java基础primitive 类型

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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      bitsbytes  封装类 warpper   默认值 *boolean 1Boolean  false * char16  2Character  '/u0000'(null) * byte8  1Byte  (byte)0 * short16   2Short  (short)0 *  int 32  4Integer  0 *  long 64  8Long0L *  float32  4Float0.0f *  double64  8Double0.0d *   *  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 Environmentsun.boot.library.path=E:\Program Files (x86)\Java\jdk1.8.0_...java.vm.version=25.25-b02java.vm.vendor=Oracle Corporationjava.vendor.url=http://java.oracle.com/path.separator=;java.vm.name=Java HotSpot(TM) Client VMfile.encoding.pkg=sun.iouser.script=user.country=CNsun.java.launcher=SUN_STANDARDsun.os.patch.level=Service Pack 1java.vm.specification.name=Java Virtual Machine Specificationuser.dir=F:\系统文件夹\Documents\java\Eclipse for j...java.runtime.version=1.8.0_25-b18java.awt.graphicsenv=sun.awt.Win32GraphicsEnvironmentjava.endorsed.dirs=E:\Program Files (x86)\Java\jdk1.8.0_...os.arch=x86java.io.tmpdir=C:\Users\ADMINI~1\AppData\Local\Temp\line.separator=java.vm.specification.vendor=Oracle Corporationuser.variant=os.name=Windows 7sun.jnu.encoding=GBKjava.library.path=E:\Program Files (x86)\Java\jdk1.8.0_...java.specification.name=Java Platform API Specificationjava.class.version=52.0sun.management.compiler=HotSpot Client Compileros.version=6.1user.home=C:\Users\Administratoruser.timezone=Asia/Shanghaijava.awt.printerjob=sun.awt.windows.WPrinterJobfile.encoding=GBKjava.specification.version=1.8user.name=Administratorjava.class.path=F:\系统文件夹\Documents\java\Eclipse for j...java.vm.specification.version=1.8sun.arch.data.model=32java.home=E:\Program Files (x86)\Java\jdk1.8.0_25sun.java.command=com.hjh.ThinkInJava01.Property_2_7_1java.specification.vendor=Oracle Corporationuser.language=zhawt.toolkit=sun.awt.windows.WToolkitjava.vm.info=mixed mode, sharingjava.version=1.8.0_25java.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 Corporationfile.separator=\java.vendor.url.bug=http://bugreport.sun.com/bugreport/sun.cpu.endian=littlesun.io.unicode.encoding=UnicodeLittlesun.desktop=windowssun.cpu.isalist=pentium_pro+mmx pentium_pro pentium+m...---Memory Usage---Total Memory = 16252928 Free Memory =14893032

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