Core Java Interview Questions
1. Why threads block or enters to waiting state on I/O?
Threads enters to waiting state or block on I/O because other threads can execute while the I/O operations are
performed.
2. What is List interface?
List is an ordered collection of objects.
3. What is the difference between yield() and sleep()?
When a object invokes yield() it returns to ready state. But when an object invokes sleep() method enters to not
ready state.
8. What are Wrapper Classes ?
They are wrappers to primitive data types. They allow us to access primitives as objects.
9. Can we call finalize() method ?
Yes. Nobody will stop us to call any method , if it is accessible in our class. But a garbage collector cannot call an
object's finalize method if that object is reachable.
10. What is the difference between time slicing and preemptive scheduling ?
In preemptive scheduling, highest priority task continues execution till it enters a not running state or a higher
priority task comes into existence. In ime slicing, the task continues its execution for a predefined period of time
and reenters the pool of ready tasks.
11. What is the initial state of a thread when it is created and started?
The thread is in ready state.
12. Can we declare an anonymous class as both extending a class and implementing an interface?
No. An anonymous class can extend a class or implement an interface, but it cannot be declared to do both
13. What are the differences between boolean && operator and & operator?
When an expression containing the & operator is evaluated, both operands are evaluated. And the & operator is
applied to the operand. When an expression containing && operator is evaluated, the first operand is evaluated. If
the first operand returns a value of true then only the second operand is evaluated otherwise the second part will
not get executed. && is also called short cut and.
14.What is the use of the finally block?
Finally is the block of code that executes always. The code in finally block will execute even if an exception is
occurred. finally will not execute when the user calls System.exit().
15. What is an abstract method ?
An abstract method is a method that don't have a body. It is declared with modifier abstract.
An abstract method is a method whose implementation is deferred to a subclass.
16. What is the difference between System.err and System.out?
We can redirect System.out to another file but we cannot redirect System.err stream
17. What are the differences between an abstract class and an interface?
An abstract class can have concrete method, which is not allowed in an interface. Abstract class can have private
or protected methods and variables and only public methods and variables are allowed in interface. We can
implement more than one interface , but we can extend only one abstract class. Interfaces provides loose coupling
where as abstract class provides tight coupling.
18. What is the difference between synchronized block and synchronized method?
Synchronized blocks place locks for the specified block where as synchronized methods place locks for the entire
method.
19. How can you force garbage collection in java?
You cannot force Garbage Collection, but you can request for it by calling the method System.gc(). But it doesn't
mean that Garbage Collection will start immediately. The garbage collection is a low priority thread of JVM.
20. How can you call a constructor from another constructor ?
By using this() reference.
21. How can you call the constructor of super class ?
By using super() syntax.
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22. What's the difference between normal methods and constructors?
Constructors must have the same name of the class and can not have a return type. They are called only once,
while regular methods can be called whenever required. We cannot explicitly call a constructor.
23. What is the use of packages in java ?
,Packages are a way to organize files in java when a project consists of more than one module. It helps in resolving
name conflicts when different modules have classes with the same names.
24. What must be the order of catch blocks when catching more than one exception?
The sub classes must come first. Otherwise it will give a compile time error.
25. How can we call a method or variable of the super class from child class ?
We can use super.method() or super.variable syntax for this purpose.
26. If you are overriding equals() method of a class, what other methods you might need to override ?
hashCode
27. How can you create your own exception?
Our class must extend either Exception or its sub class
28. What is serialization?
Serialization is the process of saving the state of an object.
29. What is de-serialization?
De-serialization is the process of restoring the state of an object.
30. What is externalizable?
It is an interface that extends Serializable. It is having two different methods writeExternal() and readExternal. This
interface allows us to customize the output.
31. Does garbage collection guarantee that a program will not run out of memory?
Garbage collection does not guarantee that a program will not run out of memory. It is also possible for programs to
create objects that are not subject to garbage collection. And there is no guarantee that Garbage Collection thread
will be executed.
