java基础-常用类-day11
1. 包装类
基础数据类型 进行封装 产生一个新的类 (属性 方法 构造器)
基本数据类型 包装类 继承关系
byte Byte Number-->Object
short Short Number-->Object
int Integer Number-->Object
long Long Number-->Object
float Float Number-->Object
double Double Number-->Object
char Character Object
boolean Boolean Object
为什么要有封装类
java是面型对象语言 操作类对象方便
可以放引用数据类型
数组 int[] String[] Student[]
集合 只能装引用数据类型
2. Integer类
public class Test {
public static void main(String[] args) {
System.out.println(Integer.MAX_VALUE);
System.out.println(Integer.MIN_VALUE);
System.out.println("物极必反");
System.out.println(Integer.MAX_VALUE+1);
System.out.println(Integer.MIN_VALUE-1);
}
}
Integer构造器
public class Test {
public static void main(String[] args) {
System.out.println(Integer.MAX_VALUE);
System.out.println(Integer.MIN_VALUE);
System.out.println("物极必反");
System.out.println(Integer.MAX_VALUE+1);
System.out.println(Integer.MIN_VALUE-1);
Integer i1 = new Integer(12);
System.out.println(i1);
}
}
入参为String的构造器
public Integer(String s) throws NumberFormatException {
this.value = parseInt(s, 10);
}
包装类的 装箱 与拆箱
装箱:int 自动转 Integer
拆箱: Integer --> int
如何快速使用快捷键 变量接收
new 类()
alt+enter
再按一次enter
打断点阅读源码
public static Integer valueOf(int i) {
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}
-128--127之间会去cache缓存里面取 之外的才会new对象
compareTo方法
public static int compare(int x, int y) {
return (x < y) ? -1 : ((x == y) ? 0 : 1);
}
x<y -1
x==y 0
x>y 1
public class Test04 {
public static void main(String[] args) {
//compareTo
Integer integer = new Integer(6);
Integer integer2 = new Integer(12);
System.out.println(integer.compareTo(integer2));
}
}
package com.msb2;
/**
* @Auther: jack.chen
* @Date: 2023/9/17 - 09 - 17 - 16:54
* @Description: com.msb2
* @version: 1.0
*/
public class Test04 {
public static void main(String[] args) {
//compareTo
Integer integer = new Integer(6);
Integer integer2 = new Integer(12);
System.out.println(integer.compareTo(integer2));
Integer integer3 = new Integer(12);
Integer integer4 = new Integer(12);
System.out.println(integer3 == integer4);//对地址的比较 两个不同的对象 返回false
System.out.println(integer3.equals(integer4));//对封装的value的比较 true
//自动装箱 int-->Integer
//装箱 int自动转Integer对象可以具备很多方法
//只有装箱后才有 equal... 才具有很多类的很多方法 供调用
Integer i5 = 4;
Integer i6 = 4;
System.out.println(i5 == i6);//这里为啥返回true 在 -128--127之间 指向的是缓存里面的同一个对象
System.out.println(i5.equals(i6)); //true
Integer i7 = 130;
Integer i8 = 130;
System.out.println(i7 == i8);//这里为啥返回false // 不在 -128--127之间
System.out.println(i7.equals(i8)); //true
}
}
Integer --> int 拆箱 只保留里面有用的value 其他的都不要扒掉 仅仅暴露本原
Integer i9 = 130; //装箱
int i10 = i9.intValue(); //方式1
int i11 = Integer.parseInt("120");//方式2
System.out.println(i9.toString());
3.util.date
public class Test {
public static void main(String[] args) {
Date date = new Date();
System.out.println(date);
System.out.println(date.toString());
System.out.println(date.toGMTString());
System.out.println(date.toLocaleString());
System.out.println(date.getYear());//1200开始
System.out.println(date.getMonth());//开始月为0
System.out.println(date.getTime());// 1970-1-1 00:00:00 到现在的时间 ms为单位
System.out.println(System.currentTimeMillis());//系统时间
// 常用的还是System.currentTimeMillis()
// 计算代码的运行时间
long startTime = System.currentTimeMillis();
for (int i = 0; i < 100000; i++) {
System.out.println(i);
}
long endTime = System.currentTimeMillis();
System.out.println(endTime-startTime);
}
}
4. java.sql.Date
package com.msb;
import java.sql.Date;
/**
* @Auther: jack.chen
* @Date: 2023/9/17 - 09 - 17 - 17:20
* @Description: com.msb
* @version: 1.0
*/
public class Test02 {
public static void main(String[] args) {
// java.sql.Date
Date d = new Date(1592055964263L);
