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Core Java Interview Questions & Answers - Learning Mode
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Core Java Interview Questions & Answers - Learning Mode

"Core Java" is Sun's term, used to refer to Java SE, the standard edition and a set of related technologies, like the Java VM, CORBA, et cetera. This is mostly to differentiate from, say, Java ME or Java EE. Also note that they're talking about a set of libraries rather than the programming language. When people talk about core java they usually refer to the features of the java language and the standard libraries alone. Advanced Java is probably everything else you can add, such as Java Enterprise Edition libraries.

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Core Java Interview Questions & Answers - Learning Mode
Try Core Java Interview Questions & Answers - Exam Mode
Question: What do you understand by Synchronization?

Answer: Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption. Source: CoolInterview.com
Question: What do u mean by "method" in java ?

Answer: Java methods are similar to functions or procedures in other programming languages.

Every Java program must have one main() method.

Here is the main() method from a Java program which prints
"Hello World":

public static void main (String[] args) {
// This Java program prints "Hello World!"
System.out.println{"Hello World!");
Source: CoolInterview.com
Question: Why would you use a synchronized block vs. synchronized method?

Answer: Synchronized blocks place locks for shorter periods than synchronized methods. Source: CoolInterview.com
Question: What is Encapsulation?

Answer: Encapsulation is a technique used for hiding the properties and behaviors of an object and allowing outside access only as appropriate. It prevents other objects from directly altering or accessing the properties or methods of the encapsulated object. Source: CoolInterview.com
Question: How are this() and super() used with constructors?

Answer: * Constructors use this to refer to another constructor in the same class with a different parameter list.
* Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must use it in the first line; otherwise, the compiler will complain.
Source: CoolInterview.com
Question: What does it mean that a method or field is ?static??

Answer: Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class. Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That?s how library methods like System.out.println() work. out is a static field in the j Source: CoolInterview.com
Question: What is the difference between throw and throws?

Answer: Throw:it is used to raise exception explicitly
that means it is use when a user defined exception is raised.

Throws:if a method is capable of throwing an exception but
it does not handle the exception that must be specified by using "throws" class. Source: CoolInterview.com
Question: What is an abstract class?

Answer: Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared, but contains no implementation.
Note:

* If even a single method is abstract, the whole class must be declared abstract.
* Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.
* You can?t mark a class as both abstract and final.
Source: CoolInterview.com
Question: What are the differences between Contructors and Methods?

Answer: Constructors Methods

Purpose


Create an instance of a class


Group Java statements

Modifiers


Cannot be abstract, final, native, static, or synchronized


Can be abstract, final, native, static, or synchronized

Return Type


No return type, not even void


void or a valid return type

Name


Same name as the class (first letter is capitalized by convention) -- usually a noun


Any nam Source: CoolInterview.com
Question: Why operator overloading is not there in java?

Answer: C++ has proven by example that operator overloading makes code almost impossible to maintain. In fact there very nearly wasn?t even method overloading in Java, but it was thought that this was too useful for some very basic methods like print(). Note that some of the classes like DataOutputStream have unoverloaded methods like writeInt() and writeByte(). Source: CoolInterview.com
Question: What modifiers are allowed for methods in an Interface?

Answer: Only public and abstract modifiers are allowed for methods in interfaces. Source: CoolInterview.com
Question: What is Constructor?

Answer: * A constructor is a special method whose task is to initialize the object of its class.
* It is special because its name is the same as the class name.
* They do not have return types, not even void and therefore they cannot return values.
* They cannot be inherited, though a derived class can call the base class constructor.
* Constructor is invoked whenever an object of its associated class is created.
Source: CoolInterview.com
Question: What are static methods?

Answer: Methods declared with the keyword static as modifier are called static methods or class methods. They are so called because they affect a class as a whole, not a particular instance of the class. Static methods are always invoked without reference to a particular instance of a class.
Note:The use of a static method suffers from the following restrictions:

* A static method can only call other static methods.
* A static method must only access static data.
* A static m Source: CoolInterview.com
Question: What modifiers are allowed for methods in an Interface?

Answer: Only public and abstract modifiers are allowed for methods in interfaces. Source: CoolInterview.com
Question: What is method overriding?

Answer: Method overriding occurs when sub class declares a method that has the same type arguments as a method declared by one of its superclass. The key benefit of overriding is the ability to define behavior that?s specific to a particular subclass type.<br>Note:<br><br> The overriding method cannot have a more restrictive access modifier than the method being overridden (Ex: You can?t override a method marked public and make it protected).<br> * You cannot override a method Source: CoolInterview.com
Question: Explain the different forms of Polymorphism.

Answer: Polymorphism exists in three distinct forms in Java:

1.Method overloading
2. Method overriding through inheritance
3. Method overriding through the Java interface
Source: CoolInterview.com
Question: Explain the Polymorphism principle.

Answer: The meaning of Polymorphism is something like one name many forms. Polymorphism enables one entity to be used as general category for different types of actions. The specific action is determined by the exact nature of the situation. The concept of polymorphism can be explained as "one interface, multiple methods". Source: CoolInterview.com
Question: What are the differences between Class Methods and Instance Methods?

Answer: Class Methods Instance Methods
Class methods are methods which are declared as static. The method can be called without creating an instance of the class Instance methods on the other hand require an instance of the class to exist before they can be called, so an instance of a class needs to be created by using the new keyword.
Instance methods operate on specific instances of classes.
Class methods can only operate on class members and not on instance members as class methods are un Source: CoolInterview.com
Question: What is similarities between an Abstract class and Interface?

Answer: Neither Abstract classes or Interface can be instantiated. Source: CoolInterview.com
Question: what is main purpose of abstract class?

Answer: if a class contains abstract method,that class must be declared as a abstract class. Abstract methods contains only declarations. it does not contain objects. Source: CoolInterview.com

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