32. What is a native method?
native method is a method that is implemented in a language other than Java.
33. What are different type of exceptions in Java?
There are two types of exceptions in java. Checked exceptions and Unchecked exceptions. Any exception that is is
derived from Throwable and Exception is called checked exception except RuntimeException and its sub classes.
The compiler will check whether the exception is caught or not at compile time. We need to catch the checked
exception or declare in the throws clause. Any exception that is derived from Error and RuntimeException is called
unchecked exception. We don't need to explicitly catch a unchecked exception.
34. Can we catch an error in our java program ?
Yes. We can . We can catch anything that is derived from Throwable. Since Error is a sub class of Throwable we
can catch an error also.
35. What is thread priority?
Thread Priority is an integer value that identifies the relative order in which it should be executed with respect to
others. The thread priority values ranging from 1- 10 and the default value is 5. But if a thread have higher priority
doesn't means that it will execute first. The thread scheduling depends on the OS.
36. How many times may an object's finalize() method be invoked by the garbage collector? Only once.
37. What is the difference between a continue statement and a break statement?
Break statement results in the immediate termination of the statement to which it applies (switch, for, do, or while).
A continue statement is used to end the current loop iteration and return control to the loop statement.
38. What must a class do to implement an interface?
It must identify the interface in its implements clause. Also it must provide definition for all the methods in the
interface otherwise it must be declared abstract.
39. What is an abstract class?
An abstract class is an incomplete class. It is declared with the modifier abstract. We cannot create objects of the
abstract class. It is used to specify a common behavioral protocol for all its child classes.
40. What is the difference between notify and notifyAll method ?
notify wakes up a single thread that is waiting for object's monitor. If any threads are waiting on this object, one of
them is chosen to be awakened. The choice is arbitrary and occurs at the discretion of the implementation. notifyAll
Wakes up all threads that are waiting on this object's monitor. A thread waits on an object's monitor by calling one
of the wait methods.
41. What does wait method do ?
It causes current thread to wait until either another thread invokes notify or notifyAll method of the current object, or
a specified amount of time has elapsed.
42. What are the different states of a thread?
The different thread states are ready, running, waiting and dead.
43. What is the difference between static and non static inner class?
non-static inner class can have an object instances that are associated with instances of the class's outer class. A
static inner class can not have any object instances.
44. What is the difference between String and StringBuffer class ?
Strings are immutable (constant), their values cannot be changed after they are created. StringBuffer supports
mutable objects.
45. Which is the base class for all classes?
java.lang.Object.
46. What is the difference between readers and streams?
, Readers are character oriented where streams are byte oriented. The readers are having full support for Unicode
data.
47. What is constructor chaining?
When a constructor of a class is executed it will automatically call the default constructor of the super class (if no
explicit call to any of the super class constructor) till the root of the hierarchy.
48. What are the different primitive data types in java?
There are 8 primitive types in java. boolean , char, byte, short, int long, float, double.
49. What is static?
static means one per class. static variables are created when the class loads. They are associated with the class.
In order to access a static we don't need objects. We can directly access static methods and variable by calling
classname.variablename.
50. Why we cannot override static methods?
Static means they are associated with a class. In static methods, the binding mechanism is static binding. So it
must be available at the compile time.
51. What is the difference between static and non static variables?
A static variable is associated with the class as a whole rather than with specific instances of a class. There will be
only one value for static variable for all instances of that class. Non-static variables take on unique values with
each object instance.
52. When does a compiler supplies a default constructor for a class?
If there is no other constructor exist in a class, the compiler will supply a default constructor.
53. What are the restrictions placed on overriding a method ?
The overridden method have the exact signature of the super class method, including the return type. The access
specified cannot be less restrictive than the super class method. We cannot throw any new exceptions in
overridden method.
54. What are the restrictions placed on overloading a method?
Overloading methods must differ in their parameter list, or number of parameters.
55. What is casting?
Casting means converting one type to another. There are mainly two types of casting. Casting between primitive
types and casting between object references.
