other most frequently asked java questions

difference between concurrenthashmap and hashtable

In short, hashtable is synchronized at whole map level where as concurrenthashmap is synchronized at bucket level. Here are the differences:

  • read is completely not synchronized in concurrenthashmap
  • Iteratable/Enumerator does not throw concurrent modification exception while iterting in case the map is modified while iterating
  • write is synchronized only at the bucket level instead at the table level

importance of volatility, atomicity and synchronization

volatile: When a variable is declared volatile, any update to that variable is made directly to main memory instead of CPU cache and hence it guarantees the visibility of changes across threads

private volatile boolean flag;

public void run() {
while(!flag) {
//do some work

public void markFlag() {
flag = true;

atomicity: When a variable is declared atomic, it ensures the operations made on the variable are atomic in nature, it uses compare-and-swap low level cpu operation to accomplish this without the need of synchronization.

synchronization: traditional way of acquiring locks to do read/write operation on state (which is by design time consuming operation)

Serialization Mechanism

Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a FileOutputStream. This will save the object to a file.

The serializable interface is an empty interface. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process.

The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized along with the original object.One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.

There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are

  • Serialization ignores static fields, because they are not part of any particular state.
  • Base class fields are only handled if the base class itself is serializable.
  • Transient fields.

Joins and Implementation of relationships in sql

We have three types of relationships in SQL.

  • one-to-one
  • one-to-many
  • many-to-many
  • To implement these relationships

    One-to-one: Use foreign key to the referenced table
    Student: sid, fname, lname, addressId
    Address: addressId, adress, city, sid

    One-to-many: Use foreign key on the “many” side of the relationship linking back to the “one” side.
    Teacher: tid, fname, lname
    Classes: cid, cname, tid    
    Note: Here table ‘Teacher’ is one side and table ‘Classes’ is on many side

    Many-to-many: Use junction table.
    The junction table stores the primary keys of each table that is involved in the relationship.
    Student: sid, fname, lname
    classes: cid, cname, tid
    student_classes: cid, sid


    Here is a very good chart of all joins (and its variations) in SQL
    Reference: http://stackoverflow.com/questions/38549/difference-between-inner-and-outer-joins


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