Java基础学习笔记二 Java基础语法详解编程语言

//这是一条单行注释  
public int i;

/* 这是 
* 一段注释， 
* 它跨越了多个行 
*/  
public void f() {} 
}

/** The first Thinking in Java example program. 
* Lists system information on current machine. 
* @author Bruce Eckel 
* @author http://www.BruceEckel.com 
* @version 1.0 
*/ 
public class Property { 
/** Sole entry point to class & application 
* @param args array of string arguments 
* @return No return value 
* @exception exceptions No exceptions thrown 
*/ 
public static void main(String[] args) { 
System.out.println(new Date()); 
Properties p = System.getProperties(); 
p.list(System.out); 
System.out.println("--- Memory Usage:"); 
Runtime rt = Runtime.getRuntime(); 
System.out.println("Total Memory = " 
+ rt.totalMemory() 
+ " Free Memory = " 
+ rt.freeMemory()); 
}59 
}

public class Main { 
    public static void main(String[] args) { 
        //输出整数 十进制 
        System.out.println(50);//50 
        //输出整数，二进制, 数字开头0B 
        System.out.println(0B11);//3 
        //输出整数，八进制，数字开头0 
        System.out.println(051);//41 
        //输出整数，十六进制，数组开头0X  0-9 A-F 
        System.out.println(0XE);//14 
        //输出浮点数据 
        System.out.println(5.0);//5.0 
        //输出布尔数据，只有2个值，true，false 关键字 
        System.out.println(true);//true 
        System.out.println(false);//false 
        //输出字符常量，单引号包裹，只能写1个字符 
        System.out.println('a');//a 
        //输出字符串常量，双引号包裹，可以写0-n个字符 
        System.out.println("HelloWorld");//HelloWorld 
    } 
}

数据类型  变量名  =  数据值； 
int         a    =  100;

int x; 
x = 20; //为x赋值20

public static void main(String[] args) { 
    int x = 20; 
    { 
        int y = 20; 
    } 
    System.out.println(x);//读取x变量中的值，再打印 
    System.out.println(y);//读取y变量中的值失败，失败原因，找不到y变量，因为超出了y变量作用范围，所以不能使用y变量 
}

public static void main(String[] args){ 
      int x = 10; 
      double x = 5.5;//编译失败，变量重复定义 
}

范围大的数据类型 变量 = 范围小的数据类型值； 
比如：double d = 1000; 

范围小的数据类型  变量 = (范围小的数据类型) 范围大的数据类型值; 
比如：int  i = (int)6.718;   //i的值为6

int n = (3>2 && 4>6) ? 100 : 200; 
//逻辑运算后的结果为false，运算结果为表达式2的值200,然后将结果200赋值给了变量n

public class Main { 
    public static void main(String[] args) { 
        //苹果笔记本电脑 
        String macBrand = "MacBookAir"; 
        double macSize = 13.3; 
        double macPrice = 6988.88; 
        int macCount = 5; 
 
        //联想Thinkpad笔记本电脑 
        String thinkpadBrand = "ThinkpadT450"; 
        double thinkpadSize = 14.0; 
        double thinkpadPrice = 5999.99; 
        int thinkpadCount = 10; 
 
        //华硕ASUS笔记本电脑 
        String ASUSBrand = "ASUS-FL5800"; 
        double ASUSSize = 15.6; 
        double ASUSPrice = 4999.50; 
        int ASUSCount = 18; 
        //统计所有库存商品数量与金额 
        //统计库存总个数、库存总金额 
        int totalCount = macCount + thinkpadCount + ASUSCount; 
        double totalMoney = (macCount * macPrice) + (thinkpadCount * thinkpadPrice) + (ASUSCount * ASUSPrice); 
        //打印库存清单顶部信息 
        System.out.println("------------------------------商城库存清单-----------------------------"); 
        System.out.println("品牌型号    尺寸    价格    库存数"); 
        //打印库存清单中部信息 
        System.out.println(macBrand + "    " + macSize + "    " + macPrice + "    " + macCount); 
        System.out.println(thinkpadBrand + "    " + thinkpadSize + "    " + thinkpadPrice + "    " + thinkpadCount); 
        System.out.println(ASUSBrand + "    " + ASUSSize + "    " + ASUSPrice + "    "+ASUSCount); 
        //打印库存清单底部信息 
        System.out.println("-----------------------------------------------------------------------"); 
        System.out.println("总库存数：" + totalCount); 
        System.out.println("库存商品总金额：" + totalMoney); 
    } 
}