Java常用类

1.String

import org.junit.Test;/* * String:代表不可变的字符序列；底层使用char[]存放 * String是final的 */public class testString {@Testpublic void test5(){String str1 = "520";int i = Integer.parseInt(str1);System.out.println(i);double temp =  520.52052603d;float  temp2 = 520.5205667f;//520.52057String doubleString = String.valueOf(temp);String floatString = String.valueOf(temp2);System.out.println("double:" + doubleString);System.out.println("float:" + floatString);}@Testpublic void test4(){/* *  public String[] split(String str) : 按照字符串str对当前字符串进行拆分，整体返回值是String[] */String str1 = "a*dd*f-d*ad*156";//因为+、*、|、\等符号在正则表达示中有相应的不同意义，所以在使用时要进行转义处理。String str2 = "\\*";//或者 str2 = "[*]";String[] str = str1.split(str2);for(int i = 0;i < str.length;i++){System.out.println(str[i]);}System.out.println("------");String[] str3 = str1.split("-");for(String strTemp:str3){System.out.println(strTemp);}}@Testpublic void test3(){/* * public String trim() : 去除当前字符串中首尾的空格，若有多个，就去除多个 */String str1 = "  a  bcc   ";String str2 = str1.trim();System.out.println("---" + str1 + "--");System.out.println("---" + str2 + "--");/* * public String concat(String str) : 将当前字符串与str连接 */String str3 = "我是一个小伙子";String str4 = "、长得也还可以。";System.out.println(str3.concat(str4));}@Testpublic void test2(){/* * public String substring(int start,int end) : 返回从start开始到end结束的左闭右开的字符串 *  */String str1 = "我是一个 热爱学习、并且长得也还不错的小伙子。";String str2 = str1.substring(0, str1.length());String str3 = "0123456789";String str4 = str3.substring(0,7);System.out.println(str2);System.out.println(str4);}@Testpublic void test1(){String str1 = "JavaEE";String str2 = "JavaEE";String str3 = new String("JavaEE");String str4 = "JavaEE" + "Android";String str5 = "Android";String str6 = str1 + str5;str5 = str5 + "Handoop";String str7 = str6.intern();String str8 = "JavaEEAndroid";System.out.println(str1 == str2);//trueSystem.out.println(str1 == str3);//falseSystem.out.println(str1.equals(str3));//trueSystem.out.println(str4 == str6);//falseSystem.out.println(str4 == str7);//trueSystem.out.println(str4 == str8);Person p1 = new Person("AA");Person p2 = new Person("AA");System.out.println("^_^"+ (p1.name == p2.name));}}class Person{String name;public Person(String name) {this.name = name;}}

2.StringBuffer,StringBuilder

java.lang.StringBuffer:可变字符序列，可以对字符序列内容进行增删
 *     >线程安全
 *     >效率相对低
 * java.lang.StringBuilder:可变字符序列
 *     >线程不安全
 *  >效率相对高
 *
 *
 *
 * 添加:append();
 * 插入:public StringBuffer insert(int index,String str)
 * 反转:public StringBuffer reverse()
 * 删除:StringBuffer delete(int startIndex,int endIndex)   左闭右开
 * 修改:public void setCharAt(int index,char ch)
 * 取代:StringBuffer replace(int startIndex,int endIndex,String str)   左闭右开
 * 剪切:public String substring(int start,int end)         左闭右开

public static void main(String[] args) {/* * 对比StringBuffer StringBuild String 三者在添加上的效率 * StringBuild > StringBuffer > String */String text = "";long startTime = 0L;long endTime = 0L;StringBuffer buffer = new StringBuffer("");StringBuilder builder = new StringBuilder("");startTime = System.currentTimeMillis();for(int i = 1;i <= 50000;i++)buffer.append(i);endTime = System.currentTimeMillis();System.out.println("StringBuffer 所用的时间是:" + (endTime - startTime));startTime = System.currentTimeMillis();for(int i = 1;i <= 50000;i++)builder.append(i);endTime = System.currentTimeMillis();System.out.println("StringBuilder 所用的时间是:" + (endTime - startTime));startTime = System.currentTimeMillis();for(int i = 1;i <= 50000;i++)text += i + "";endTime = System.currentTimeMillis();System.out.println("String 所用的时间是:" + (endTime - startTime));}

3.Date

/* * 与时间相关的类 * 1.System 类下的 currentTimeMillis(); * 2.Date类:java.util.Date 以及其子类 java.sql.Date * 3.SimpleDateFormat类 * 4.Calendar 类 */public class TestDate {//日历类@Testpublic void test4(){Calendar c = Calendar.getInstance();int day = c.get(Calendar.DAY_OF_MONTH);System.out.println(day);c.add(Calendar.DAY_OF_MONTH, -1);day = c.get(Calendar.DAY_OF_MONTH);System.out.println(day);c.set(Calendar.DAY_OF_MONTH, 25);Date d = c.getTime();System.out.println(d);}/* * "三天打渔，两天晒网"  "1990-01-01" XXXX-XX-XX 打渔？晒网？ */public int getTime(String date1,String date2) throws ParseException{SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");Date d1 = sdf.parse(date1);Date d2 = sdf.parse(date2);long millTime = d2.getTime() - d1.getTime();long time = millTime/1000/3600/24;//double time = Math.ceil((double)millTime/1000/3600/24);//System.out.println("^_^"+millTime/1000/3600/24);return  (int) time+1;//time=0表示第一天，所以这里应该加一才对}@Testpublic void test3() throws ParseException{String date1 = "1990-01-01";String date2 = "1990-01-0";for(int i = 1;i<=9;i++){//date2 = date2 + i+"";//System.out.println(date2+i);int temp = getTime(date1, date2+i);if(temp % 5 == 0 || temp % 5 == 4){System.out.println(i+":晒网");}else{System.out.println(i+":打渔");}}}/* * 利于国际化:日期-->文本 *     解析:文本-->日期 */@Testpublic void test2() throws ParseException{//默认格式化SimpleDateFormat sdf = new SimpleDateFormat();String date = sdf.format(new Date());System.out.println(date); //17-11-18 下午11:45//指定格式化SimpleDateFormat sdf2 = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");String date2 = sdf2.format(new Date());System.out.println(date2);//2017-11-18 23:52:28 sdf2 = new SimpleDateFormat("EEE, d MMM yyyy HH:mm:ss Z");date2 = sdf2.format(new Date());System.out.println(date2);//星期六, 18 十一月 2017 23:57:07 +0800//解析Date date3 = sdf.parse("17-11-18 下午11:45");/* * 传入的参数必须和当前对象的构造器一致  * 错误写法： * sdf.parse("EEE, d MMM yyyy HH:mm:ss Z") */System.out.println(date3);//Sat Nov 18 23:45:00 CST 2017date3 = sdf2.parse("星期六, 18 十一月 2017 23:57:07 +0800");System.out.println(date3);//Sat Nov 18 23:57:07 CST 2017}@Testpublic void test1(){Date d1 = new Date();System.out.println(d1);//Sat Nov 18 23:36:50 CST 2017System.out.println(d1.getTime());//1511019410086Date d2 = new Date(1511019410086L);System.out.println(d2);//Sat Nov 18 23:36:50 CST 2017//SQL中的日期类，传入一个long类型的值，返回一个日期，如：2017-11-18java.sql.Date d3 = new java.sql.Date(1511019374333L);System.out.println(d3);}public static void main(String[] args) {// TODO Auto-generated method stub}}