新建helloworld.java文件
// 定义一个类helloworld,类名必须与文件名相同 public class helloworld { public static void main(string[] args) { system.out.println("hello, world!"); } }
编译和运行
javac helloworld.java //编译 java helloworld //运行
(1)所有关键字
(2)标识符
是指在程序中,我们自己定义的内容,比如类名,方法名和变量名等等
命名规则
命名规范
基本数据类型
变量的定义
public class variable { public static void main(string[] args){ int i = 12; system.out.println(i); float f = 0.5f; system.out.println(f); char c = 'b'; system.out.println(c); } }
自动转换
public class datatype { public static void main(string[] args){ int i = 5; byte b = 2; // byte类型比int小,做运算的时候会自动把byte类型转换成int类型 int j = i + b; system.out.println(j); } }
强制转换
public class datatype { public static void main(string[] args){ double i = 2.33; system.out.println(i); //2.33 //把double类型强制转换成int类型 int j = (int) 2.33; system.out.println(j); //2 } }
判断语句
public class demoifelse { public static void main(string[] args){ int score = 74; if(score<0 || score>100){ system.out.println("数据错误"); }else if(score>=80 && score<=100){ system.out.println("优秀"); }else if(score>=60 && score<80){ system.out.println("及格"); }else{ system.out.println("不及格"); } } }
选择语句
public class demoswitch { public static void main(string[] args){ int i = 2; switch(i){ case 1: system.out.println("星期一"); break; case 2: system.out.println("星期二"); break; case 3: system.out.println("星期三"); break; default: system.out.println("数据错误"); break; } } }
public class demofor{ public static void main(string[] args){ //for循环 for(int i=1;i<=10;i++){ system.out.println(i); } //while循环 int j = 1; while(j<=10){ system.out.println(j); j++; } //do..while循环 int a = 1; do{ system.out.println(a); a++; }while(a<=10); } }
package derek.day04.demo02; public class demomethod { public static void main(string[] args) { system.out.println(sum(2, 3)); } public static int sum(int a, int b) { int result = a + b; return result; } }
package derek.day04.demo02; public class demoarray { public static void main(string[] args) { int[] arra = new int[] {1,2,3,4,5}; // int[] arra = {1,2,3,4,5} int num = arra[1]; system.out.println(num); //2 int[] arrb = new int[3]; arrb[0] = 11; //添加元素 arrb[1] = 22; arrb[2] = 33; //数组长度 system.out.println(arrb.length); //3 //数组的遍历 for (int i = 0; i < arrb.length; i++) { system.out.println(arrb[i]); } } }
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