Java Interview Questions 24

116. 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();
}

117. 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;
}

118. 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
}

119. 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.

120. 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.