2. System Class

2. System Class
	Chapter 5. Essencial Classes

Often, a program requires access to system resources such as properties, standard input and output streams, or the current time:

Figure 5.3. System Class

System class include:

standard input, output, and error streams;
system properties;
garbage collection;
loading dynamic libraries;
miscellany, including copying arrays, getting the current time, exiting the runtime environment, and using the security manager.

	Note
	All of System's methods and variables are class methods and class variables.

A set of properties - key/value pairs - that define traits or attributes of the current working environment.

Table 5.1. System Properties

Key	Meaning	Applet Access
"file.separator"	File separator (e.g., "/")	yes
"java.class.path"	Java classpath	no
"java.class.version"	Java class version number	yes
"java.home"	Java installation directory	no
"java.vendor"	Java vendor-specific string	yes
"java.vendor.url"	Java vendor URL	yes
"java.version"	Java version number	yes
"line.separator"	Line separator	yes
"os.arch"	Operating system architecture	yes
"os.name"	Operating system name	yes
"path.separator"	Path separator (e.g., ":")	yes
"user.dir"	User's current working directory	no
"user.home"	User home directory	no
"user.name"	User account name	no

System.getProperty("path.separator");

System.getProperty("subliminal.message", "Buy Java Now!");

The getProperties() returns a Properties object.

The setProperties() method: takes a Properties object.

System.runFinalization();

This method calls the finalize() methods on all objects that are waiting to be garbage collected.

System.gc();

It asks the garbage collector to run at any time by calling System's gc() method.

It bypasses the system-independent interface of the System class and use system resources directly from the runtime environment.

Figure 5.4. Runtime Object

The oval labelled Runtime in the diagram represents the current runtime environment and is an instance of the Runtime Class.

	Note
	Messaging the Runtime object directly compromises your ability to run your program on different systems. You should do this only in special situation.


1. String Class		3. Input/Output Streams