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Java Interview Questions and Answers
What is the purpose of the System class?
The purpose of the System class is to provide access to system
resources. What is the
purpose of the finally clause of a try-catch-finally statement?
The finally clause is used to provide the capability to execute code no
matter whether or not an exception is thrown or caught.
What is the Locale class?
The Locale class is used to tailor program output to the conventions of
a particular geographic, political, or cultural region.
What is an abstract method?
An abstract method is a method whose implementation is deferred to a
subclass. Or, a method that has no implementation.
What is the difference between interface and abstract class?
interface contains methods that must be abstract; abstract class may
contain concrete methods. interface contains variables that must be
static and final; abstract class may contain non-final and final
variables. members in an interface are public by default, abstract class
may contain non-public members. interface is used to "implements";
whereas abstract class is used to "extends". interface can be used to
achieve multiple inheritance; abstract class can be used as a single
inheritance. interface can "extends" another interface, abstract class
can "extends" another class and "implements" multiple interfaces.
interface is absolutely abstract; abstract class can be invoked if a
main() exists. interface is more flexible than abstract class because
one class can only "extends" one super class, but "implements" multiple
interfaces. If given a choice, use interface instead of abstract class.
What is a static method?
A static method is a method that belongs to the class rather than any
object of the class and doesn't apply to an object or even require that
any objects of the class have been instantiated.
What is a protected method?
A protected method is a method that can be accessed by any method in its
package and inherited by any subclass of its class.
What is the difference between a static and a non-static inner class?
A non-static inner class may have object instances that are associated
with instances of the class's outer class. A static inner class does not
have any object instances.
What is an object's lock and which object's have locks?
An object's lock is a mechanism that is used by multiple threads to
obtain synchronized access to the object. A thread may execute a
synchronized method of an object only after it has acquired the object's
lock. All objects and classes have locks. A class's lock is acquired on
the class's Class object.When can an object reference be cast to an interface reference?
An object reference can be cast to an interface reference when the
object implements the referenced interface.
What is the difference between a Window and a Frame?
The Frame class extends Window to define a main application window that
can have a menu bar.
What is the difference between a Window and a Frame?
Heavy weight components like Abstract Window Toolkit (AWT), depend on
the local windowing toolkit. For example, java.awt.Button is a heavy
weight component, when it is running on the Java platform for Unix
platform, it maps to a real Motif button. In this relationship, the
Motif button is called the peer to the java.awt.Button. If you create
two Buttons, two peers and hence two Motif Buttons are also created. The
Java platform communicates with the Motif Buttons using the Java Native
Interface. For each and every component added to the application, there
is an additional overhead tied to the local windowing system, which is
why these components are called heavy weight.
Which package has light weight components?
javax.Swing package. All components in Swing, except JApplet, JDialog,
JFrame and JWindow are lightweight components.
What are peerless components?
The peerless components are called light weight components.
What is the difference between the Font and FontMetrics classes?
The FontMetrics class is used to define implementation-specific
properties, such as ascent and descent, of a Font object
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