Can an inner class be built in an Interface?• Yes,an inner class may be built an Interface
public interface xyz
static int p=0;
public static void main(String c)
Why is explicit object casting needed?In order to assign a superclass object in a variable to a subclass,one needs to do explicit casting.
man = (Man)person;
An automatic casting takes place when we typecast a object in subclass as parent class object.
Define Externalizable.• Externalizable is coined as an Interface
• It extends the Serializable Interface.
• It also sends data into the Streams.
• Externalizable sends data in a Compressed Format.
• Externalizable is having two methods,for e.g. writeExternal(ObjectOuput out) & readExternal(ObjectInput in)
How can a collection object be sorted?• // Sort
• // Sort case-insensitive
• // SortReverse-order
Collections.sort(list, Collections.reverseOrder ());
• // Reverse-order sort case-insensitive
Define local, member and a class variable.• Within a method variables declared are called “local” variables.
• Variables declared in the class i.e not in any methods are “member” variables (global variables).
• Variables declared in the class i.e not in any methods and are called as “static” are class variables
Name the different identifier states of a Thread.Different types of identifiers of a Thread are:
• R - Running or runnable thread
• S - Suspended thread
• CW - Thread waiting on a condition variable
• MW - Thread waiting on a monitor lock
• MS - Thread suspended waiting on a monitor lock
Define Vector class? Differentiate the Vector and ArrayList.• Vector canbe said a legacy class which has been introduced to implement the List interface since Java 2 platform v1.2
• Vector is always synchronized but ArrayList is not.
• When Vector class is synchronized, if we will run in multithreading environment we've to use ArrayList with Collections.
• Vector has a default size i.e 10 while arrayList has no default size .
• ArraayList is not having any method returning Enumerations where as vector list is having.
Differentiate between Enumeration and Iterator interface• In java.util package the Enumeration and Iterator are available.
• The Enumeration interface is replicated by the Iterator interface.
• In preference to Enumeration new implementations should consider using Iterator .
The difference of Iterators from enumerations are:
• Enumeration has 2 methods namely hasMoreElements() & nextElement() where the Iterator contained three methods namely hasNext(), next(),remove().
• An optional remove operation is added in Iterator,and has shorter method names. We Use remove() to delete the objects but the Enumeration interface does not support this feature.
• The legacy classes use Enumeration interface .Vector.elements() & Hashtable.elements() method results Enumeration. All Java Collections Framework classes returns iterator. java.util.Collection.iterator() method returning an instance of Iterator.
What are the alternatives to inheritance?• Delegation is an alternative to inheritance.
• Delegation denotes that you include an instance of any class as an instance variable, and forward messages to the instance.
• It is safer than inheritance because it ceases you to think about forwarded message , because the instance is of a known class, rather than a new class, and because it doesn’t force you to accept all the methods of the super class: you can provide only the methods that really make sense.
• On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).
Number the bits, used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?• Unicode requires 16 bits
• ASCII require 7 bits. but it is usually represented as 8 bits.
• UTF-8 represents characters using 8, 16, and 18 bit patterns.
• UTF-16 uses 16-bit and larger bit patterns.
Define reflection• Reflection allows program related access to information about the fields, methods and constructors of loaded classes
• It use reflected fields, methods, and constructors
• It helps to operate on their underlying counterparts on objects,
• It operates within security restrictions.
What is RMI and how it is useful?• Remote method invocation is called RMI.
• One can work with remote object using RMI .
• It gives a impression that you are working with a object that resides within your own JVM though it is somewhere.
• The protocol used by RMI is RMI-IIOP
Define a Collection API.• The set of classes and interfaces supporting the operation on collections of objects is the Collection API .
• Than the vectors, arrays, and hashtables if effectively replaces,these classes and interfaces are more flexible, more powerful, and more regular
• class examples: HashSet, TreeMap, ArrayList, LinkedList,HashMap and TreeMap.
• interface examples: Set,List ,Collection and Map.
How many forms of Polymorphism are there?polymorphism exists in three different forms in Java:
• Method overloading
• Method overriding through inheritance
• Method overriding through the Java interface
Define the wrapper classes in Java and name a few.• Wrapper class is wraps around the primitive data type.
• list of the primitive types and the corresponding wrapper classes:
Differentiate between JDK ,JRE & JVM• JDK stands for Java Development Kit.
It is the most widely used Java Software Development Kit.
• JRE stands for Java Runtime Environment
It is an implementation of the Java Virtual Machine which executes Java programs
• JVM stands for Java Virtual Machine
It is an interpreter.
Why will you use Comparator and Comparable interfaces?• java.util.Comparator
• java.util.Comparator compares some other class’s instances,
• java.lang.Comparable compares another object with itself .
Differentiate between final, finally and finalize.• The keyword is final.sss
It is used for declaring a constant
It prevents a class from producing subclasses.
• finally is a code.
It always executes when the try block is finished,
Unless System.exit() has been called.
• finalize() is a method,
Before discarding by the garbage collector it is invoked .
Differentiate JAR and WAR filesJAR files:
• JAR files is the acronym stands for Java ARchive fles.
• JAR files allow aggregating many files into one,
• JAR is usually used to hold Java classes in a library.
• WAR files is the acronym stands for Web ARchive fles.
• WAR stores XML, java classes, and JavaServer pages
• WAR is mainly used for Web Application purposes.`
In a Java , how can you send program messages on the system console, but error messages, to a file?• The class System has a variable out that denotes the standard output,
• the standard error device represents the variable err .
• Naturally, they both point at the system console.
In this way, the standard output can be sent to the file:
Stream x = new Stream(new FileOutputStream("error.txt"));