System.out.println(d);
/*
1. java.sql.Date extends java.util.Date
2. 输出
sql.Date 年月日
util.Date 年月日 时分秒
*/
java.util.Date d2 = new java.util.Date();
System.out.println(d2.toLocaleString());
//转换 向下 转型 util.Date--sql.Date 父类转子类
java.util.Date d3 = new Date(1592055964263L);// 多态 父类指向子类的引用
Date d4 = (Date)d3; // Date 为java.sql.Date;
/*
Animal a = new Dog();
Dog d = (Dog) a;
* */
Date d5 = new Date(d3.getTime());// d3 为util.Date对象
Date d6 = new Date(1592055964263L);// sql.Date
java.util.Date d7 = d6;
Date d9 = Date.valueOf("2023-09-17");
System.out.println(d9);
}
}
5. SimpleDateFormat
String --> java.util.Date
package com.msb;
/**
* @Auther: jack.chen
* @Date: 2023/9/17 - 09 - 17 - 21:36
* @Description: com.msb
* @version: 1.0
*/
public class Test03 {
public static void main(String[] args) {
// java.sql.Date d = java.sql.Date.valueOf("2023-09-17 19:01:01");
// java.sql.Date d = java.sql.Date.valueOf("2023/09/17");
java.sql.Date d = java.sql.Date.valueOf("2023-09-17");
java.util.Date d2 = d;
System.out.println(d2.toString());
}
}
注:上面的代码有局限性 只能转 yyyy-MM-dd 这样的形式的日期
学习新的类 SimpleDateFormat
见名知意 用于日期的简单转换
package com.msb;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
/**
* @Auther: jack.chen
* @Date: 2023/9/17 - 09 - 17 - 21:43
* @Description: com.msb
* @version: 1.0
*/
public class Test04 {
public static void main(String[] args) {
// SimpleDateFormat 继承 DateFormat-抽象类
SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
//String --> Date
Date d = null;
try {
d = simpleDateFormat.parse("2023-10-01 10:00:00");
} catch (ParseException e) {
e.printStackTrace();
}
System.out.println(d);
// Date-->String
String s = simpleDateFormat.format(new Date());
System.out.println(s);
Date d2 = new Date();
System.out.println(d2.toString());
System.out.println(d2.toLocaleString());
System.out.println(d2.toGMTString());
}
}
6. Calendar
package com.msb;
import java.util.Calendar;
import java.util.GregorianCalendar;
/**
* @Auther: jack.chen
* @Date: 2023/9/17 - 09 - 17 - 21:49
* @Description: com.msb
* @version: 1.0
*/
public class Test05 {
public static void main(String[] args) {
// Calendar 是一个抽象类 不能实例化对象
// new Calendar()
// GregorianCalendar 是Calendar的子类 extends Calendar
Calendar cal = new GregorianCalendar();
Calendar cal2 = Calendar.getInstance();
System.out.println(cal.get(Calendar.YEAR));//年 2023
System.out.println(cal.get(Calendar.MONTH));// 月 8 (从0开始)
System.out.println(cal.get(Calendar.DATE));// 日 17
System.out.println(cal.get(Calendar.DAY_OF_WEEK));//星期中的第几天 1
System.out.println(cal.get(Calendar.DAY_OF_MONTH));// 月里面的第几天 17
System.out.println(cal.getActualMaximum(Calendar.DATE));//日期的最大天数 30
System.out.println(cal.getActualMinimum(Calendar.DATE));//日期的最小天数 1
// set
cal.set(Calendar.YEAR, 1900);
cal.set(Calendar.MONTH, 2);
cal.set(Calendar.DATE, 20);
System.out.println(cal);
// String转Calendar
java.sql.Date d = java.sql.Date.valueOf("2023-09-17");
cal.setTime(d);
System.out.println(cal);
}
}
练习:输入日志 按格式 输出日历
package com.msb;
import java.util.Calendar;
import java.util.Scanner;
/**
* @Auther: jack.chen
* @Date: 2023/9/17 - 09 - 17 - 22:03
* @Description: com.msb
* @version: 1.0
*/
public class Test06 {
public static void main(String[] args) {
Calendar cal = Calendar.getInstance();
Scanner sc = new Scanner(System.in);
System.out.println("请输入日期,格式:2019-3-7");
String strDate = sc.next();
// java.sql.Date d = java.sql.Date.valueOf("2023-09-16");
java.sql.Date d = java.sql.Date.valueOf(strDate);
cal.setTime(d);
System.out.println("日\t一\t二\t三\t四\t五\t六");
int maxDate = cal.getActualMaximum(Calendar.DATE);
int nowDay = cal.get(Calendar.DATE);
// 把一号对应 星期的第几天求出来
cal.set(Calendar.DATE, 1);
