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Core Java Interview Questions and Answers

Ques 301. What is keyword in Core Java?

Ans. A keyword is a word with a predefined meaning in Java programming language syntax. Reserved for Java, keywords may not be used as identifiers for naming variables, classes, methods or other entities.


There are 50 reserved keywords in the Java programming language.

abstract
The abstract keyword is used to declare a class or method to be abstract. An abstract method has no implementation; all classes containing abstract methods must themselves be abstract, although not all abstract classes have abstract methods. Objects of a class which is abstract cannot be instantiated, but can be extended by other classes. All subclasses of an abstract class must either provide implementations for all abstract methods, or must also be abstract.
assert
The assert keyword, which was added in J2SE 1.4 is used to make an assertion—a statement which the programmer believes is always true at that point in the program. If assertions are enabled when the program is run and it turns out that an assertion is false, an AssertionError is thrown and the program terminates. This keyword is intended to aid in debugging.
boolean
The boolean keyword is used to declare a variable that can store a boolean value; that is, either true or false. This keyword is also used to declare that a method returns a value of the primitive type boolean.
break
Used to resume program execution at the statement immediately following the current enclosing block or statement. If followed by a label, the program resumes execution at the statement immediately following the enclosing labeled statement or block.
byte
The byte keyword is used to declare a variable that can store an 8-bit signed two's complement integer. This keyword is also used to declare that a method returns a value of the primitive type byte.
case
The case keyword is used to create individual cases in a switch statement; see switch.
catch
Defines an exception handler—a group of statements that are executed if an exception is thrown in the block defined by a preceding try keyword. The code is executed only if the class of the thrown exception is assignment compatible with the exception class declared by the catch clause.
char
The char keyword is used to declare a variable that can store a 16-bit Unicode character. This keyword is also used to declare that a method returns a value of the primitive type char.
class
A type that defines the implementation of a particular kind of object. A class definition defines instance and class fields, methods, and inner classes as well as specifying the interfaces the class implements and the immediate superclass of the class. If the superclass is not explicitly specified, the superclass is implicitly Object.
const
Although reserved as a keyword in Java, const is not used and has no function. For defining constants in java, see the 'final' reserved word.
continue
Used to resume program execution at the end of the current loop body. If followed by a label, continue resumes execution at the end of the enclosing labeled loop body.
default
The default keyword can optionally be used in a switch statement to label a block of statements to be executed if no case matches the specified value; see switch. Alternatively, the default keyword can also be used to declare default values in a Java annotation.
do
The do keyword is used in conjunction with while to create a do-while loop, which executes a block of statements associated with the loop and then tests a boolean expression associated with the while. If the expression evaluates to true, the block is executed again; this continues until the expression evaluates to false.
double
The double keyword is used to declare a variable that can hold a 64-bit double precision IEEE 754 floating-point number. This keyword is also used to declare that a method returns a value of the primitive type double.
else
The else keyword is used in conjunction with if to create an if-else statement, which tests a boolean expression; if the expression evaluates to true, the block of statements associated with the if are evaluated; if it evaluates to false, the block of statements associated with the else are evaluated.
enum (as of J2SE 5.0)
A Java keyword used to declare an enumerated type. Enumerations extend the base class Enum.
extends
Used in a class declaration to specify the superclass; used in an interface declaration to specify one or more superinterfaces. Class X extends class Y to add functionality, either by adding fields or methods to class Y, or by overriding methods of class Y. An interface Z extends one or more interfaces by adding methods. Class X is said to be a subclass of class Y; Interface Z is said to be a subinterface of the interfaces it extends.
Also used to specify an upper bound on a type parameter in Generics.
final
Define an entity once that cannot be changed nor derived from later. More specifically: a final class cannot be subclassed, a final method cannot be overridden, and a final variable can occur at most once as a left-hand expression. All methods in a final class are implicitly final.
finally
Used to define a block of statements for a block defined previously by the try keyword. The finally block is executed after execution exits the try block and any associated catch clauses regardless of whether an exception was thrown or caught, or execution left method in the middle of the try or catch blocks using the return keyword.
float
The float keyword is used to declare a variable that can hold a 32-bit single precision IEEE 754 floating-point number. This keyword is also used to declare that a method returns a value of the primitive type float.
for
The for keyword is used to create a for loop, which specifies a variable initialization, a boolean expression, and an incrementation. The variable initialization is performed first, and then the boolean expression is evaluated. If the expression evaluates to true, the block of statements associated with the loop are executed, and then the incrementation is performed. The boolean expression is then evaluated again; this continues until the expression evaluates to false.
As of J2SE 5.0, the for keyword can also be used to create a so-called "enhanced for loop", which specifies an array or Iterable object; each iteration of the loop executes the associated block of statements using a different element in the array or Iterable.
goto
Although reserved as a keyword in Java, goto is not used and has no function.
if
The if keyword is used to create an if statement, which tests a boolean expression; if the expression evaluates to true, the block of statements associated with the if statement is executed. This keyword can also be used to create an if-else statement; see else.
