Posted in: Faqs
• What is a JVM?JVM is Java Virtual Machine which is a run time environment for the compiled java class files.
• How to define a constant variable in Java?
The variable should be declared as static and final. So only one copy of the variable exists for all instances of the class and the value can't be changed also.
static final int PI = 2.14; is an example for constant.
• I don't want my class to be inherited by any other class. What should i do?
You should declared your class as final. But you can't define your class as final, if it is an abstract class. A class declared as final can't be extended by any other class.
• What are the restriction imposed on a static method or a static block of code?
A static method should not refer to instance variables without creating an instance and cannot use "this" operator to refer the instance.
• What does synchronized means in Hashtable context?
Synchronized means only one thread can modify a hash table at one point of time. Any thread before performing an update on a hashtable will have to acquire a lock on the object while others will wait for lock to be released.
• What is HashMap and Map?
Map is Interface which is part of Java collections framework. This is to store Key Value pair, and Hashmap is class that implements that using hashing technique.
• How can we make Hashmap synchronized?
HashMap can be synchronized by Map m = Collections.synchronizedMap(hashMap);
• What is an enumeration?
An enumeration is an interface containing methods for accessing the underlying data structure from which the enumeration is obtained. It is a construct which collection classes return when you request a collection of all the objects stored in the collection. It allows sequential access to all the elements stored in the collection.
• What is immutable object in Java? Can you change values of a immutable object?
A Java object is considered immutable when its state cannot change after it is created. Use of immutable objects is widely accepted as a sound strategy for creating simple, reliable code. Immutable objects are particularly useful in concurrent applications. Since they cannot change state, they cannot be corrupted by thread interference or observed in an inconsistent state. java.lang.String and java.lang.Integer classes are the Examples of immutable objects from the Java Development Kit. Immutable objects simplify your program, since they :
1. are simple to use test and construct.
2. are automatically thread-safe.
3. do not require a copy constructor.
4. do not require an implementation of clone.
5. allow hashCode to use lazy initialization, and to cache its return value.
6. do not need to be copied defensively when used as a field.
7. are good Map keys and Set elements (these objects must not change state while stored in the collection).
8. have their class invariant established once upon construction, and it never needs to be checked again.
• Is Java Pass by Reference or Pass by Value?
The Java Spec says that everything in Java is pass-by-value. There is no such thing as "pass-by-reference" in Java. The difficult thing can be to understand that Java passes "objects as references" passed by value.
• What is Metadata and why should I use it?
JDBC API has 2 Metadata interfaces DatabaseMetaData & ResultSetMetaData.
The DatabaseMetaData provides Comprehensive information about the database as a whole. This interface is implemented by driver vendors to let users know the capabilities of a Database Management System (DBMS) in combination with the driver based on JDBC technology ("JDBC driver") that is used with it. Below is a sample code which demonstrates how we can use the DatabaseMetaData.
• What does setAutoCommit(false) do?
A JDBC connection is created in auto-commit mode by default. This means that each individual SQL statement is treated as a transaction and will be automatically committed as soon as it is executed. If you require two or more statements to be grouped into a transaction then you need to disable auto-commit mode using below command.
• Explain different way of using thread?
A Java thread could be implemented by using Runnable interface or by extending the Thread class. The Runnable is more advantageous, when you are going for multiple inheritance.
• What is the difference between Thread.start() & Thread.run() method?
Thread.start() method (native method) of Thread class actually does the job of running the Thread.run() method in a thread. If we directly call Thread.run() method it will executed in same thread, so does not solve the purpose of creating a new thread.
• What happens when I make a static method as synchronized?
Synchronized static methods have a lock on the class "Class", so when a thread enters a synchronized static method, the class itself gets locked by the thread monitor and no other thread can enter any static synchronized methods on that class. This is unlike instance methods, as multiple threads can access "same synchronized instance methods" at same time for different instances.