public class IntegerDemo { public static void main(String[] args) { int a = 10; Integer j = new Integer(10); //拆箱与装箱的过程 Integer j1 = Integer.valueOf(a);//装箱 int j2 = j.intValue();//拆箱 Integer i1 = 100; Integer i2 = 100; Integer i3 = 200; Integer i4 = 200; System.out.println(i1==i2);//true System.out.println(i3==i4);//false //Integer源码: // public static Integer valueOf(int i) { // if (i >= Integer.IntegerCache.low && i <= Integer.IntegerCache.high) // m和n均从缓存中取得同一个127(范围:-128~+127) // return Integer.IntegerCache.cache[i + (-Integer.IntegerCache.low)]; //超出范围,自动创建一个新的对象,地址不再一样 // return new Integer(i); // } Double d1 = 1.0; Double d2 = 1.0; Double d3 = 2.0; Double d4 = 2.0; System.out.println(d1==d2);//false System.out.println(d3==d4);//false //Double源码: // public static Double valueOf(double d) { // return new Double(d); // } } }String类
public class StringDemo { public static void main(String[] args) { String str1 = "abc"; String str2 = new String("abc"); System.out.println(str1.equals(str2));//比较两个字符串是否相等 true System.out.println(str1.charAt(1));//取出字符串中某个特定元素 b System.out.println(str1.concat("cde"));//字符数组的复制+拼接 abccde System.out.println(str1.compareTo(str2));//比较字符串 -1 前面小 0 相等 1 后面小 String str3 = "ABC"; System.out.println(str1.compareToIgnoreCase(str3));//将字符串全部转为小写并比较 System.out.println(str1.indexOf("b"));//从字符串中获取指定元素的下标 1 String str4 = "abcdefghigklmn"; System.out.println(str4.substring(6));//指定开始,直到字符串结尾,截取字符串 ghigklmn System.out.println(str4.substring(2, 6));//指定开始,指定结束,截取字符串 cdef System.out.println(str4.length());//返回字符数组(字符串)长度 // String a = "abcd"; // String b = new String("abcd"); //是当前的字符对象(通过new出来的对象)可以使用intern方法从常量池中获取, // 如果常量池中不存在该字符串,那么就新建一个这样的字符串放到常量池中。 // b = b.intern(); // System.out.println(a==b);//true String a = "abc"; String b = "def"; String c = "abcdef"; String d = a+b; String e = "abc"+"def"; System.out.println(c==d);//false System.out.println(c==e);//true String f = "a"+"b"+"c"; String a1 = "a"; String a2 = "b"; String a3 = "c"; String a4 = a1+a2+a3; //一共生成几个对象:4个 “abc”、“a”、“b”、“c” } }StringBuffer类和StringBuilder类
public class StringBufferDemo { public static void main(String[] args) { StringBuffer stringBuffer = new StringBuffer(); stringBuffer.append(1).append(1.234).append("abc").append(true); System.out.println(stringBuffer);//拼接字符串 System.out.println(stringBuffer.length());//剩余容量 System.out.println(stringBuffer.capacity());//总容量 System.out.println(stringBuffer.delete(1,2 ));//删除某位置的值 System.out.println(stringBuffer.insert(1, 1));//往指定位置插入元素 StringBuilder stringBuilder = new StringBuilder(); stringBuilder.append("123").append(1).append(false); } }日期处理相关类
import java.text.DateFormat; import java.text.ParseException; import java.text.SimpleDateFormat; import java.util.Calendar; import java.util.Date; public class DateDemo { public static void main(String[] args) throws ParseException { Date date = new Date(); System.out.println(date); System.out.println(date.getTime());//毫秒值 DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); String str = dateFormat.format(date); //dateFormat.format() 将Date类按照规定的日期格式返回一个字符串 System.out.println(str); //将字符串转换成对应的日期类 Date d1 = dateFormat.parse("2010-10-10 20:20:20"); System.out.println(d1); //一种全新创建类的方式 Calendar calendar = Calendar.getInstance(); //获取当前系统的时间 System.out.println(calendar);//格林威治标准时间(GMT) //获取年月日,时分秒 calendar.setTime(d1); System.out.println(calendar); System.out.println(calendar.get(Calendar.YEAR)); System.out.println(calendar.get(Calendar.MONTH));//9 从零开始,表示10月份 System.out.println(calendar.get(Calendar.DAY_OF_MONTH)); System.out.println(calendar.get(Calendar.HOUR_OF_DAY)); System.out.println(calendar.get(Calendar.MINUTE)); System.out.println(calendar.get(Calendar.SECOND)); } }Math类
public class MathDemo { public static void main(String[] args) { System.out.println(Math.abs(-1));//取绝对值 System.out.println(Math.sqrt(4));//开方 System.out.println(Math.ceil(3.14));//向上取整 System.out.println(Math.floor(3.14));//向下取整 System.out.println(Math.pow(2, 3));//a的b次方 } }枚举类
最初级的枚举类用法
public enum Gender { 男,女 }
public class Test { Gender gender1 = Gender.女; Gender gender2 = Gender.男; }
进阶
public enum EventEnum { LAUNCH("launch"),PAGEVIEW("pagevien"),EVENT("event"); EventEnum(String name){ this.name = name; } private String name; public void show(){ System.out.println(this.name); EventEnum[] ee = values(); for(int i=0;i public class Test { public static void main(String[] args) { EventEnum ee = EventEnum.LAUNCH;//launch ee.show(); //LAUNCH //PAGEVIEW //EVENT String name = EventEnum.PAGEVIEW.name(); System.out.println(name); //PAGEVIEW } } static void main(String[] args) { EventEnum ee = EventEnum.LAUNCH;//launch ee.show(); //LAUNCH //PAGEVIEW //EVENT String name = EventEnum.PAGEVIEW.name(); System.out.println(name); //PAGEVIEW } }欢迎分享,转载请注明来源:内存溢出
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