Casting between primitive numeric types is used to convert larger data types to smaller data types. Casting
between object references is used to refer to an object by a compatible class, interface, or array type reference.
56. What is the difference between == and equals?
The equals method can be considered to perform a deep comparison of the value of an object, whereas the ==
operator performs a shallow comparison. If we are not overriding the equals method both will give the same result.
== will is used to compare the object references. It is used to check whether two objects are points to the same
reference.
57. What is a void return type ?
void indicates that the method will not return anything.
58. What will happen if an exception is not caught ?
An uncaught exception results in the uncaughtException() method of the thread's ThreadGroup, which results in
the termination of the program.
59. What are the different ways in which a thread can enter into waiting state?
There are three ways for a thread to enter into 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.
60. What is a ResourceBundle class?
The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to create the
program's appearance to the particular locale in which it is being run.
61. What is numeric promotion?
Numeric promotion is the conversion of a smaller numeric type to a larger numeric type. In numerical promotion,
byte, char, and short values are converted to int values. The int, long and float values are converted to the desired
types if required.
62. What is the difference between the prefix and postfix forms of the ++ operator?
The prefix form first performs the increment operation and then returns the value of the increment operation. The
postfix form first returns the current value of the expression and then performs the increment operation on that
value.
63. What are synchronized methods and synchronized statements?
Synchronized methods are methods that are declared with the keyword synchronized. A thread executes a
synchronized method only after it has acquired the lock for the method's object or class. Synchronized statements
are similar to synchronized methods. It is a block of code declared with synchronized keyword.
A synchronized statement can be executed only after a thread has acquired the lock for the object or class
referenced in the synchronized statement.
64. How can we create a thread?
thread can be created by extending Thread class or by implementing Runnable interface. Then we need to override
the method public void run().
65. What is the difference between a switch statement and an if statement?
1. Why threads block or enters to waiting state on I/O?
Threads enters to waiting state or block on I/O because other threads can execute while the I/O operations are
performed.
2. What is List interface?
List is an ordered collection of objects.
3. What is the difference between yield() and sleep()?
When a object invokes yield() it returns to ready state. But when an object invokes sleep() method enters to not
ready state.
8. What are Wrapper Classes ?
They are wrappers to primitive data types. They allow us to access primitives as objects.
9. Can we call finalize() method ?
Yes. Nobody will stop us to call any method , if it is accessible in our class. But a garbage collector cannot call an
object's finalize method if that object is reachable.
10. What is the difference between time slicing and preemptive scheduling ?
In preemptive scheduling, highest priority task continues execution till it enters a not running state or a higher
priority task comes into existence. In ime slicing, the task continues its execution for a predefined period of time
and reenters the pool of ready tasks.
11. What is the initial state of a thread when it is created and started?
The thread is in ready state.
12. Can we declare an anonymous class as both extending a class and implementing an interface?
No. An anonymous class can extend a class or implement an interface, but it cannot be declared to do both
13. What are the differences between boolean && operator and & operator?
When an expression containing the & operator is evaluated, both operands are evaluated. And the & operator is
applied to the operand. When an expression containing && operator is evaluated, the first operand is evaluated. If
the first operand returns a value of true then only the second operand is evaluated otherwise the second part will
not get executed. && is also called short cut and.
14.What is the use of the finally block?
Finally is the block of code that executes always. The code in finally block will execute even if an exception is
occurred. finally will not execute when the user calls System.exit().
15. What is an abstract method ?
An abstract method is a method that don't have a body. It is declared with modifier abstract.
An abstract method is a method whose implementation is deferred to a subclass.
16. What is the difference between System.err and System.out?
We can redirect System.out to another file but we cannot redirect System.err stream
17. What are the differences between an abstract class and an interface?
An abstract class can have concrete method, which is not allowed in an interface. Abstract class can have private
or protected methods and variables and only public methods and variables are allowed in interface. We can
implement more than one interface , but we can extend only one abstract class. Interfaces provides loose coupling
where as abstract class provides tight coupling.
