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Java System Class --- a Utility often used

Java System Class

The System class  is a final class and  contains many useful fields and methods which are quite useful utilities like:
standard input, output and error, for loading files and libraries.
It helps to provide access to externally defined properties and variables.
It also contain utility methods to quickly copy an array
To get the environment variables.

Fields in the System Class:

static PrintStream err : The standard error output stream
static InputStream in : The standard input stream
static PrintStream out: The standard output stream

The class has many useful methods , lets see few important methods:
static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length): It is used to copy array from src array to destination array starting at srcPosition and starts copying at destPosition and copies the length specified from source to destination.

System Properties Methods:
It contains method to get , set and clear system properties.
It can get   full set of properties or the specified properties.
You can get and print all set of properties as below :


Properties sysProps = System.getProperties();
Set<Object> keySet = sysProps.keySet();
for(Object obj : keySet){
    String key = (String) obj;
    System.out.println("{"+obj+"="+sysProps.getProperty(key)+"}");
}
 

For setting multiple properties below is the method

static void setProperties(Properties props)

Few important system properties are like : "user.home" to provide the home directory for the user.
similarly "user.dir" provides the default user directory
"file.separator" : to provide the file separator used in the current environment.
"os.name" : current operating system being used

public static Console console() : Returns the unique Console object associated.


System Environment Variables:
You can also get environment variables.
It can return a specified environment variable value or a MAP of all the values. Below are the methods to get that

public static String getenv(String name)
public static Map <String, String> getenv()

SecurityManager: We can get and set the security Manager using system class.
Security Manger is used to implement the security policy.
For example, while getting and setting the system properties,  first it checks if there is any security manager. Then it calls checkPropertiesAccess() which will threw a SecurityException if the calling thread is not allowed to access or modify the system properties.
Similarly while calling getenv or setenv it calls  its checkPermission method is called with a RuntimePermission("getenv."+name)

How does System.out.println() works:
System is a final class which contains a public static final member refrence 'out', of class PrintStream .
The PrintStream class contains the println().
There are multiple println methods with different arguments (overloading). Every println makes a call to print method and adds a newline. print calls write() and the story goes on like that.











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