Java基础---常用类

谁借莪１个温暖的怀抱￠ 2021-09-23 05:34 351阅读 0赞

# 1、字符串相关的类 #

## String类 ##

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## String常用的方法 ##

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## 常用方法测试 ##

package com.lcy;
    
    public class StringTest { 
        public static void main(String[] args) { 
            String str = "12hello34world5java7891mysql456";
            //把字符串中的数字替换成,，如果结果中开头和结尾有，的话去掉
            String string = str.replaceAll("\\d+", ",").replaceAll("^,|,$", "");
            System.out.println(string);
            System.out.println("----------------------------");
            String str1 = "12345";
            //判断str字符串中是否全部有数字组成，即有1 - n个数字组成
            boolean matches = str1.matches("\\d+");
            System.out.println(matches);
            System.out.println("----------------------------");
    
    
            String tel = "0571-4534289";
            //判断这是否是一个杭州的固定电话
            boolean result = tel.matches("0571-\\d{7,8}");
            System.out.println(result);
            System.out.println("----------------------------");
    
            String str2 = "hello|world|java";
            String[] strs = str2.split("\\|");
            for (int i = 0; i < strs.length; i++) { 
                System.out.println(strs[i]);
            }
            System.out.println("----------------------------");
    
            String str3 = "hello.world.java";
            String[] strs2 = str3.split("\\.");
            for (int i = 0; i < strs2.length; i++) { 
                System.out.println(strs2[i]);
            }
        }
    }

## 测试结果： ##

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## StringBuffer类 ##

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## StringBuilder类 ##

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# 2、日期时间API #

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package com.lcy;
    
    import java.util.Date;
    
    public class StringBuffer { 
        public static void main(String[] args) { 
    
            Date date = new Date();
            System.out.println(date);     //Wed Jan 29 12:35:40 CST 2020
            System.out.println(System.currentTimeMillis()); //1580272540654
            System.out.println(date.getTime()); //1580272540637
            Date date1 = new Date(date.getTime());
            System.out.println(date1.getTime());   //1580272540637
            System.out.println(date1.toString());   //Wed Jan 29 12:35:40 CST 2020
        }
    }

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package com.lcy;
    
    import java.text.SimpleDateFormat;
    import java.util.Date;
    
    public class Data { 
        public static void main(String[] args) { 
            Date date = new Date();
            SimpleDateFormat formater = new SimpleDateFormat();
            System.out.println(formater.format(date));  //20-1-29 下午5:50
            SimpleDateFormat formater2 = new SimpleDateFormat("yyyy年MM月dd日EEE HH:mm:ss");
            System.out.println(formater2.format(date));//2020年01月29日星期三 17:50:42
        }
    }

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package com.lcy;
    
    import java.text.SimpleDateFormat;
    import java.util.Calendar;
    import java.util.Date;
    
    public class Data { 
        public static void main(String[] args) { 
    
            System.out.println("-------------");
    
            Calendar calendar = Calendar.getInstance();
    // 从一个Calendar 对象中获取Date对象
            Date date = calendar.getTime();
    // 使用给定的Date 设置此Calendar 的时间
            date = new Date(234234235235L);
            calendar.setTime(date);
            calendar.set(Calendar.DAY_OF_MONTH, 8);
            System.out.println("当前时间日设置为8后,时间是:" + calendar.getTime());
            calendar.add(Calendar.HOUR, 2);
            System.out.println("当前时间加2小时后,时间是:" + calendar.getTime());
            calendar.add(Calendar.MONTH, -2);
            System.out.println("当前日期减2个月后,时间是:" + calendar.getTime());
        }
    }

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# 3、 Java比较器 #

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# 4、System类 #

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# 5 Math类 #

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# 6 BigInteger与BigDecimal #

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package com.lcy;
    
    import java.math.BigDecimal;
    import java.math.BigInteger;
    import java.text.SimpleDateFormat;
    import java.util.Calendar;
    import java.util.Date;
    
    public class Data { 
        public static void main(String[] args) { 
            BigInteger bi = new BigInteger("12433241123");
            BigDecimal bd = new BigDecimal("12435.351");
            BigDecimal bd2 = new BigDecimal("11");
            System.out.println(bi);
    // System.out.println(bd.divide(bd2));
            System.out.println(bd.divide(bd2, BigDecimal.ROUND_HALF_UP));
            System.out.println(bd.divide(bd2, 15, BigDecimal.ROUND_HALF_UP));
        }
    }

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