18. What is the difference between synchronized block and synchronized method?
Synchronized blocks place locks for the specified block where as synchronized methods place locks for the entire
method.
19. How can you force garbage collection in java?
You cannot force Garbage Collection, but you can request for it by calling the method System.gc(). But it doesn't
mean that Garbage Collection will start immediately. The garbage collection is a low priority thread of JVM.
20. How can you call a constructor from another constructor ?
By using this() reference.
21. How can you call the constructor of super class ?
By using super() syntax.
The Package Young man mails himself in a box to his girlfriend... naked. Domain Names Helpful links on buying a
domain name. Message Boards - $4.99 Message board hosting. Fully customizable, no advertising. Parental
Control Software Keep Your Kids Safe Online With PCTattletale Internet Monitoring.
22. What's the difference between normal methods and constructors?
Constructors must have the same name of the class and can not have a return type. They are called only once,
while regular methods can be called whenever required. We cannot explicitly call a constructor.
23. What is the use of packages in java ?
,Packages are a way to organize files in java when a project consists of more than one module. It helps in resolving
name conflicts when different modules have classes with the same names.
24. What must be the order of catch blocks when catching more than one exception?
The sub classes must come first. Otherwise it will give a compile time error.
25. How can we call a method or variable of the super class from child class ?
We can use super.method() or super.variable syntax for this purpose.
26. If you are overriding equals() method of a class, what other methods you might need to override ?
hashCode
27. How can you create your own exception?
Our class must extend either Exception or its sub class
28. What is serialization?
Serialization is the process of saving the state of an object.
29. What is de-serialization?
De-serialization is the process of restoring the state of an object.
30. What is externalizable?
It is an interface that extends Serializable. It is having two different methods writeExternal() and readExternal. This
interface allows us to customize the output.
31. Does garbage collection guarantee that a program will not run out of memory?
Garbage collection does not guarantee that a program will not run out of memory. It is also possible for programs to
create objects that are not subject to garbage collection. And there is no guarantee that Garbage Collection thread
will be executed.
32. What is a native method?
native method is a method that is implemented in a language other than Java.
33. What are different type of exceptions in Java?
There are two types of exceptions in java. Checked exceptions and Unchecked exceptions. Any exception that is is
derived from Throwable and Exception is called checked exception except RuntimeException and its sub classes.
The compiler will check whether the exception is caught or not at compile time. We need to catch the checked
exception or declare in the throws clause. Any exception that is derived from Error and RuntimeException is called
unchecked exception. We don't need to explicitly catch a unchecked exception.
34. Can we catch an error in our java program ?
Yes. We can . We can catch anything that is derived from Throwable. Since Error is a sub class of Throwable we
can catch an error also.
35. What is thread priority?
Thread Priority is an integer value that identifies the relative order in which it should be executed with respect to
others. The thread priority values ranging from 1- 10 and the default value is 5. But if a thread have higher priority
doesn't means that it will execute first. The thread scheduling depends on the OS.
36. How many times may an object's finalize() method be invoked by the garbage collector? Only once.
37. What is the difference between a continue statement and a break statement?
Break statement results in the immediate termination of the statement to which it applies (switch, for, do, or while).
A continue statement is used to end the current loop iteration and return control to the loop statement.
38. What must a class do to implement an interface?
It must identify the interface in its implements clause. Also it must provide definition for all the methods in the
interface otherwise it must be declared abstract.
39. What is an abstract class?
An abstract class is an incomplete class. It is declared with the modifier abstract. We cannot create objects of the
abstract class. It is used to specify a common behavioral protocol for all its child classes.
40. What is the difference between notify and notifyAll method ?
notify wakes up a single thread that is waiting for object's monitor. If any threads are waiting on this object, one of
them is chosen to be awakened. The choice is arbitrary and occurs at the discretion of the implementation. notifyAll
Wakes up all threads that are waiting on this object's monitor. A thread waits on an object's monitor by calling one
of the wait methods.
