129 Core Java Interview Questions and Answers
Java is a platform independent language.
Platform independence means that we can write and compile the java code in one platform (eg Windows) and can execute the class in any other supported platform eg (Linux,Solaris,etc).
JVM’s are not platform independent. JVM’s are platform specific run time implementation provided by the vendor.
JVM is Java Virtual Machine which is a run time environment for the compiled java class files.
JDK is Java Development Kit which is for development purpose and it includes execution environment also. But JVM is purely a run time environment and hence you will not be able to compile your source files using a JVM.
Pointer is a reference handle to a memory location. Improper handling of pointers leads to memory leaks and reliability issues hence Java doesn’t support the usage of pointers.
java.lang.Object
Java doesn’t support multiple inheritance.
Java uses primitive data types and hence is not a pure object oriented language.
In Java, Arrays are objects.
Path and Classpath are operating system level environment variales. Path is used define where the system can find the executables(.exe) files and classpath is used to specify the location .class files.
Local varaiables are those which are declared within a block of code like methods. Local variables should be initialised before accessing them.
Instance variables are those which are defined at the class level. Instance variables need not be initialized before using them as they are automatically initialized to their default values.
The variable should be declared as static and final. So only one copy of the variable exists for all instances of the class and the value can’t be changed also.
static final int PI = 2.14; is an example for constant.
No not required. main method should be defined only if the source class is a java application.
Main method doesn’t return anything hence declared void.
main method is called by the JVM even before the instantiation of the class hence it is declared as static.
main method accepts an array of String object as arguement.
Yes. You can have any number of main methods with different method signature and implementation in the class.
Yes. Any inheriting class will not be able to have it’s own default main method.
No it doesn’t matter but void should always come before main().
Yes a single source file can contain any number of Class declarations but only one of the class can be declared as public.
Package is a collection of related classes and interfaces. package declaration should be first statement in a java class.
java.lang package is imported by default even without a package declaration.
Not possible.
A class can’t be declared as protected. only methods can be declared as protected.
A protected method can be accessed by the classes within the same package or by the subclasses of the class in any package.
A final variable’s value can’t be changed. final variables should be initialized before using them.
A method declared as final can’t be overridden. A sub-class can’t have the same method signature with a different implementation.
You should declared your class as final. But you can’t define your class as final, if it is an abstract class. A class declared as final can’t be extended by any other class.
java.lang.String,java.lang.Math are final classes.
final is a modifier which can be applied to a class or a method or a variable. final class can’t be inherited, final method can’t be overridden and final variable can’t be changed.
finally is an exception handling code section which gets executed whether an exception is raised or not by the try block code segment.
finalize() is a method of Object class which will be executed by the JVM just before garbage collecting object to give a final chance for resource releasing activity.
No a class cannot be defined as static. Only a method,a variable or a block of code can be declared as static.
When a method needs to be accessed even before the creation of the object of the class then we should declare the method as static.
A static method should not refer to instance variables without creating an instance and cannot use “this” operator to refer the instance.
Print the statement inside a static block of code. Static blocks get executed when the class gets loaded into the memory and even before the creation of an object. Hence it will be executed before the main method. And it will be executed only once.
static variables are class level variables where all objects of the class refer to the same variable. If one object changes the value then the change gets reflected in all the objects.
Static varaibles are class level variables and they can’t be declared inside a method. If declared, the class will not compile.
A Class which doesn’t provide complete implementation is defined as an abstract class. Abstract classes enforce abstraction.
Not possible. An abstract class without being inherited is of no use and hence will result in compile time error.
Variables can’t be declared as abstract. only classes and methods can be declared as abstract.
Not possible. Abstract classes can’t be instantiated.
Yes it’s possible. This is basically to avoid instance creation of the class.
No not possible. Class C should provide implementation for all the methods in the Interface I. Since Class C didn’t provide implementation for m1 method, it has to be declared as abstract. Abstract classes can’t be instantiated.
No not possible. Doing so will result in compilation error. public and abstract are the only applicable modifiers for method declaration in an interface.
Intefaces doesn’t provide implementation hence a interface cannot implement another interface.
Yes an Interface can inherit another Interface, for that matter an Interface can extend more than one Interface.
Not possible. A Class can extend only one class but can implement any number of Interfaces.
Basically Java doesn’t allow multiple inheritance, so a Class is restricted to extend only one Class. But an Interface is a pure abstraction model and doesn’t have inheritance hierarchy like classes(do remember that the base class of all classes is Object). So an Interface is allowed to extend more than one Interface.
Not possible. Doing so so will result in compilation error.
Yes it’s possible.
Yes it’s possible.
An Interface which doesn’t have any declaration inside but still enforces a mechanism.
Polymorphism.
