System类

rt.jar/java.lang包下

public final class System

System类
概念:表示系统类
类常量:out,err
类方法:
		获取当前时间戳:		currentTimeMillis(1秒=1000毫秒)	
		获取当前纳秒:		nanoTime(1纳秒=0.000001毫秒)
		获取系统环境相关:	getenv
		获取系统属性:		getProperties,getProperty
		数组复制:			arraycopy
		程序退出:			exit(0是正常退出,非0是非正常退出)
		垃圾回收:			gc

这里只介绍其中部分常用方法

public static native long currentTimeMillis()
public static native long nanoTime()
public static java.util.Map getenv()
public static String getenv(String name)
public static Properties getProperties() 
public static String getProperty(String key)
public static native void arraycopy(Object src,  int  srcPos,Object dest, int destPos,int length)    
public static void exit(int status)
public static void gc()     

//类常量:out,err
System.out.print("HelloWorld");
System.out.println();
System.out.printf("test %d %s", 1,"test");
System.out.println();
System.err.print("HelloWorld");
System.err.println();
System.err.printf("test %d %s", 1,"test");
System.err.println();
//获取当前时间戳
System.out.println(System.currentTimeMillis());
//获取当前纳秒
System.out.println(System.nanoTime());
//获取系统环境相关
Map map = System.getenv();
for (Entry entry : map.entrySet()) {
    System.out.println(entry.getKey() + " => " + entry.getValue());
}

System.out.println("MAVEN_HOME  " + System.getenv("MAVEN_HOME"));
//获取系统属性
for (Entry entry : System.getProperties().entrySet()) {
    System.out.println(entry.getKey().toString() + " => " + entry.getValue().toString());
}

System.out.println(System.getProperty("java.home"));
//数组复制
int[] a = {1,3,5,7,9};
int[] b = new int[4];//{0,0,0,0}
System.arraycopy(a, 1, b, 2, 2);
System.out.println(Arrays.toString(b));
//程序退出
System.exit(1);
//垃圾回收
System.gc();