A database index is a data structure that improves the speed of operations in a table. Indices can be created using one or more columns, providing the basis for both rapid random lookups and efficient ordering of access to records. The disk space required to store the index is typically less than the storage of the table (since indices usually contain only the key-fields according to which the table is to be arranged, and excludes all the other details in the table), yielding the possibility to store indices into memory from tables that would not fit into it. In a relational databasean index is a copy of part of a table. Some databases extend the power of indexing by allowing indices to be created on functions or expressions. For example, an index could be created on upper(last_name), which would only store the uppercase versions of the last_name field in the index. Another option sometimes supported is the use of "filtered" indices, where index entries are created only for those records that satisfy some conditional expression. A further aspect of flexibility is to permit indexing on user-defined functions, as well as expressions formed from an assortment of built-in functions.

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Boxing converts a value-type to a reference-type, thus storing the object on the heap.  Unboxing converts a reference-type to a value-type, thus storing the value on the stack.

Deletes it from the memory.
To Do: answer better.  The current answer is not entirely correct.

StringBuilder is more efficient in cases where there is a large amount of string manipulation.  Strings are immutable, so each time a string is changed, a new instance in memory is created.

The data value may not be changed.  Note: The variable value may be changed, but the original immutable data value was discarded and a new data value was created in memory.

Unsafe code bypasses type safety and memory management.

Garbage collection eliminates uneeded objects. the new statement allocates memory for an object on the heap.When no objects reference the object it may be removed from the heap (this is a non deterministic process).Finalize is called just before the memory is released.

Allocate a block of memory on the stack (used in unsafe mode). 

An object used by a servlet to track a user's interaction with a Web application across multiple HTTP requests. What is session bean

An enterprise bean that is created by a client and that usually exists only for the duration of a single client-server session. A session bean performs operations, such as calculations or database access, for the client. Although a session bean can be transactional, it is not recoverable should a system crash occur. Session bean objects either can be stateless or can maintain conversational state across methods and transactions. If a session bean maintains state, then the EJB container manages this state if the object must be removed from memory. However, the session bean object itself must manage its own persistent data.