int weekNum = cal.get(cal.DAY_OF_WEEK);
// System.out.println("ss:"+weekNum);
int num = weekNum-1;
for (int i = 0; i < num; i++) {
System.out.print("\t");
}
int count = 0;
count = count + num;
for (int i = 1; i < maxDate+1; i++) {
if (count % 7 ==0){
System.out.print("\n");
}
if (i == nowDay){
System.out.print(i+"*"+"\t");
}else{
System.out.print(i+"\t");
}
count ++;
}
}
}
7. Math
package com.msb;
/**
* @Auther: jack.chen
* @Date: 2023/9/18 - 09 - 18 - 20:26
* @Description: com.msb
* @version: 1.0
*/
public class Test07 {
public static void main(String[] args) {
System.out.println(Math.PI);
System.out.println(Math.random());// [0.0, 1.0)
System.out.println(Math.abs(-12));
System.out.println(Math.ceil(9.1));
System.out.println(Math.floor(9.9));
System.out.println(Math.round(3.5));
System.out.println(Math.max(2, 6));
System.out.println(Math.min(2, 6));
}
}
Math.Random
package com.msb;
import java.util.Random;
/**
* @Auther: jack.chen
* @Date: 2023/9/18 - 09 - 18 - 20:30
* @Description: com.msb
* @version: 1.0
*/
public class Test08 {
public static void main(String[] args) {
System.out.println(Math.random());
Random r1 = new Random(System.currentTimeMillis());
int i = r1.nextInt();
System.out.println(i);
Random r2 = new Random();
System.out.println(r2.nextInt(10));
System.out.println(r2.nextDouble()); // 0--1之间
}
}
8. String 字符串
value是用于存放字符串的数组
package com.msb;
/**
* @Auther: jack.chen
* @Date: 2023/9/18 - 09 - 18 - 20:34
* @Description: com.msb
* @version: 1.0
*/
public class TestString01 {
public static void main(String[] args) {
String s = "abc";
}
}
基本接口
package com.msb;
import java.util.Arrays;
/**
* @Auther: jack.chen
* @Date: 2023/9/18 - 09 - 18 - 20:34
* @Description: com.msb
* @version: 1.0
*/
public class TestString01 {
public static void main(String[] args) {
String s = "abc";
System.out.println(s);
String s1 = new String();
String s2 = new String("abc");
String s3 = new String(new char[]{'a','b','c'});
System.out.println(s1.isEmpty());
System.out.println(s2.charAt(0));
System.out.println(s1.equals(s2));
System.out.println(s1.compareTo(s2));
String s10 = "abcdefhijk";
System.out.println(s10.substring(3));//从第三个开始截取到最后 包括第三个
System.out.println(s10.substring(3, 6));
System.out.println(s10.concat("pppppp"));
System.out.println(s10.replace('a', 'z'));
String s11 = "a-b-c-d-e-f";
String[] s11s = s11.split("-");
System.out.println(Arrays.toString(s11s));
String s12 = "abc";
System.out.println(s12.toUpperCase());
String s14 = " 123 ";
System.out.println(s14.trim());
String s15 = "Abc";
System.out.println(s15.toString());
System.out.println(String.valueOf(false));
}
}
9.StringBuilder
package com.msb;
/**
* @Auther: jack.chen
* @Date: 2023/9/18 - 09 - 18 - 20:51
* @Description: com.msb
* @version: 1.0
*/
public class StringBuilderTest {
public static void main(String[] args) {
StringBuilder sb = new StringBuilder("nihaojavawodeshijie");
sb.append("这是梦想");
System.out.println(sb);
sb.delete(3, 6);
System.out.println(sb);
sb.deleteCharAt(0);
System.out.println(sb);
StringBuilder s2 = new StringBuilder("23445980947");
s2.insert(3, ",");
System.out.println(s2);
StringBuilder s3 = new StringBuilder("$2你好吗5980947");
s3.replace(3, 5, "我好累");
System.out.println(s3);
s3.setCharAt(3, '!');
System.out.println(s3);
StringBuilder sb3 = new StringBuilder("asfda");
for (int i = 0; i < sb3.length(); i++) {
System.out.print(sb3.charAt(i)+"\t");
}
System.out.println();
String str = sb3.substring(2, 4);//得到新的string
System.out.println(str);
System.out.println(sb3);
}
}
String 不可变
StringBuilder StringBuffer 可变 append insert resvert setCharAt 改变内容 可以 使用toString
StringBuilder 效率高 线程不安全
StringBuffer 效率低 线程安全