java新手笔记2 数据类型

/**  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

}

}