implements
Included in a class declaration to specify one or more interfaces that are implemented by the current class. A class inherits the types and abstract methods declared by the interfaces.
import
Used at the beginning of a source file to specify classes or entire Java packages to be referred to later without including their package names in the reference. Since J2SE 5.0, import statements can import static members of a class.
instanceof
A binary operator that takes an object reference as its first operand and a class or interface as its second operand and produces a boolean result. The instanceof operator evaluates to true if and only if the runtime type of the object is assignment compatible with the class or interface.
int
The int keyword is used to declare a variable that can hold a 32-bit signed two's complement integer. This keyword is also used to declare that a method returns a value of the primitive type int.
interface
Used to declare a special type of class that only contains abstract methods, constant (static final) fields and static interfaces. It can later be implemented by classes that declare the interface with the implements keyword.
long
The long keyword is used to declare a variable that can hold a 64-bit signed two's complement integer. This keyword is also used to declare that a method returns a value of the primitive type long.
native
Used in method declarations to specify that the method is not implemented in the same Java source file, but rather in another language.
new
Used to create an instance of a class or array object.
package
A group of types. Packages are declared with the package keyword.
private
The private keyword is used in the declaration of a method, field, or inner class; private members can only be accessed by other members of their own class.
protected
The protected keyword is used in the declaration of a method, field, or inner class; protected members can only be accessed by members of their own class, that class's subclasses or classes from the same package.
public
The public keyword is used in the declaration of a class, method, or field; public classes, methods, and fields can be accessed by the members of anyclass.
return
Used to finish the execution of a method. It can be followed by a value required by the method definition that is returned to the caller.
short
The short keyword is used to declare a field that can hold a 16-bit signed two's complement integer. This keyword is also used to declare that a method returns a value of the primitive type short.
static
Used to declare a field, method, or inner class as a class field. Classes maintain one copy of class fields regardless of how many instances exist of that class. static also is used to define a method as a class method. Class methods are bound to the class instead of to a specific instance, and can only operate on class fields. (Classes and interfaces declared as static members of another class or interface are actually top-level classes and are not inner classes.)
strictfp (as of J2SE 1.2)
A Java keyword used to restrict the precision and rounding of floating point calculations to ensure portability.
super
Used to access members of a class inherited by the class in which it appears. Allows a subclass to access overridden methods and hidden members of its superclass. The super keyword is also used to forward a call from a constructor to a constructor in the superclass.
Also used to specify a lower bound on a type parameter in Generics.
switch
The switch keyword is used in conjunction with case and default to create a switch statement, which evaluates a variable, matches its value to a specific case, and executes the block of statements associated with that case. If no case matches the value, the optional block labelled by default is executed, if included.
synchronized
Used in the declaration of a method or code block to acquire the mutex lock for an object while the current thread executes the code. For static methods, the object locked is the class' Class. Guarantees that at most one thread at a time operating on the same object executes that code. The mutex lock is automatically released when execution exits the synchronized code. Fields, classes and interfaces cannot be declared as synchronized.
this
Used to represent an instance of the class in which it appears. this can be used to access class members and as a reference to the current instance. The this keyword is also used to forward a call from one constructor in a class to another constructor in the same class.
throw
Causes the declared exception instance to be thrown. This causes execution to continue with the first enclosing exception handler declared by the catch keyword to handle an assignment compatible exception type. If no such exception handler is found in the current method, then the method returns and the process is repeated in the calling method. If no exception handler is found in any method call on the stack, then the exception is passed to the thread's uncaught exception handler.
throws
Used in method declarations to specify which exceptions are not handled within the method but rather passed to the next higher level of the program. All uncaught exceptions in a method that are not instances of RuntimeException must be declared using the throws keyword.
transient
Declares that an instance field is not part of the default serialized form of an object. When an object is serialized, only the values of its non-transient instance fields are included in the default serial representation. When an object is deserialized, transient fields are initialized only to their default value. If the default form is not used, e.g. when a serialPersistentFields table is declared in the class hierarchy, all transient keywords are ignored.
try
Defines a block of statements that have exception handling. If an exception is thrown inside the try block, an optional catch block can handle declared exception types. Also, an optional finally block can be declared that will be executed when execution exits the try block and catch clauses, regardless of whether an exception is thrown or not. A try block must have at least one catch clause or a finally block.
void
The void keyword is used to declare that a method does not return any value.
volatile
Used in field declarations to specify that the variable is modified asynchronously by concurrently running threads. Methods, classes and interfaces thus cannot be declared volatile, nor can local variables or parameters.
while
The while keyword is used to create a while loop, which tests a boolean expression and executes the block of statements associated with the loop if the expression evaluates to true; this continues until the expression evaluates to false. This keyword can also be used to create a do-while loop.

