/**  doc注释

*    类说明信息

*/

//声明类  文件名与类名一致

public class World {//类定界符

//声明方法 main方法

    public static void main(String[] args) {

         System.out.println("World World!");

         //System.out.println("welcome java world!");

         //注释的代码不执行  单行注释

      /*  多行注释

      System.out.println("World World!");

      System.out.println("World World!");

      System.out.println("World World!");

      System.out.println("World World!");

      */

    }

}

public class InputDemo {

    public static void main(String[] args) {

      //输出

      System.out.println("TR21" );

      System.out.print("javase\n");

      System.out.printf("%-5d%d  %.5f\n",15,3,3.14);

      System.out.println("\"大家好\" xml\toracle j\bavase");

      int a ;//声明变量

      a = 20;//赋值

      int b = 10;

      a = a + b;

      System.out.println("a = " + a);//变量不能同名

      final double PI = 3.14;//常量 final

      //PI = 3.15;

      System.out.println("PI = " + PI);

    }

}

public class DataDemo {

    public static void main(String[] args) {

       //System.out.println("Hello...");

       int _a3 = 10;

       //整型  byte short int  long

       int a = -5;//3.14;

       System.out.println(Integer.MAX_VALUE);

       short b = 20;

       System.out.println(Short.MAX_VALUE);

       long c = 123;

      System.out.println(Long.MAX_VALUE);

      byte d = 127;

      System.out.println("d = " + d);

      //d = 128;

      c = 1234567890123456L;//默认整型 转换long类型

      //浮点型

      float f = 3.14f;//默认double

      double dou = 3.14;

      System.out.println("f = " + f);

      System.out.println("dou = " + dou);

      char ch = 'A';

      System.out.println("ch = " + ch);

      ch = '中';//变量赋值

      System.out.println("ch = " + ch);

      boolean boo = true;//false

      int num1 = 123;

      int num8 = 010;

      int num16 = 0xa;

      System.out.println("num1 = " + num1);

      System.out.println("num8 = " + num8);

      System.out.println("num16 = " + num16);

    }

}

public class DataConvert {

    public static void main(String[] args) {

       int a = 10;

       long b = a;//隐式类型转换  低精度--》高精度

       byte c = (byte)b;//强制类型转换

      System.out.println("a = " + a);

      System.out.println("b = " + b);

      System.out.println("c = " + c);

      long result = c + b + a;

      System.out.println("result = " +  result);// long

    }

}