JAVA中常用的类

public static void main(String[] args) {String str1;str1 = "你好";// 使用字符串常量构造一个字符串String str2 = new String("你好");// String类中重写了父类Object类的equals方法// String类中的equals方法比较的是字符串的内容// 使用String类中的equals方法时，建议将字符串常量写在前面String str3 = null;System.out.println("你好".equals(str3));//正确写法//System.out.println(str3.equals("你好"));//错误写法// 使用equals方法System.out.println(str1.equals(str2));//trueSystem.out.println(str1 == str2);//false// 使用char数组构造一个字符串char [] ch1 = {'a', 'b', 'c'};String str4 = new String(ch1); System.out.println("str4: " + str4);char [] ch2 = {'a', 'b', 'c', 'd', 'e', 'f'}; String str5 = new String(ch2, 2, 2); System.out.println("str5: " + str5);// 使用byte数组构造一个字符串//byte [] by1 = {-50, 3, 100};byte [] by1 = "测试".getBytes();String str6 = new String(by1); System.out.println("str6: " + str6);// String类中的equals方法和等号String str7 = "西安网星";String str8 = "西安网星";System.out.println(str7.equals(str8));//trueSystem.out.println(str7 == str8);//true// length()：获取字符串的长度System.out.println(str7.length());//4// isEmpty()：判断字符串手不是为空System.out.println(str7.isEmpty());//false// charAt(int index)：获取指定位置上的字符System.out.println(str7.charAt(2));//网// equalsIgnoreCase：忽略大小写比较String str9 = "abccdabc";String str10 = "ABccdabc";System.out.println(str9.equals(str10));//falseSystem.out.println(str9.equalsIgnoreCase(str10));//tru e// startsWith：判断某个字符串以什么开始// endsWith：判断某个字符串以什么结尾System.out.println(str9.startsWith("a"));//trueSystem.out.println(str9.startsWith("a", 2));//falseSystem.out.println(str9.endsWith("c"));//true//String str9 = "abccdabc";// indexOf：从开始位置查找出指定字符串在已有字符串中的位置// lastIndexOf：从结尾位置查找出指定字符串在已有字符串中的位置System.out.println(str9.indexOf("bc"));//1System.out.println(str9.indexOf("ac"));//-1System.out.println(str9.indexOf("bc", 2));//6System.out.println(stIndexOf("ab"));//5System.out.println(stIndexOf("ab", 4));//0// substring：截取字符串，包括指定开始位置本身System.out.println(str9.substring(4));//dabc// substring：截取字符串，包括指定开始位置本身，结束位置不包括System.out.println(str9.substring(4, 7));//dabSystem.out.println(str9.substring(4,str9.length()));//dabcString str11 = "test.jpg";System.out.println(str11.substring(4,str11.length()));//.jpg// concat：连接字符串System.out.println(str9.concat(str11));//abccdabctest.j pgSystem.out.println(str9 + str11);//abccdabctest.jpg// contains：包含某个字符串System.out.println(str11.contains("."));//true// replace：字符串替换System.out.println(str11.replace("test", "my"));//my.jpg// split：分割字符串String str12 = "test.jpg";//String[] strs = str12.split("\\.");String[] strs = str12.split("t");System.out.println(strs.length);for(int i = 0; i < strs.length; i++){System.out.println("strs[" + i + "]=" + strs[i]);//str[0]=aa}// toLowerCase()：转换成小写// toUpperCase()：转换成大写System.out.println(str12.toLowerCase());//test.jpgSystem.out.println(str12.toUpperCase());//TEST.JPG // trim()：去前后空格String str13 = " abc ";System.out.println(str13.length());//5String str14 = str13.trim();System.out.println(str14);System.out.println(str14.length());//3// regionMatches：测试两个字符串区域是否相等。

String str15 = "abcdefghijk";String str16 = "abcdefghijk";boolean flag = str15.regionMatches(0, str16, 0, str16.length());System.out.println("flag: " + flag);//trueflag = str15.regionMatches(1, str16, 1, str16.length() - 1);System.out.println("flag: " + flag);//trueflag = str15.regionMatches(3, str16, 3, 6);//str16="defg"System.out.println("flag: " + flag);//true// 将“数字”组成的字符串转换为基本数据类型int a = Integer.parseInt("1111");System.out.println(a);float f = Float.parseFloat("23.45F");System.out.println(f);double d = Double.parseDouble("11.22");System.out.println(d);// 将基本数据类型转换为字符串String s1 = String.valueOf(a);String s2 = String.valueOf(f);String s3 = String.valueOf(d);/*// equals()// ==A a1 = new A();A a2 = new A();System.out.println(a1.equals(a2));//falseSystem.out.println(a1 == a2);//false*/}}public static void main(String[] args) {// 字符串与字节数组相互转化String s1 = "欢迎来abc到网星!";// 将字符串转换成字节数组byte [] bytes = s1.getBytes();// 将字节数组转换成字符串String s2 = new String(bytes);System.out.println(s2);System.out.println("---------------------");try {// 将字符串转换成字节数组bytes = s1.getBytes("GBK");// ISO-8859-1 UTF-8 GBK// 将字节数组转换成字符串s2 = new String(bytes,"GBK");System.out.println(s2);System.out.println("---------------------");bytes = s1.getBytes("UTF-8");// ISO-8859-1 UTF-8 GBKs2 = new String(bytes,"UTF-8");System.out.println(s2);System.out.println("---------------------");bytes = s1.getBytes("ISO-8859-1");// ISO-8859-1 UTF-8 GBKs2 = new String(bytes,"ISO-8859-1");System.out.println(s2);//乱码} catch (UnsupportedEncodingException e) {e.printStackTrace();}}}public static void main(String[] args) {String s1 = "你好";String s2 = s1.concat("张三");System.out.println(s2);// 动态修改字符串的内容StringBuilder sb1 = new StringBuilder("你好"); sb1.append("张三");System.out.println(sb1);// 获取指定位置上的字符char ch = sb1.charAt(3);//三System.out.println(ch);// 在指定位置插入字符串sb1.insert(2, "干啥");System.out.println(sb1);//你好干啥张三// 字符串反转sb1.reverse();System.out.println(sb1);//三张啥干好你// 在指定位置删除字符串sb1.delete(2, 4);System.out.println(sb1);//三张好你sb1.reverse();// 在指定位置替换字符串sb1.replace(2, 4, "李四");System.out.println(sb1);//你好李四}}public static void main(String[] args) {Date d1 = new Date();System.out.println(d1);//d1.toString()Date d2 = new Date(1000);System.out.println(d2);//d2.toString()Date d3 = new Date(00L);System.out.println(d3);//d3.toString()System.out.println(System.currentTimeMillis());//00 Date d4 = new Date(35L);System.out.println(d4);//d4.toString()// 日期格式化SimpleDateFormat sdf = new SimpleDateFormat("yyyy年MM月dd日HH:mm:ss.SSS E");// 将日期类型转换为字符串String dateStr = sdf.format(d4);System.out.println("dateStr: " + dateStr);// 把(日期)字符串转换为日期类型try {Date d5 = sdf.parse(dateStr);System.out.println("d5: " + d5);} catch (ParseException e) {e.printStackTrace();}System.out.println("------------------------------------------");// 日期格式化SimpleDateFormat sdf1 = new SimpleDateFormat("yyyy年MM月dd日HH:mm:ss", );// 将日期类型转换为字符串String dateStr1 = sdf1.format(d4);System.out.println("dateStr1: " + dateStr1);public static void main(String[] args) {//c产生绝对值System.out.println(m);double d1 = Math.random();System.out.println(d1);// 产生1到8之间的随机数int n = (int)(Math.random() * 8) + 1;System.out.println(n);// 产生1到100之间的随机数n = (int)(Math.random() * 100) + 1;System.out.println(n);int a = Math.round(3.5643F);System.out.println("a: " + a);a = Math.round(3.4643F);System.out.println("a: " + a);double d2 = Math.sqrt(8);//求平方根System.out.println("d2: " + d2);System.out.println("--------------------------");// 数字格式化：普通数字、百分比、货币形式NumberFormat nFormat = NumberFormat.getInstance();nFormat.setMaximumFractionDigits(7);//设置小数部分的最大位数nFormat.setMinimumIntegerDigits(3);//设置整数部分的最小位数String s = nFormat.format(d2);System.out.println("8的平方根格式化后："+s);System.out.println("--------------------------");// 设置语言地区//Locale locale = Locale.getDefault();//java.util.Locale//Locale locale = ;//zh_CN, en_US, en_GB, zh_HKLocale locale = new Locale("en", "GB");System.out.println(locale);/*nFormat = NumberFormat.getCurrencyInstance();Currency currency = Currency.getInstance(locale);nFormat.setCurrency(currency);*/nFormat = NumberFormat.getCurrencyInstance(locale);String string = nFormat.format(0.35);System.out.println(string);//$0.35//HK$0.35//￥0.35System.out.println("--------------------------");Random random = new Random();// java.util.Randomn = random.nextInt(8) + 1;System.out.println("n=" + n);/ 正则表达式public class RegexDemo {public static void main(String[] args) {、String regex = "\\dhello";boolean flag = s1.matches(regex);String result = flag ? "匹配成功" : "匹配失败";System.out.println(result);System.out.println("-----------------------");s1 = "029-";regex = "[0]\\d{2,3}-[2-8]\\d{6,7}";flag = s1.matches(regex);result = flag ? "匹配成功" : "匹配失败";System.out.println(result);}。