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Q1.
How could Java classes direct program messages to the system console, but error
messages, say to a file?
A. The class System has a variable out that represents the standard
output, and the variable err that represents the standard error device.
By default, they both point at the system console. This how the standard output
could be re-directed:
Stream
st = new Stream(new FileOutputStream("output.txt")); System.setErr(st);
System.setOut(st);
*Q2. What's the difference between an interface and an abstract class?
A. An abstract class may contain code in method bodies, which is not allowed in
an interface. With abstract classes, you have to inherit your class from it and
Java does not allow multiple inheritance. On the other hand, you can implement
multiple interfaces in your class.
*Q3. Why would you use a synchronized block vs. synchronized method?
A. Synchronized blocks place locks for shorter periods than synchronized
methods.
*Q4. Explain the usage of the keyword transient?
A. This keyword indicates that the value of this member variable does not have
to be serialized with the object. When the class will be de-serialized, this
variable will be initialized with a default value of its data type (i.e. zero
for integers).
*Q5. How can you force garbage collection?
A. You can't force GC, but could request it by calling System.gc(). JVM does
not guarantee that GC will be started immediately.
*Q6. How do you know if an explicit object casting is needed?
A. If you assign a superclass object to a variable of a subclass's data type,
you need to do explicit casting. For example:
Object
a; Customer b; b = (Customer) a;
When you assign a subclass to a variable having a supeclass type, the casting
is performed automatically.
*Q7. What's the difference between the methods sleep() and wait()
A. The code sleep(1000); puts thread aside for exactly one second. The code
wait(1000), causes a wait of up to one second. A thread could stop waiting
earlier if it receives the notify() or notifyAll() call. The method wait() is
defined in the class Object and the method sleep() is defined in the class
Thread.
*Q8. Can you write a Java class that could be used both as an applet as well as
an application?
A. Yes. Add a main() method to the applet.
*Q9. What's the difference between constructors and other methods?
A. Constructors must have the same name as the class and can not return a
value. They are only called once while regular methods could be called many
times.
*Q10. Can you call one constructor from another if a class has multiple
constructors
A. Yes. Use this() syntax.
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