packagecom.itheima.demo01;publicclassHelloWorld{
publicstaticvoidmain(String[] args){
short s =10;// 错误 s + 3,3是int,所以s提升成int,导致把int赋值到short上面报错。// s = s + 3;// 正确,强制类型转换
s =(short)(s +3);// 正确,+=包含了强制类型转换,相当于上面
s +=3;}}
packagecom.itheima.demo01;publicclassHelloWorld{
publicstaticvoidmain(String[] args){
int time =10;if(time >0&& time <=8){
System.out.println("早上好");}elseif(time >8&& time <=17){
System.out.println("下午好");}else{
System.out.println("晚上好");}}}
5. for循环
packagecom.itheima.demo01;publicclassHelloWorld{
publicstaticvoidmain(String[] args){
for(int i=1; i <=5; i++){
System.out.println(i);}}}
6. while循环
packagecom.itheima.demo01;publicclassHelloWorld{
publicstaticvoidmain(String[] args){
int i =0;while(i<5){
System.out.println("hello world");
i++;}}}
7. 生成随机数
packagecom.itheima.demo01;publicclassHelloWorld{
publicstaticvoidmain(String[] args){
int num;for(int i =1; i <=5; i++){
// Math.random();可以获取0.0 ~ 1.0之间所有的数据, 包左不包右.
num =(int)(Math.random()*10+1);System.out.println(num);}}}
packagecom.itheima.demo07_extends;publicclassCatextendsAnimal{
publicCat(){
super();}int num =20;publicvoidshow(){
int num =10;System.out.println(num);System.out.println(this.num);System.out.println(super.num);}@Overridepublicvoideat(){
System.out.println("猫吃鱼!");}}
packagecom.itheima.demo08_multi;//动物类publicclassAnimal/*extends Object*/{
int age =30;publicvoideat(){
System.out.println("动物会吃!");}}
packagecom.itheima.demo08_multi;publicclassCatextendsAnimal{
int age =20;@Overridepublicvoideat(){
System.out.println("猫吃鱼");}publicvoidcatchMouse(){
System.out.println("捉老鼠");}}
packagecom.itheima.demo08_multi;publicclassDemo2{
publicstaticvoidmain(String[] args){
Cat c =newCat();printAnimal(c);Dog d =newDog();printAnimal(d);}publicstaticvoidprintAnimal(Animal an){
an.eat();}}
17. 静态属性和final关键词
packagecom.itheima.demo09_final_static;publicclassStudent{
String name;staticString graduateFrom ="黑马";publicstaticvoidshow(){
// 报错 public static void show 静态方法不能获取动态属性// System.out.println(name);// 正确,可以获取静态成员System.out.println(graduateFrom);}}
packagecom.demo08_date;importjava.util.Date;publicclassDemo01{
publicstaticvoidmain(String[] args){
Date d =newDate();System.out.println(d);// Wed Apr 26 16:50:28 CST 2023System.out.println(d.getTime());//1682499028354System.out.println("--------------------");Date d2 =newDate(1631180490798L);//d2.setTime(1631180490798L);System.out.println(d2);//Thu Sep 09 17:41:30 CST 2021System.out.println("--------------------");//获取当前时间毫秒值, 实际开发做法.long time =System.currentTimeMillis();System.out.println(time);//1682499028397}}
27.2 Date和String互转
packagecom.demo08_date;importjava.text.ParseException;importjava.text.SimpleDateFormat;importjava.util.Date;publicclassDemo02{
publicstaticvoidmain(String[] args)throwsParseException{
//演示: Date -> String: 格式化.Date d =newDate();System.out.println(d);SimpleDateFormat sdf =newSimpleDateFormat("yyyy/MM/dd HH:mm:ss");String result = sdf.format(d);System.out.println(result);// 2023/04/26 16:56:10System.out.println("-----------------------");String s ="2021年09月09日 17:50:02";SimpleDateFormat sdf2 =newSimpleDateFormat("yyyy年MM月dd日 HH:mm:ss");Date d2 = sdf2.parse(s);System.out.println(d2);//Thu Sep 09 17:50:02 CST 2021}}
packagecom.demo08_date;importjava.text.ParseException;importjava.text.SimpleDateFormat;importjava.util.Date;publicclassDemo03{
publicstaticvoidmain(String[] args)throwsParseException{
Date d =newDate();String s1 =DateUtils.date2String(d,"yyyy/MM/dd HH:mm:ss");System.out.println(s1);System.out.println("-----------------------");String s ="2021年09月09日 17:50:02";Date d2 =DateUtils.string2Date(s,"yyyy年MM月dd日 HH:mm:ss");System.out.println(d2);}}
27.3 Calendar对象
packagecom.demo08_date;importjava.util.Calendar;publicclassDemo04{
publicstaticvoidmain(String[] args){
Calendar c =Calendar.getInstance();System.out.println(c.get(Calendar.YEAR));System.out.println(c.get(Calendar.MONTH));//8, Java中月份范围: 0 ~ 11System.out.println(c.get(Calendar.DATE));//9System.out.println(c.get(Calendar.DAY_OF_MONTH));//9System.out.println(c.get(Calendar.DAY_OF_YEAR));//252System.out.println("-----------------------------");}}