IGNOU BCA MCA Students - VIVA Question Answer for JAVA or JSP
JAVA APPLET or JSP Selected Question Answer - PART VIII
Java Applets
48. What is an Applet ? A java applet is program that can be included in a HTML page and be executed in a java enabled client browser. Applets are used for creating dynamic and interactive web applications.
49. Explain the life cycle of an Applet. An applet may undergo the following states:
· Init: An applet is initialized each time is loaded.
· Start: Begin the execution of an applet.
· Stop: Stop the execution of an applet.
· Destroy: Perform a final cleanup, before unloading the applet.
50. What happens when an applet is loaded ? First of all, an
instance of the applet’s controlling class is created. Then, the applet
initializes itself and finally, it starts running.
51. What is the difference between an Applet and a Java
Application ? Applets are executed within a java enabled browser, but a
Java application is a standalone Java program that can be executed outside of a
browser. However, they both require the existence of a Java Virtual Machine
(JVM). Furthermore, a Java application requires a main method with a specific
signature, in order to start its execution. Java applets don’t need such a
method to start their execution. Finally, Java applets typically use a
restrictive security policy, while Java applications usually use more relaxed
security policies.
52. What are the restrictions imposed on Java applets ? Mostly due to
security reasons, the following restrictions are imposed on Java applets:
·
An applet cannot load libraries or define native methods.
·
An applet cannot ordinarily read or write files on the execution
host.
·
An applet cannot read certain system properties.
·
An applet cannot make network connections except to the host
that it came from.
·
An applet cannot start any program on the host that’s executing
it.
53. What are untrusted applets ? Untrusted
applets are those Java applets that cannot access or execute local system
files. By default, all downloaded applets are considered as untrusted.
54. What is the difference between applets loaded over the
internet and applets loaded via the file system ?Regarding the case
where an applet is loaded over the internet, the applet is loaded by the applet
classloader and is subject to the restrictions enforced by the applet security
manager. Regarding the case where an applet is loaded from the client’s local
disk, the applet is loaded by the file system loader. Applets loaded via the
file system are allowed to read files, write files and to load libraries on the
client. Also, applets loaded via the file system are allowed to execute
processes and finally, applets loaded via the file system are not passed
through the byte code verifier.
55. What is the applet class loader, and what does it provide ? When an applet
is loaded over the internet, the applet is loaded by the applet classloader.
The class loader enforces the Java name space hierarchy. Also, the class loader
guarantees that a unique namespace exists for classes that come from the local
file system, and that a unique namespace exists for each network source. When a
browser loads an applet over the net, that applet’s classes are placed in a
private namespace associated with the applet’s origin. Then, those classes
loaded by the class loader are passed through the verifier.The verifier checks
that the class file conforms to the Java language specification . Among other
things, the verifier ensures that there are no stack overflows or underflows
and that the parameters to all bytecode instructions are correct.
56. What is the applet security manager, and what does it
provide ? The
applet security manager is a mechanism to impose restrictions on Java applets.
A browser may only have one security manager. The security manager is
established at startup, and it cannot thereafter be replaced, overloaded,
overridden, or extended.
Swing
57. What is the difference between a Choice and a List ? A Choice is
displayed in a compact form that must be pulled down, in order for a user to be
able to see the list of all available choices. Only one item may be selected
from a Choice. A List may be displayed
in such a way that several List items are visible. A List supports the
selection of one or more List items.
58. What is a layout manager ? A layout manager is the used to organize
the components in a container.
59. What is the difference between a Scrollbar and a JScrollPane
? A Scrollbar is a Component, but not aContainer. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling.
60. Which Swing methods are thread-safe ? There are only
three thread-safe methods: repaint, revalidate, and invalidate.
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