4. Virtual reference "virtual reference" the name suggests, is non-existent, and several other references are different, virtual reference and does not determine the objects life cycle. If an object only holds virtual reference, then it is and without any references, at any time may be garbage collected. Virtual reference is mainly used to track the object is garbage collection activities. Virtual reference and soft and weak references a difference: the virtual reference and the reference queue to be used in combination. When the garbage collector for recovery of an object, if found that it also has virtual reference, the memory will be in the recovery of the object before it is added to the virtual reference references associated with the queue. Program can determine whether the reference queue has joined the virtual reference, to understand
System V UNIX platforms for major contributions include: Bourne shell enhanced features, including shell function; because Solaris was originally developed by Berkeley BSD derived, and therefore, SunOS 4.x command style and the other is very similar to BSD, but Since then switch to the System V version 4, which makes the SunOS 5.x changed. BSD UNIX SVR4 and the main differences can be summarized as follows: Root System V script for / etc / init.d, for the BSD / etc / rc.d. In the System V default shell for the Bourne shell, in the BSD for the C shell. System V file system database for the / etc / mnttab, for the BSD / etc / mtab. In the System V kernel name for the / UNIX, the BSD for / vmUNIX.