41. What does wait method do ?
It causes current thread to wait until either another thread invokes notify or notifyAll method of the current object, or
a specified amount of time has elapsed.
42. What are the different states of a thread?
The different thread states are ready, running, waiting and dead.
43. What is the difference between static and non static inner class?
non-static inner class can have an object instances that are associated with instances of the class's outer class. A
static inner class can not have any object instances.
44. What is the difference between String and StringBuffer class ?
Strings are immutable (constant), their values cannot be changed after they are created. StringBuffer supports
mutable objects.
45. Which is the base class for all classes?
java.lang.Object.
46. What is the difference between readers and streams?
, Readers are character oriented where streams are byte oriented. The readers are having full support for Unicode
data.
47. What is constructor chaining?
When a constructor of a class is executed it will automatically call the default constructor of the super class (if no
explicit call to any of the super class constructor) till the root of the hierarchy.
48. What are the different primitive data types in java?
There are 8 primitive types in java. boolean , char, byte, short, int long, float, double.
49. What is static?
static means one per class. static variables are created when the class loads. They are associated with the class.
In order to access a static we don't need objects. We can directly access static methods and variable by calling
classname.variablename.
50. Why we cannot override static methods?
Static means they are associated with a class. In static methods, the binding mechanism is static binding. So it
must be available at the compile time.
51. What is the difference between static and non static variables?
A static variable is associated with the class as a whole rather than with specific instances of a class. There will be
only one value for static variable for all instances of that class. Non-static variables take on unique values with
each object instance.
52. When does a compiler supplies a default constructor for a class?
If there is no other constructor exist in a class, the compiler will supply a default constructor.
53. What are the restrictions placed on overriding a method ?
The overridden method have the exact signature of the super class method, including the return type. The access
specified cannot be less restrictive than the super class method. We cannot throw any new exceptions in
overridden method.
54. What are the restrictions placed on overloading a method?
Overloading methods must differ in their parameter list, or number of parameters.
55. What is casting?
Casting means converting one type to another. There are mainly two types of casting. Casting between primitive
types and casting between object references.
Casting between primitive numeric types is used to convert larger data types to smaller data types. Casting
between object references is used to refer to an object by a compatible class, interface, or array type reference.
56. What is the difference between == and equals?
The equals method can be considered to perform a deep comparison of the value of an object, whereas the ==
operator performs a shallow comparison. If we are not overriding the equals method both will give the same result.
== will is used to compare the object references. It is used to check whether two objects are points to the same
reference.
57. What is a void return type ?
void indicates that the method will not return anything.
58. What will happen if an exception is not caught ?
An uncaught exception results in the uncaughtException() method of the thread's ThreadGroup, which results in
the termination of the program.
59. What are the different ways in which a thread can enter into waiting state?
There are three ways for a thread to enter into 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.
60. What is a ResourceBundle class?
The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to create the
program's appearance to the particular locale in which it is being run.
61. What is numeric promotion?
Numeric promotion is the conversion of a smaller numeric type to a larger numeric type. In numerical promotion,
byte, char, and short values are converted to int values. The int, long and float values are converted to the desired
types if required.
62. What is the difference between the prefix and postfix forms of the ++ operator?
The prefix form first performs the increment operation and then returns the value of the increment operation. The
postfix form first returns the current value of the expression and then performs the increment operation on that
value.
63. What are synchronized methods and synchronized statements?
Synchronized methods are methods that are declared with the keyword synchronized. A thread executes a
synchronized method only after it has acquired the lock for the method's object or class. Synchronized statements
are similar to synchronized methods. It is a block of code declared with synchronized keyword.
A synchronized statement can be executed only after a thread has acquired the lock for the object or class
referenced in the synchronized statement.
64. How can we create a thread?
thread can be created by extending Thread class or by implementing Runnable interface. Then we need to override
the method public void run().
65. What is the difference between a switch statement and an if statement?
