30. What is the purpose of finalization?
The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.
31. Which class is the superclass for every class.
32. What invokes a thread’s run() method?
After a thread is started, via its start() method or that of the Thread class, the JVM invokes the thread’s run() method when the thread is initially executed.
33. What is the difference between the Boolean & operator and the && operator?
If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the & operator is applied to the operand. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. The && operator is then applied to the first and second operands. If the first operand evaluates to false, the evaluation of the second operand is skipped.
Operator & has no chance to skip both sides evaluation and && operator does. If asked why, give details as above.
34. What is the GregorianCalendar class?
The GregorianCalendar provides support for traditional Western calendars.
35. What is the SimpleTimeZone class?
The SimpleTimeZone class provides support for a Gregorian calendar.
36. Which Container method is used to cause a container to be laid out and redisplayed?
37. What is the Properties class?
The properties class is a subclass of Hashtable that can be read from or written to a stream. It also provides the capability to specify a set of default values to be used.
38. What is the purpose of the Runtime class?
The purpose of the Runtime class is to provide access to the Java runtime system.
39. What is the purpose of the System class?
The purpose of the System class is to provide access to system resources.
40. 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.
41. 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.
42. What must a class do to implement an interface?
It must provide all of the methods in the interface and identify the interface in its implements clause.
43. What is the purpose of the wait(), notify(), and notifyAll() methods?
The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to communicate each other.
44. What is an abstract method?
An abstract method is a method whose implementation is deferred to a subclass.
45. What are the high-level thread states?
The high-level thread states are ready, running, waiting, and dead.
46. 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.
47. 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.
48. When can an object reference be cast to an interface reference?
An object reference be cast to an interface reference when the object implements the referenced interface.
49. 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.
50. What do heavy weight components mean?
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 heavyweight.