt = LocalDateTime.parse("2019-11-19T15:16:17");
LocalDate d = LocalDate.parse("2019-11-19");
LocalTime t = LocalTime.parse("15:16:17");
System.out.println(dt);
//DateTimeFormatter
//自定义格式化
DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy/MM/dd HH:mm:ss");
System.out.println(dtf.format(LocalDateTime.now()));
//自定义格式解析
LocalDateTime dt2 = LocalDateTime.parse("2019/11/30 15:16:17", dtf);
System.out.println(dt2);
//日子加减链式调用
LocalDateTime dt = LocalDateTime.of(2019,10, 26, 20, 30, 59);
System.out.println(dt);
LocalDateTime dt2 = dt.plusDays(5).minusHours(3);
System.out.println(dt2);
LocalDateTime dt3 = dt2.minusMonths(1);
System.out.println(dt3);
//日期调整 with
LocalDateTime dt = LocalDateTime.of(2019, 10, 26, 20, 30, 59);
System.out.println(dt);
LocalDateTime dt2 = dt.withDayOfMonth(31);
System.out.println(dt2);
LocalDateTime dt3 = dt2.withMonth(9);
System.out.println(dt3);
//本月第一天0：00时刻：
LocalDateTime firstDay = LocalDate.now().withDayOfMonth(1).<以上是关于Java Util 类的主要内容，如果未能解决你的问题，请参考以下文章