System.out.println

System class has hasA realationship with PrintStream class so System class access the object of PrintStream class i.e to out object.
like :

class System
{
PrintStream out;
}

class PrintStream
{
println()
{
}
}

System class  Variables

1. err::=Standard Error Reporting Stream::PrintStream. [ PrintStream ]
2. out::=Standard Output stream:: PrintStream. [ PrintStream ]
3. in::=Standard Input stream::InputStream

System class  Methods

1. arraycopy(Object, int, Object, int, int)::= Copies an array from the source array, beginning at the specified position, to the specified position of the destination array.
2. exit(int)::= Exits the virtual machine with an exit code. [ Array Copy ]
3. currentTimeMillis()::= Returns the current time in milliseconds GMT since the epoch (00:00:00 UTC, January 1, 1970). See Also [ Date ]
4. gc()::= Runs the garbage collector.
5. getProperties()::= Gets the System properties.
6. getProperty(String)::= Gets the System property indicated by the specified key.
7. getProperty(String, String)::= Gets the System property indicated by the specified key and def.
8. getSecurityManager()::= Gets the system security interface.
9. load(String)::= Loads a dynamic library, given a complete path name.
10. loadLibrary(String)::= Loads a dynamic library with the specified library name.
11. runFinalization()::= Runs the finalization methods of any objects pending finalization.
12. setProperties(Properties)::= Sets the System properties to the specified properties.
13. setSecurityManager(SecurityManager)::= Sets the System security. 

for getting full detail see the whole  package