java新手笔记2 数据类型

1.注释

/**  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!");
      */
    }
}

 2.变量

public class InputDemo {

    public static void main(String[] args) {
      //输出 
      System.out.println("TR21" );
      System.out.print("javase
");
      System.out.printf("%-5d%d  %.5f
",15,3,3.14);

      System.out.println(""大家好" xml	oracle javase");
      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);
    }

}

 3.数据类型

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);
    }
   
}

 4.数据类型转换

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
      
    }

}