Java Related

Question 1

Why is Java not truly OOP?

Answer 1

Java language is not a Pure Object Oriented Language as it contain these properties:

1. Java is not fully object oriented because it supports primitive data type like it,byte,long etc.,which are not objects.
2. The static keyword: When we declares a class as static then it can be used without the use of an object in Java. If we are using static function or static variable then we can’t call that function or variable by using dot(.) or class object defying object oriented feature.

Question 2

How do you create an immutable class?

Answer 2

• The class must be declared as final so that child classes can’t be created.
• Data members in the class must be declared private so that direct access is not allowed.
• Data members in the class must be declared as final so that we can’t change the value of it after object creation. (-Make all fields final and private.)
• Don’t provide “setter” methods — methods that modify fields or objects referred to by fields.

Question 3

What is Synchronization in java?

Question 4

Hashset adn hasmap difference

Question 5

Overloading or Overriding which one is runtime polymorphism.

Answer 5

Overloading is used in compile-time polymorphism. Overriding is implemented in runtime polymorphism.

Question 6

What is Functional interface

Answer 6

A functional interface is an interface that contains only one abstract method. A functional interface can have any number of default methods.

Question 7

What is a marker interface? give examples

Answer 7

A marker interface is an interface that has no methods or constants inside it. It provides run-time type information about objects.

E.g. Serializable, Cloneable, Remote etc

Question 8

What is ADT

Answer 8

class of objects whose logical behavior is defined by a set of values and a set of operations. ADT only mentions what operations are to be performed but not how these operations will be implemented.

Question 9

What are the different types of memory areas allocated by JVM?

Answer 9

In java, JVM allocates memory to different processes, methods and
objects. Some of the memory areas allocated by JVM are:
1. ClassLoader: It is a component of JVM used to load class
files.
2. Class (Method) Area: It stores per-class structures such as
the runtime constant pool, field and method data, and the
code for methods.
3. Heap: Heap is created a runtime and it contains the runtime
data area in which objects are allocated.
4. Stack: Stack stores local variables and partial results at
runtime. It also helps in method invocation and return
value. Each thread creates a private JVM stack at the time
of thread creation.
5. Program Counter Register: This memory area contains the
address of the Java virtual machine instruction that is
currently being executed.
6. Native Method Stack: This area is reserved for all the
native methods used in the application.

Question 10

What is JIT compiler?

Answer 10

591

A JIT compiler runs after the program has started and compiles the code (usually bytecode or some kind of VM instructions) on the fly (or just-in-time, as it’s called) into a form that’s usually faster, typically the host CPU’s native instruction set. A JIT has access to dynamic runtime information whereas a standard compiler doesn’t and can make better optimizations like inlining functions that are used frequently.

Question 11

Who changes java code to bytecode

Answer 11

Javac

Question 12

What is classLoader in java

Answer 12

ClassLoader has a memory space allocated by jVM. Class loaders are responsible for loading Java classes dynamically to the JVM (Java Virtual Machine) during runtime. They’re also part of the JRE (Java Runtime Environment). Therefore, the JVM doesn’t need to know about the underlying files or file systems in order to run Java programs thanks to class loaders.

Question 13

What is the difference between

byte and char data types in Java?

Answer 13

Both byte and char are numeric data types in Java. They are used to
represent numbers in a specific range.
Major difference between them is that a byte can store raw binary
data where as a char stores characters or text data.
Usage of char is E.g. char ch = ‘x’;
Byte values range from -128 to 127.
A byte is made of 8 bits. But a char is made of 16 bits. So it is
equivalent to 2 bytes.

Question 14

What are the main principles of

Object Oriented Programming?

Answer 14

Main principles of Object Oriented Programming (OOPS) are:

1. Abstraction
2. Encapsulation
3. Inheritance
4. Polymorphism

Question 15

What is the difference between Object Oriented Programming

language and Object Based Programming language?

Answer 15

Object Oriented Programming languages like Java and C++ follow
concepts of OOPS like- Encapsulation, Abstraction, Polymorphism
and Inheritance etc.
Object Based Programming languages follow some features of
OOPS but they do not provide support for Polymorphism and
Inheritance. Egg. JavaScript, VBScript etc

Question 16

Can we inherit a Constructor?

Answer 16

No, Java does not support inheritance of constructor.

