java基础primitive 类型

一.java的基本数据类型 的相关

java的8大主要类型 即primitive类型 对应的字节 和封装类，默认值

另外，引用也是primitive

BigInteger 也算是封装类，但并不对应任何一种primitive类型

/**
*
* @author Administrator
*  type      bits		bytes  	封装类 warpper   默认值
*	boolean 	1				Boolean		  false
* 	char		16		  2		Character	  '/u0000'(null)
* 	byte		8		  1		Byte		  (byte)0
* 	short		16 		  2		Short		  (short)0
*  int 		32		  4		Integer		  	0
*  long 		64		  8		Long			0L
*  float		32		  4		Float			0.0f
*  double		64		  8		Double			0.0d
*
*  							BigInteger		构造方法需要参数
*  							BigDecimal		构造方法需要参数
*
*/

二.java 基础相关知识 

+= 和 = 的区别

short s = 1;
s = s+1 //报错 因为s+1返回值是int型，将int赋给s(short)类型会报错
s+=1;	//不报错 等于(short)s+1;

System.out 是一个 static PrintStream 对象

三.System.getProperties()

这个方法的作用是获取系统参数,代码如下

public class Property{
public static void main(String[] args) {
System.out.println(new Date());
//得到
Properties p = System.getProperties();

p.list(System.out);//输出系统参数 list是方法
//输出runtime 内存使用情况
System.out.println("---Memory Usage---");

Runtime runtime = Runtime.getRuntime();

System.out.println("Total Memory = "
+runtime.totalMemory()
+" Free Memory ="
+ runtime.freeMemory()
);

}
}

结果如下

Wed Jul 27 20:14:12 CST 2016
-- listing properties --
java.runtime.name=Java(TM) SE Runtime Environment
sun.boot.library.path=E:\Program Files (x86)\Java\jdk1.8.0_...
java.vm.version=25.25-b02
java.vm.vendor=Oracle Corporation
java.vendor.url=http://java.oracle.com/
path.separator=;
java.vm.name=Java HotSpot(TM) Client VM
file.encoding.pkg=sun.io
user.script=
user.country=CN
sun.java.launcher=SUN_STANDARD
sun.os.patch.level=Service Pack 1
java.vm.specification.name=Java Virtual Machine Specification
user.dir=F:\系统文件夹\Documents\java\Eclipse for j...
java.runtime.version=1.8.0_25-b18
java.awt.graphicsenv=sun.awt.Win32GraphicsEnvironment
java.endorsed.dirs=E:\Program Files (x86)\Java\jdk1.8.0_...
os.arch=x86
java.io.tmpdir=C:\Users\ADMINI~1\AppData\Local\Temp\
line.separator=

java.vm.specification.vendor=Oracle Corporation
user.variant=
os.name=Windows 7
sun.jnu.encoding=GBK
java.library.path=E:\Program Files (x86)\Java\jdk1.8.0_...
java.specification.name=Java Platform API Specification
java.class.version=52.0
sun.management.compiler=HotSpot Client Compiler
os.version=6.1
user.home=C:\Users\Administrator
user.timezone=Asia/Shanghai
java.awt.printerjob=sun.awt.windows.WPrinterJob
file.encoding=GBK
java.specification.version=1.8
user.name=Administrator
java.class.path=F:\系统文件夹\Documents\java\Eclipse for j...
java.vm.specification.version=1.8
sun.arch.data.model=32
java.home=E:\Program Files (x86)\Java\jdk1.8.0_25
sun.java.command=com.hjh.ThinkInJava01.Property_2_7_1
java.specification.vendor=Oracle Corporation
user.language=zh
awt.toolkit=sun.awt.windows.WToolkit
java.vm.info=mixed mode, sharing
java.version=1.8.0_25
java.ext.dirs=E:\Program Files (x86)\Java\jdk1.8.0_...
sun.boot.class.path=E:\Program Files (x86)\Java\jdk1.8.0_...
java.vendor=Oracle Corporation
file.separator=\
java.vendor.url.bug=http://bugreport.sun.com/bugreport/
sun.cpu.endian=little
sun.io.unicode.encoding=UnicodeLittle
sun.desktop=windows
sun.cpu.isalist=pentium_pro+mmx pentium_pro pentium+m...
---Memory Usage---
Total Memory = 16252928 Free Memory =14893032