Data Structures

1. What is data structure?
   A data structure is a way of organizing data that considers not only the items stored, but also their relationship to each other. Advance knowledge about the relationship between data items allows designing of efficient algorithms for the manipulation of data.
2. List out the areas in which data structures are applied extensively?
   1. Compiler Design,
   2. Operating System,
   3. Database Management System,
   4. Statistical analysis package,
   5. Numerical Analysis,
   6. Graphics,
   7. Artificial Intelligence,
   8. Simulation
3. What are the major data structures used in the following areas : RDBMS, Network data model and Hierarchical data model.
   1. RDBMS = Array (i.e. Array of structures)
   2. Network data model = Graph
   3. Hierarchical data model = Trees

Advanced Java

1. What is a transient variable?
   A transient variable is a variable that may not be serialized.
2. Which containers use a border Layout as their default layout?
   The Window, Frame and Dialog classes use a border layout as their default layout.
3. Why do threads block on I/O?
   Threads block on I/O (that is enters the waiting state) so that other threads may execute while the I/O Operation is performed.
   

Operating Systems

1. Explain the concept of Reentrancy?
   It is a useful, memory-saving technique for multiprogrammed timesharing systems. A Reentrant Procedure is one in which multiple users can share a single copy of a program during the same period. Reentrancy has 2 key aspects: The program code cannot modify itself, and the local data for each user process must be stored separately. Thus, the permanent part is the code, and the temporary part is the pointer back to the calling program and local variables used by that program. Each execution instance is called activation. It executes the code in the permanent part, but has its own copy of local variables/parameters. The temporary part associated with each activation is the activation record. Generally, the activation record is kept on the stack.

   Note: A reentrant procedure can be interrupted and called by an interrupting program, and still execute correctly on returning to the procedure.
2. Explain Belady's Anomaly?
   Also called FIFO anomaly. Usually, on increasing the number of frames allocated to a process virtual memory, the process execution is faster, because fewer page faults occur. Sometimes, the reverse happens, i.e., the execution time increases even when more frames are allocated to the process. This is Belady's Anomaly. This is true for certain page reference patterns.
3. What is a binary semaphore? What is its use?
   A binary semaphore is one, which takes only 0 and 1 as values. They are used to implement mutual exclusion and synchronize concurrent processes.