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Ques 302. What is identifier in java?
Ans. An identifier is a sequence of one or more characters. The first character must be a valid first character (letter, $_) in an identifier of the Java programming language. Each subsequent character in the sequence must be a valid nonfirst character (letter, digit, $_) in a Java identifier.
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Ques 303. What is Checked Exception and its use in java?
Ans.
Checked Exception in Java is all those Exception which requires being catches and handled during compile time. If Compiler doesn’t see try or catch block handling a Checked Exception, it throws Compilation error. All the Exception which are direct sub Class of Exception but not inherit RuntimeException are Checked Exception.

Use of Checked Exception:
1) All Operation where chances of failure is more e.g. IO Operation, Database Access or Networking operation can be handled with Checked Exception.
2) When you know what to do (i.e. you have alternative) when an Exception occurs, may be as part of Business Process.
3) Checked Exception is a reminder by compiler to programmer to handle failure scenario.

Example of checked Exception in Java API
Following are some Examples of Checked Exception in Java library:
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Ques 304. What is Unchecked Exception in java?
Ans.
Unchecked Exception in Java is those Exceptions whose handling is not verified during Compile time. Unchecked Exceptions mostly arise due to programming errors like accessing method of a null object, accessing element outside an array bonding or invoking method with illegal arguments. In Java, Unchecked Exception is direct sub Class of RuntimeException. What is major benefit of Unchecked Exception is that it doesn't reduce code readability and keeps the client code clean.

Use of UnCheckedException in Java
A good strategy of Exception handling in Java is wrapping a checked Exception into UnCheckedException. Since most of Database operation throws SQLException but it’s not good to let SQLException propagate from your DAO layer to up higher on business layer and client code provide exception handling you can handle SQLException in DAO layer and you can wrap the cause in a RuntimeException to propagate through client code. Also as I said earlier unchecked exceptions are mostly programming errors and to catch them is real hard until you do a load test with all possible input and scenario.

Example of unchecked Exception in Java API
Here are few examples of Unchecked Exception in Java library:
NullPointerException
ArrayIndexOutOfBound
IllegalArgumentException
IllegalStateException
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Ques 305. What is Anonymous (inner) Class in java?
Ans.
An anonymous inner class can come useful when making an instance of an object which certain "extras" such as overloading methods, without having to actually subclass a class.

I tend to use it as a shortcut for attaching an event listener:
button.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e)
    {
        // do something.
    }
});
Using this method makes coding a little bit quicker, as I don't need to make an extra class that implements ActionListener - I can just instantiate an anonymous inner class without actually making a separate class.

I only use this technique for "quick and dirty" tasks where making an entire class feels unnecessary. Having multiple anonymous inner classes that do exactly the same thing should be refactored to an actual class, be it an inner class or a separate class.
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