Question 17

Why constructors cannot be final

Answer 17

If we set a method as final it means we do not want any class to
override it. But the constructor (as per Java Language
Specification) cannot be overridden. So there is no use of marking it
final.

Question 18

If there are no pointers in Java, then why do we get NullPointerException?

Answer 18

In Java, the pointer equivalent is Object reference. When we use a .
it points to object reference. So JVM uses pointers but
programmers only see object references.

In case an object reference points to null object, and we try to
access a method or member variable on it, then we get
NullPointerException.

Question 19

Is it possible to use this() and super() both in same constructor?

Answer 19

No, Java does not allow using both super() and this() in same
constructor. As per Java specification, super() or this() must be the
first statement in a constructor.

Question 20

What is the meaning of object

cloning in Java

Answer 20

Object.clone() method is used for creating an exact copy of the
object in Java. It acts like a copy constructor. It creates and returns
a copy of the object, with the same class and with all the fields
having same values as of the original object.

One disadvantage of cloning is that the return type is an Object. It
has to be explicitly cast to actual type.

Question 21

CloneNotSupportedException

Answer 21

Thrown to indicate that the clone method in class Object has been called to clone an object, but that the object’s class does not implement the Cloneable

Question 22

Why do we mark main method as

static in Java?

Answer 22

The main method in Java is marked as static, so that JVM can call it
to start the program. If main method is not static, then which
constructor will be called by Java process?

Question 23

In what scenario do we use a static

block?

Answer 23

At times, there is a class that has static member variables. These
variables need some complicated initialization. At this time static
block helps as a tool to initialize complex static member variable
initialization.

static {
    Map map = new HashMap()
    map.put("AEN", "Alfred E. Newman");
    // etc.
    initials = Collections.unmodifiableMap(map);
}

Question 24

How will you implement method

overloading in Java?

Answer 24

1. Different number of parameters
2. Different data type of parameters
3. Different sequence of data type of parameters

Question 25

What kinds of argument variations are allowed in Method Overloading?

Answer 25

Number of parameters Data type of parameters Sequence of data type of paramet

Question 26

Is it allowed to overload main() | method in Java?

Answer 26

Yes, Java allows users to create many methods with same name ‘main’. But only public static void main(String[] args) method is used for execution

Question 27

virtual function???

Answer 27

A virtual function is a member function in the base class that we expect to redefine in derived classes.

Question 28

covariant return type

Answer 28

Covariant return, means that when one overrides a method, the return type of the overriding method is allowed to be a subtype of the overridden method's return type.

Question 29

Types of annotation creation in Java?

Answer 29

1. Class level @Target(ElementType.Type) 2. field level @Target(ElementType.Field) 3, method level @Target(ElementType.Method)

Question 30

What is a blank final variable in Java? How can we initialize a blank final variable?

Answer 30

When we declare a final variable without giving any initial value, then it is called blank final variable. A blank final instance variable can be initialized in a constructor. A blank final static variable can be initialized in the static block of class

Question 31

What is the purpose of package in | Java?

Answer 31

A package is used to encapsulate a group of classes, interfaces and sub-packages. Often, it is a hierarchical structure of storing information. It is easier to organize the related classes and subpackages in this manner. A Package also provides access protection for classes and interfaces. A package also helps in removing naming collision

Question 32

What is java.lang package?

Answer 32

``` In Java, java.lang package contains the classes that are fundamental to the design of Java programming language. The most important class in this package is Object class. It also contains wrapper classes like- Integer, Boolean, Character etc. It provides Math class for mathematical operations. ```

Question 33

Is it mandatory to import java.lang | package every

Answer 33

No. By default, JVM loads it internally

Question 34

What is Locale in Java?

Answer 34

A Locale object represents a specific geographical, political, or cultural region. It is used to locale-sensitive operations in Java.

Question 35

What is the serialization?

Answer 35

Serialization is a process converting an object into a byte array. This byte array represents the class, version and internal state of the object. JVM can use this byte array to transmit/read the object over a network.

Question 36

What is the purpose of | serialization?

Answer 36

1. Communication: It is used for transmitting an object over network between two machines. 2. Persistence: We can store the object’s state in a database and retrieve it from database later on. 3. Caching: Serialization can be used for caching to improve performance. We may need 10 minutes to build an object, but it may take just 10 seconds to de-serialize the object. 4. Cross JVM Synchronization: It can be used in same way across multiple JVM that follow different architecture.

Question 37

Externalizable interface ???

Answer 37

Externalizable is an interface that enables you to define custom rules and your own mechanism for serialization.

Question 38

What is the difference between Serializable and Externalizable interface?

Answer 38

Serializable is a marker interface but Externalizable is not a marker interface.

Question 39

What is Reflection in Java?

Answer 39

