Questions 1 to 40
1.Is “abc” a primitive value?
Ans: The String literal “abc” is not a primitive value. It is a String object.
2.What restrictions are placed on the values of each case of a switch statement?
Ans: During compilation, the values of each case of a switch statement must evaluate to a value that can be promoted to an int value.
3.What modifiers may be used with an interface declaration?
Ans: An interface may be declared as public or abstract.
4.Is a class a subclass of itself?
Ans: A class is a subclass of itself.
5.What is the difference between a while statement and a do statement?
Ans: A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.
6.What modifiers can be used with a local inner class?
Ans: A local inner class may be final or abstract.
7.What is the purpose of the File class?
Ans: The File class is used to create objects that provide access to the files and directories of a local file system.
8.Can an exception be re-thrown?
Ans: Yes, an exception can be re-thrown.
9.When does the compiler supply a default constructor for a class?
Ans: The compiler supplies a default constructor for a class if no other constructors are provided.
10.If a method is declared as protected, where may the method be accessed?
Ans: A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.
11.Which non-Unicode letter characters may be used as the first character of an identifier?
Ans: The non-Unicode letter characters $ and _ may appear as the first character of an identifier
12.What restrictions are placed on method overloading?
Ans: Two methods may not have the same name and argument list but different return types.
13.What is casting?
Ans: There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.
14.What is the return type of a program’s main() method?
Ans: A program’s main() method has a void return type.
15.What class of exceptions are generated by the Java run-time system?
Ans: The Java run-time system generates RuntimeException and Error exceptions.
16.What class allows you to read objects directly from a stream?
Ans: The ObjectInputStream class supports the reading of objects from input streams.
17.What is the difference between a field variable and a local variable?
Ans: A field variable is a variable that is declared as a member of a class. A local variable is a variable that is declared local to a method.
18.How are this() and super() used with constructors?
Ans: this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor
19.What is the relationship between a method’s throws clause and the exceptions that can be thrown during the method’s execution?
Ans: A method’s throws clause must declare any checked exceptions that are not caught within the body of the method.
20.Why are the methods of the Math class static?
Ans: So they can be invoked as if they are a mathematical code library.
21.What are the legal operands of the instanceof operator?
Ans: The left operand is an object reference or null value and the right operand is a class, interface, or array type.
22.What an I/O filter?
Ans: An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.
23.If an object is garbage collected, can it become reachable again?
Ans: Once an object is garbage collected, it ceases to exist. It can no longer become reachable again.
24.What are E and PI?
Ans: E is the base of the natural logarithm and PI is mathematical value pi.
25.Are true and false keywords?
Ans: The values true and false are not keywords.
26.What is the difference between the File and RandomAccessFile classes?
Ans: The File class encapsulates the files and directories of the local file system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file.
27.What happens when you add a double value to a String?
Ans: The result is a String object.
28.What is your platform’s default character encoding?
Ans: If you are running Java on English Windows platforms, it is probably Cp1252. If you are running Java on English Solaris platforms, it is most likely 8859_1.
29.Which package is always imported by default?
Ans: The java.lang package is always imported by default.
30.What interface must an object implement before it can be written to a stream as an object?
Ans: An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.
31.How can my application get to know when a HttpSession is removed?
Ans: Define a Class HttpSessionNotifier which implements HttpSessionBindingListener and implement the functionality what you need in valueUnbound() method. Create an instance of that class and put that instance in HttpSession.
32.Whats the difference between notify() and notifyAll()?
Ans: notify() is used to unblock one waiting thread; notifyAll() is used to unblock all of them. Using notify() is preferable (for efficiency) when only one blocked thread can benefit from the change (for example, when freeing a buffer back into a pool). notifyAll() is necessary (for correctness) if multiple threads should resume (for example, when releasing a “writer” lock on a file might permit all “readers” to resume).
33.Why can’t I say just abs() or sin() instead of Math.abs() and Math.sin()?
Ans: The import statement does not bring methods into your local name space. It lets you abbreviate class names, but not get rid of them altogether. That’s just the way it works, you’ll get used to it. It’s really a lot safer this way.
However, there is actually a little trick you can use in some cases that gets you what you want. If your top-level class doesn’t need to inherit from anything else, make it inherit from java.lang.Math. That *does* bring all the methods into your local name space. But you can’t use this trick in an applet, because you have to inherit from java.awt.Applet. And actually, you can’t use it on java.lang.Math at all, because Math is a “final” class which means it can’t be extended.
34.Why are there no global variables in Java?
Ans: Global variables are considered bad form for a variety of reasons: Adding state variables breaks referential transparency (you no longer can understand a statement or expression on its own: you need to understand it in the context of the settings of the global variables), State variables lessen the cohesion of a program: you need to know more to understand how something works. A major point of Object-Oriented programming is to break up global state into more easily understood collections of local state, When you add one variable, you limit the use of your program to one instance. What you thought was global, someone else might think of as local: they may want to run two copies of your program at once. For these reasons, Java decided to ban global variables.
35.What does it mean that a class or member is final?
Ans: A final class can no longer be subclassed. Mostly this is done for security reasons with basic classes like String and Integer. It also allows the compiler to make some optimizations, and makes thread safety a little easier to achieve. Methods may be declared final as well. This means they may not be overridden in a subclass. Fields can be declared final, too. However, this has a completely different meaning. A final field cannot be changed after it’s initialized, and it must include an initializer statement where it’s declared. For example, public final double c = 2.998; It’s also possible to make a static field final to get the effect of C++’s const statement or some uses of C’s #define, e.g. public static final double c = 2.998;
36.What does it mean that a method or class is abstract?
Ans: 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.
37.What is a transient variable?
Ans: transient variable is a variable that may not be serialized.
38.How are Observer and Observable used?
Ans: Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects
39.Can a lock be acquired on a class?
Ans: Yes, a lock can be acquired on a class. This lock is acquired on the class’s Class object.
40.What state does a thread enter when it terminates its processing?
Ans: When a thread terminates its processing, it enters the dead state.
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