Core Java Interview Questions- 25
1) Which Component subclass is used for drawing and painting?
2) What are synchronized methods and synchronized statements?
Synchronized methods are methods that are used to control access to an object. A thread
only executes a synchronized method after it has acquired the lock for the method’s object
or class. Synchronized statements are similar to synchronized methods. A synchronized
statement can only be executed after a thread has acquired the lock for the object or class
referenced in the synchronized statement.
3) What method must be implemented by all threads?
All tasks must implement the run() method, whether they are a subclass of Thread or
implement the Runnable interface.
4) What methods are used to get and set the text label displayed by a Button object?
getLabel() and setLabel()
5) How are the elements of a GridLayout organized?
The elements of a GridBad layout are of equal size and are laid out using the squares of a
6) How is it possible for two String objects with
identical values not to be equal under the == operator?
The == operator compares two objects to determine if they are the same object in memory.
It is possible for two String objects to have the same value, but located indifferent areas of
7) What state is a thread in when it is executing?
An executing thread is in the running state.
8) What are the legal operands of the instanceof
The left operand is an object reference or null value and the right operand is a class,
interface, or array type.
9) Why are the methods of the Math class static?
So they can be invoked as if they are a mathematical code library.
10) What Checkbox method allows you to tell if a Checkbox is checked?
11) What an I/O filter?
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.
12) If an object is garbage collected, can it become
Once an object is garbage collected, it ceases to exist. It can no longer become reachable
13) Are true and false keywords?
The values true and false are not keywords.
14) What classes of exceptions may be thrown by a throw
A throw statement may throw any expression that may be assigned to the Throwable type.
15) What are E and PI?
E is the base of the natural logarithm and PI is mathematical value pi.
16) What is the difference between the File and
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.
17) What happens when you add a double value to a String?
The result is a String object.
18) What is a void return type?
A void return type indicates that a method does not return a value.
19) What is the purpose of the enableEvents() method?
The enableEvents() method is used to enable an event for a particular object. Normally, an
event is enabled when a listener is added to an object for a particular event. The
enableEvents() method is used by objects that handle events by overriding their
20) What is the Set interface?
The Set interface provides methods for accessing the elements of a finite mathematical set.
Sets do not allow duplicate elements.
21) What is your platform’s default character encoding?
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..
22) What interface is extended by AWT event listeners?
All AWT event listeners extend the java.util.EventListener interface.
23) What restrictions are placed on method overriding?
• Overridden methods must have the same name, argument list, and return type.
• The overriding method may not limit the access of the method it overrides.
• The overriding method may not throw any exceptions that may not be thrownby the
24) How can a dead thread be restarted?
A dead thread cannot be restarted.
25) What happens if an exception is not caught?
An uncaught exception results in the uncaughtException() method of the thread’s
ThreadGroup being invoked, which eventually results in the termination of the program in
which it is thrown.
26) What is a layout manager?
A layout manager is an object that is used to organize components in a container.
27) Which arithmetic operations can result in the
throwing of an ArithmeticException?
Integer / and % can result in the throwing of an ArithmeticException.
28) Which package is always imported by default?
The java.lang package is always imported by default.
29) What interface must an object implement before it
can be written to a stream as an object?
An object must implement the Serializable or Externalizable interface before it can be
written to a stream as an object.
30) How are this and super used?
this is used to refer to the current object instance. super is used to refer to the variables
and methods of the superclass of the current object instance.
31) What is the purpose of garbage collection?
The purpose of garbage collection is to identify and discard objects that are no longer
needed by a program so that their resources may be reclaimed and reused.187) What is a compilation unit?
A compilation unit is a Java source code file.
32) What is numeric promotion?
Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so
that integer and floating-point operations may take place. In numerical promotion, byte,
char, and short values are converted to int values. The int values are also converted to long
values, if necessary. The long and float values are converted to double values, as required.
33) What is the difference between a Scrollbar and a ScrollPane?
A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane
handles its own events and performs its own scrolling.
34) What are the Object and Class classes used for?
The Object class is the highest-level class in the Java class hierarchy. The Class class is
used to represent the classes and interfaces that are loaded by a Java program..
35) How does a try statement determine which catch
clause should be used to handle an exception?
When an exception is thrown within the body of a try statement, the catch clauses of the try
statement are examined in the order in which they appear. The first catch clause that is
capable of handling the exception is executed. The remaining catch clauses are ignored.
36) Can an unreachable object become reachable again?
An unreachable object may become reachable again. This can happen when the object’s
finalize() method is invoked and the object performs an operation which causes it to
become accessible to reachable objects.
