Knowledge Training #4 :Core Java Interview Questions

Question: How to define an Abstract class?
Answer: A class containing abstract method is called Abstract class. An Abstract class can't be instantiated.
Example of Abstract class:
abstract class testAbstractClass {
    protected String myString;
    public String getMyString() {
        return myString;
        }
    public abstract string anyAbstractFunction();
}
     

Question: How to define an Interface?
Answer: In Java Interface defines the methods but does not implement them. Interface can include constants. A class that implements the interfaces is bound to implement all the methods defined in Interface.
Emaple of Interface:

public interface sampleInterface {
    public void functionOne();


    public long CONSTANT_ONE = 1000;
}
   

Question: Explain the user defined Exceptions?
Answer: User defined Exceptions are the separate Exception classes defined by the user for specific purposed. An user defined can created by simply sub-classing it to the Exception class. This allows custom exceptions to be generated (using throw) and caught in the same way as normal exceptions.
Example:
class myCustomException extends Exception {
     // The class simply has to exist to be an exception
}
  

Question: Explain the new Features of JDBC 2.0 Core API?
Answer: The JDBC 2.0 API includes the complete JDBC API, which includes both core and Optional Package API, and provides inductrial-strength database computing capabilities.
New Features in JDBC 2.0 Core API:

• Scrollable result sets- using new methods in the ResultSet interface allows programmatically move the to particular row or to a position relative to its current position
• JDBC 2.0 Core API provides the Batch Updates functionality to the java applications.
• Java applications can now use the ResultSet.updateXXX methods.
• New data types - interfaces mapping the SQL3 data types
• Custom  mapping of user-defined types (UTDs)
• Miscellaneous features, including performance hints, the use of character streams, full precision for java.math.BigDecimal values, additional security, and support for time zones in date, time, and timestamp values. 

    

Question: Explain garbage collection?
Answer: Garbage collection is one of the most important feature of Java. Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. User program cann't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use. I Java on calling System.gc() and Runtime.gc(),  JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected.
   

Question: How you can force the garbage collection?
Answer: Garbage collection automatic process and can't be forced.
 

Question: What is OOPS?
Answer: OOP is the common abbreviation for Object-Oriented Programming.
 

Question: Describe the principles of OOPS.
Answer: There are three main principals of oops which are called Polymorphism, Inheritance and Encapsulation.
 

Question: Explain the Encapsulation principle.
Answer: Encapsulation is a process of binding or wrapping the data and the codes that operates on the data into a single entity. This keeps the data safe from outside interface and misuse. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper.
 

Question: Explain the Inheritance principle.
Answer: Inheritance is the process by which one object acquires the properties of another object.
 

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 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".
 

Question: Explain the different forms of Polymorphism.
Answer: From a practical programming viewpoint, polymorphism exists in three distinct forms in Java:

• Method overloading
• Method overriding through inheritance
• Method overriding through the Java interface