No it doesn’t. There should be a change in method arguements for a method to be overloaded.
Operator overloading makes the code very difficult to read and maintain. To maintain code simplicity, Java doesn’t support operator overloading.
No
Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)
Only public and abstract modifiers are allowed for methods in interfaces.
Variables declared within a method are “local” variables. Variables declared within the class i.e not within any methods are “member” variables (global variables). Variables declared within the class i.e not within any methods and are defined as “static” are class variables
An abstract method is a method whose implementation is deferred to a subclass.
The read() method returns -1 when it has reached the end of a file.
No, an object cannot be cast to a primitive value.
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.
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.
It is referred to as the modulo or remainder operator. It returns the remainder of dividing the first operand by the second operand.
An object reference be cast to an interface reference when the object implements the referenced interface.
The Object class is extended by all other classes.
The non-Unicode letter characters $ and _ may appear as the first character of an identifier
Two methods may not have the same name and argument list but different return types.
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.
void.
A private variable may only be accessed within the class in which it is declared.
These are accessibility modifiers. Private is the most restrictive, while public is the least restrictive. There is no real difference between protected and the default type (also known as package protected) within the context of the same package, however the protected keyword allows visibility to a derived class in a different package.
Downcasting is the casting from a general to a more specific type, i.e. casting down the hierarchy
A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.
Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns.
A package statement must appear as the first line in a source code file (excluding blank lines and comments).
A native method is a method that is implemented in a language other than Java.
Order of precedence determines the order in which operators are evaluated in expressions. Associatity determines whether an expression is evaluated left-to-right or right-to-left
An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.
The range of the char type is 0 to 2^16 – 1.
The range of the short type is -(2^15) to 2^15 – 1.
Because C++ has proven by example that operator overloading makes code almost impossible to maintain.
Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class. Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That’s how library methods like System.out.println() work. out is a static field in the java.lang.System class.
The null value is not a keyword.
The digits 0 through 9 may not be used as the first character of an identifier but they may be used after the first character of an identifier.
It is written x ? y : z.
The fractional part of the result is truncated. This is known as rounding toward zero.
A class that is declared without any access modifiers is said to have package access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package.
A class does not inherit constructors from any of its superclasses.
The eight primitive types are byte, char, short, int, long, float, double, and boolean.
During compilation, the values of each case of a switch statement must evaluate to a value that can be promoted to an int value.
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.
A local inner class may be final or abstract.
The compiler supplies a default constructor for a class if no other constructors are provided.
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.
The left operand is an object reference or null value and the right operand is a class, interface, or array type.
The values true and false are not keywords.
The result is a String object.
When a class is defined within a scope od another class, then it becomes inner class. If the access modifier of the inner class is static, then it becomes nested class.
An abstract class may not be declared as final
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
A public class may be accessed outside of its package. A non-public class may not be accessed outside of its package.
The default value of the boolean type is false
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.
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 thrown by the overridden method.
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.
A top-level class may be public, abstract, or final.
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 executed.
It makes no difference in the generated class files since only the classes that are actually used are referenced by the generated class file. There is another practical benefit to importing single classes, and this arises when two (or more) packages have classes with the same name. Take java.util.Timer and javax.swing.Timer, for example. If I import java.util.* and javax.swing.* and then try to use “Timer”, I get an error while compiling (the class name is ambiguous between both packages). Let’s say what you really wanted was the javax.swing.Timer class, and the only classes you plan on using in java.util are Collection and HashMap. In this case, some people will prefer to import java.util.Collection and import java.util.HashMap instead of importing java.util.*. This will now allow them to use Timer, Collection, HashMap, and other javax.swing classes without using fully qualified class names in.
No, because the methods can be called without using their return type in which case there is ambiquity for the compiler
Can’t do it. You’ll get a compilation error
As many as you want, but the static initializers and class variable initializers are executed in textual order and may not refer to class variables declared in the class whose declarations appear textually after the use, even though these class variables are in scope.
Overriding is a method with the same name and arguments as in a parent, whereas overloading is the same method name but different arguments
A child object constructor always first needs to construct its parent (which in turn calls its parent constructor.). In Java it is done via an implicit call to the no-args constructor as the first statement.
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.
The = operator is right associative.
Yes, a double value can be cast to a byte.
A break statement results in the 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.
Yes, a for statement can loop indefinitely. For example, consider the following: for(;;) ;
The default value of an String type is null.
What is the difference between a field variable and a local variable?
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.
this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.
A final class cannot be inherited. A final method cannot be overridden in a subclass. A final field cannot be changed after it’s initialized, and it must include an initializer statement where it’s declared.
An abstract class cannot be instantiated. 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 it also should be declared abstract.
transient variable is a variable that may not be serialized.
It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.
The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.
The sizeof operator is not a keyword.