``` Reflection is Java language's ability to inspect and dynamically call classes, methods, attributes etc. at Runtime. It helps in examining or modifying the Runtime behavior of a class at Runtime. ``` Used for testing, IDE, spring

Question 40

How can we create an Object | dynamically at Runtime in Java?

Answer 40

``` We can use Reflection to create an Object dynamically at Runtime in Java. We can use Class.newInstance() or Constructor.newInstance() methods for creating such Objects. ```

Question 41

What is the purpose of gc() in | Java?

Answer 41

Java provides two methods System.gc() and Runtime.gc() to request the JVM to run the garbage collection. By using these methods, programmers can explicitly send request for Garbage Collection. But JVM process can reject this request and wait for some time before running the GC

Question 42

What are the different types of | References in Java?

Answer 42

In Java, there are four types of references: 1. Strong Reference 2. Soft Reference 3. Weak Reference 4. Phantom Reference

Question 43

What is the purpose of the | Runtime class?

Answer 43

``` The purpose of the Runtime class is to provide access to the Java Runtime system. ``` 1. Runtime.freeMemory() – This method returns the value of free memory in JVM 2. Runtime.maxMemory() - This method returns the value of maximum memory that JVM can use. 3. Runtime.gc() – This method can invoke garbage collection.

Question 44

How many types of Nested classes | are in Java?

Answer 44

Java provides four types of Nested classes: 1. Member inner class 2. Local inner class 3. Anonymous inner class 4. Static nested class

Question 45

``` What is the string constant pool? Which one creates a string object in the pool? String s = "s"; String s = new String("s); ```

Answer 45

A string constant pool is a separate place in the heap memory where the values of all the literal strings which are defined in the program are stored. String s = "s"; creates a object in the pool.

Question 46

String s = new String ("test"); String x = new String ("test"); Why is s == x false?

Answer 46

The comparison here is refering to an address of two different string Strings created with the new keyword.

Question 47

What is String interning?

Answer 47

String interning refers to the concept of using only one copy of a distinct String value that is Immutable. It provides the advantage of making String processing efficient in Time as well as Space complexity. But it introduces extra time in creation of String.

Question 48

What is the basic difference between a String and StringBuffer object?

Answer 48

String is an immutable object. Its value cannot change after creation. StringBuffer is a mutable object. We can keep appending or modifying the contents of a StringBuffer in Java.

Question 49

StringBuffer and StringBuilder, which is thread safe?

Question 50

In Java, what are the differences | between a Checked and Unchecked?

Answer 50

Checked Exceptions extend Throwable class, but they do not extend RuntimeException or Error classes. UncheckedException extend RuntimeException class. Checked Exceptions are checked at compile time in Java. Unchecked Exceptions happen at Runtime, so they are not checked at compile time. IOException, SQLException etc. are examples of Checked Exceptions. NullPointerException, ArithmeticException etc. are examples of Unchecked Exceptions.

Question 51

``` What is the base class for Error and Exception classes in Java? ```

Answer 51

``` Error as well as Exception class is derived from Throwable class in Java. ```

Question 52

In what scenarios, a finally block | will not be executed?

Answer 52

There are two main scenarios in which finally block is not executed: 1. Program exits by calling system.exit() call. 2. A fatal error causes JVM to crash.

Question 53

What are the differences between Pre-emptive Scheduling | Scheduler and Time Slicing Scheduler?

Answer 53

In Pre-emptive scheduling, the highest priority task will keep getting time to execute until it goes to waiting state or dead state or a task with higher priority comes into queue for scheduling. In Time slicing scheduling, every task gets a predefined slice of time for execution, and then it goes to the pool of tasks ready for execution. The scheduler picks up the next task for execution, based on priority and various other factors.

Question 54

What does the statick {} does

Answer 54

Answer