37) When is an object subject to garbage collection?
An object is subject to garbage collection when it becomes unreachable to the program in
which it is used.
38) What is the difference between a public and a nonpublic class?
A public class may be accessed outside of its package. A non-public class may not be
accessed outside of its package.
39) To what value is a variable of the boolean type automatically initialized?
The default value of the boolean type is false.
40) What are three ways in which a thread can enter the waiting state?
A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by
unsuccessfully attempting to acquire an object’s lock, or by invoking an object’s wait()
method. It can also enter the waiting state by invoking its (deprecated) suspend() method.
41) Can an abstract class be final?
An abstract class may not be declared as final.
196) What is the ResourceBundle class?
The ResourceBundle class is used to store locale-specific resources that can be loaded by a
program to tailor the program’s appearance to the particular locale in which it is being run.
42) What is the difference between an if statement and a switch statement?
The if statement is used to select among two alternatives. It uses a boolean expression to
decide which alternative should be executed. The switch statement is used to select among
multiple alternatives. It uses an int expression to determine which alternative should be
43) What happens when you add a double value to a String?
The result is a String object.
44) What is the List interface?
The List interface provides support for ordered collections of objects.
45) What happens if a try-catch-finally statement does not have a catch clause to handle an exception that is
thrown within the body of the try statement?
The exception propagates up to the next higher level try-catch statement (if any) or results
in the program’s termination.
46) What is a Java package and how is it used?
A Java package is a naming context for classes and interfaces. A package is used to create a
separate name space for groups of classes and interfaces. Packages are also used to
organize related classes and interfaces into a single API unit and to control accessibility to
these classes and interfaces.
47) What modifiers may be used with a top-level class?
A top-level class may be public, abstract, or final.
48) Can try statements be nested?
Try statements may be tested.
49) What is the difference between the prefix and postfix
forms of the ++ operator?
The prefix form performs the increment operation and returns the value of the increment
operation. The postfix form returns the current value all of the expression and then
performs the increment operation on that value.
50) What is the purpose of a statement block?
A statement block is used to organize a sequence of statements as a single statement
51) What are the two basic ways in which classes that can be run as threads may be defined?
A thread class may be declared as a subclass of Thread, or it may implement the Runnable
52) What are the problems faced by Java programmers who don’t use layout managers?
Without layout managers, Java programmers are faced with determining how their GUI will
be displayed across multiple windowing systems and finding a common sizing and
positioning that will work within the constraints imposed by each windowing system.
53)What is an Instance in java?
Instance refers to the copy of the object at a particular time whereas object refers to the memory address of the class.
The new operator instantiates a class by allocating memory for a new object and returning a reference to that memory.
54).What is the difference between checked and unchecked exceptions?
In general, unchecked exceptions represent defects in the program (bugs), which are normally Runtime exceptions.
Furthermore, checked exceptions represent invalid conditions in areas outside the immediate control of the program.
55).How does Java allocate stack and heap memory?
Each time an object is created in Java it goes into the area of memory known as heap. The primitive variables like int and double are allocated in the stack, if they are local method variables and in the heap if they are member variables (i.e. fields of a class).
In Java methods local variables are pushed into stack when a method is invoked and stack pointer is decremented when a method call is completed.
In a multi-threaded application each thread will have its own stack but will share the same heap. This is why care should be taken in your code to avoid any concurrent access issues in the heap space.
The stack is threadsafe (each thread will have its own stack) but the heap is not threadsafe unless guarded with synchronisation through your code.
56).What is Java Reflection?
Reflection is commonly used by programs which require the ability to examine or modify the runtime behavior of applications running in the Java virtual machine.
57)What is an Encapsulation in Java ?
Encapsulation is a mechanism that binds together code and data it manipulates, and keeps both safe from outside interface and misuse. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper.Inheritance defines “is-a” hierarchy among classes in which one subclass inherits from one or more generalised super classes.
58)What is multiple inheritance and does java support?
If a child class inherits the property from multiple classes is known as multiple inheritance.
Java does not allow to extend multiple classes but to overcome this problem it allows to implement multiple Interfaces.
59)What is Composition?
Composition is a special type of aggregation relationship with a difference that its the compulsion for the OtherClass object to exist for the existence of MyMainClass.
60)What are the different forms of Polymorphism?
There are two types of polymorphism one is Compile time polymorphism and the other is run time polymorphism. Compile time polymorphism is method overloading.Runtime time polymorphism is done using inheritance and interface.