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Java Interview Questions and Answers
What does it mean that a method or class
is abstract?
An abstract class cannot be instantiated. Only its subclasses can be
instantiated. You indicate that a class is abstract with the abstract
keyword like this:
public abstract class Container extends Component {
Abstract classes may contain abstract methods. A method declared
abstract is not actually implemented in the current class. It exists
only to be overridden in subclasses. It has no body. For example,
public abstract float price();
Abstract methods may only be included in abstract classes. However, an
abstract class is not required to have any abstract methods, though most
of them do.
Each subclass of an abstract class must override the abstract methods of
its superclasses or itself be declared abstract.
What is the main difference between Java platform and other platforms?
The Java platform differs from most other platforms in that it's a
software-only platform that runs on top of other hardware-based
platforms.
The Java platform has three elements:
Java programming language
The Java Virtual Machine (Java VM)
The Java Application Programming Interface (Java API)
What is the Java Virtual Machine?
The Java Virtual Machine is a software that can be ported onto various
hardware-based platforms.
What is the Java API?
The Java API is a large collection of ready-made software components
that provide many useful capabilities, such as graphical user interface
(GUI) widgets.
What is the package?
The package is a Java namespace or part of Java libraries. The Java API
is grouped into libraries of related classes and interfaces; these
libraries are known as packages.
What is native code?
The native code is code that after you compile it, the compiled code
runs on a specific hardware platform.
Explain the user defined Exceptions?
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
}
Is Java code slower than native code?
Not really. As a platform-independent environment, the Java platform can
be a bit slower than native code. However, smart compilers, well-tuned
interpreters, and just-in-time bytecode compilers can bring performance
close to that of native code without threatening portability.
Can main() method be overloaded?
Yes. the main() method is a special method for a program entry. You can
overload main() method in any ways. But if you change the signature of
the main method, the entry point for the program will be gone.
What is the serialization?
The serialization is a kind of mechanism that makes a class or a bean
persistence by having its properties or fields and state information
saved and restored to and from storage.
Explain the new Features of JDBC 2.0 Core API?
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.
How you can force the garbage collection?
Garbage collection automatic process and can't be forced.
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