[JAVA] Easy Interview question

1. What do you understand by Java?

• Java is an object-oriented computer language.
• It is a high-level programming language developed by James Gosling in Sun Microsystems in the year 1995.
• Java is a fast, secure, and reliable language used for many games, devices, and applications.

2. Compare between Java and Python.

Criteria	Java	Python
Ease of use	Good	Excellent
Speed of coding	Average	Excellent
Data types	Static typed	Dynamically typed
Data Science and Machine Learning applications	Average	Excellent

3. Outline the major Java features.

The major features of Java programming language are explained below:

• Object-oriented: Java is based on object-oriented programming where the class and methods describe the state and behavior of an object.
• Portable: A Java program gets converted into Java bytecodes that can be executed on any platform without any dependency.
• Platform independent: Java works on the ‘write once, run anywhere’ principle as it supports multiple platforms like Windows, Linux, Mac, Sun Solaris, etc.
• Robust: Java has strong memory management as there are no pointer allocations. It has an automatic garbage collection that prohibits memory leaks.
• Interpreted: As mentioned, Java compiler converts the codes into Java bytecodes which are then interpreted and executed by Java Interpreter.

 

4. What do you mean by an object?

An object consists of methods and classes that depict its state and perform operations. A Java program contains a lot of objects instructing each other their jobs. This concept is part of core Java.

5. Distinguish between StringBuffer and StringBuilder in Java programming.

StringBuffer	StringBuilder
StringBuffer methods are synchronized.	StringBuilder is non-synchronized.
The storage area is heap and modified easily.	Storage is heap and can be modified.
StringBuffer is thread-safe.	StringBuilder is fast as it is not thread-safe.
The performance is very slow.	The performance is very fast.

 

6. Differentiate between JDK, JRE, and JVM.

• JVM stands for Java Virtual Machine which provides the runtime environment for Java bytecodes to be executed.
• JRE (Java Runtime Environment) includes the sets of files required by JVM during runtime.
• JDK (Java Development Kit) consists of JRE along with the development tools required to write and execute a program.

7. Define inheritance.

Java includes the feature of inheritance which is an object-oriented programming concept. Inheritance lets a derived class inherit the methods of a base class.

8. Explain method overloading.

When a Java program contains more than one method with the same name but with different properties, then it is called method overloading.

9. Compare overloading with overriding.

Overloading refers to the case of having two methods of the same name but different properties; whereas, overriding occurs when there are two methods of the same name and properties, but one is in the child class and the other is in the parent class.

 

10. Explain the creation of a thread-safe singleton in Java using double-checked locking.

Singleton is created with the double-checked locking as before Java 5 acts as a broker and it’s been possible to have multiple instances of singleton when multiple threads create an instance of the singleton at the same time. Java 5 made it easy to create thread-safe singleton using Enum. Using a volatile variable is essential for the same.

11. What is a class in Java?

Java encapsulates codes in various classes that define new data types. These new data types are used to create objects.

12. Differentiate between an ArrayList and a Vector.

ArrayList	Vector
An ArrayList is not synchronized.	A vector is synchronized.
An ArrayList is fast.	A vector is slow as it is thread-safe.
If an element is inserted into an ArrayList, it increases its array size by 50 percent.	A vector defaults to doubling the size of its array.
An ArrayList does not define the increment size.	A vector defines the increment size.
An ArrayList can only use Iterator for traversing.	Except for hashtable, a vector is the only other class that uses both Enumeration and Iterator.

13. Mention the difference between Iterator and Enumeration.

Iterator	Enumeration
Iterator is an interface found in the java.util package.	Enumeration is an object that generates elements one at a time.
Uses three methods to interface: hasNext() next() remove()	Uses two methods: hasMoreElements() nextElement()
Iterators allow removing elements from the given collection during the iteration with well-defined semantics.	It is used for passing through a collection, usually of unknown size.
Iterator method names have been improved.	The traversing of elements can only be done once per creation.

14. Explain the difference between the inner class and the subclass.

Inner Class	Subclass
An inner class is a class that is nested within another class.	A subclass is a class that inherits from another class called the superclass.
It provides the access rights for the class which is nesting it and that can access all variables and methods defined in the outer class.	It provides access to all public and protected methods and fields of its superclass.

15. Can we execute any code, even before the main method? Explain.

Yes, we can execute any code, even before the main method. We will be using a static block of code in the class when creating the objects at load time of the class. Any statements within this static block of code will get executed at once while loading the class, even before the creation of objects in the main method.

 

16. How can we restrict inheritance for a class?

We can restrict inheritance for a class by the following steps:

1. By using the final keyword
2. If we make all methods final, then we cannot override that
3. By using private constructors
4. By using the Javadoc comment (” // “)

17. Java doesn't support multiple inheritance. Why?

Java doesn’t support multiple inheritance because we cannot use different methods in one class; it creates an ambiguity.
Example:

class Intellipaat1 { void test() { system.out.println("test() method"); } }class Intellipaat2 { void test() { system.out.println("test() method"); } }Multiple inheritance class C extends Intellipaat1, Intellipaat2 { /* Code */ }

Intellipaat1 and Intellipaat2 test() methods are inheriting to class C. So, which test() method class C will take?

As Intellipaat1 and Intellipaat2 class test () methods are different, here we would face ambiguity.

18. Are constructors inherited? Can a subclass call the parent's class constructor?

We cannot inherit a constructor. We create an instance of a subclass using a constructor of one of its superclasses. Because overriding the superclass constructor is not our wish as if we override a superclass constructor, then we will destroy the encapsulation abilities of the language.

19. Define JSON.

The expansion of JSON is ‘JavaScript Object Notation.’ It is a much lighter and readable alternative to XML. It is independent and easily parse-able in all programming languages. It is primarily used for client–server and server–server communication.

20. What are the advantages of JSON over XML?

The advantages of JSON over XML are:

1.  JSON is lighter and faster than XML.
2.  It is easily understandable.
3.  It is easy to parse and convert to objects for information consumption.
4. JSON supports multiple data types—string, number, array, or Boolean—but XML data are all strings.

 

21. Differentiate between this() and super() in Java.

this()	super()
Represents the present instance of a class	Represents the current instance of the parent class
Calls the default constructor	Calls the base class constructor
Used to point to the current class instance	Used to point to the instance of the superclass

22. Name the methods of an object class.

• clone(): This method helps create and return a copy of an object.
• equals(): This method helps compare.
• finalize(): It is called by the garbage collector on the object.
• getClass(): It allows us to return the runtime class of the object.
• hashCode(): This method helps return a hash code value for the object.
• toString(): It lets us return a string representation of the object.
• notify(), notifyAll(), and wait(): These help in synchronizing the activities of the independently running threads in a program.

23. Define content negotiation.

If we have visited a website to search for information, we will get the information in different languages and in different formats. When a client makes an HTTP request to a server, the client can also specify the media types here. The client can specify what it can accept back from the host, and on the basis of availability the host will return to the client. This is known as content negotiation because the client and the server negotiate on the language and format of the content to be shared.

24. Can we import the same package/class twice? Will the JVM load the package twice at runtime?

A package or class can be inherited multiple times in a program code. JVM and compiler will not create any issue. Moreover, JVM automatically loads the class internally once, regardless of times it is called in the program.

25. Define an abstract class.

A class that contains the abstract keyword in its declaration is known as an abstract class. It can have abstract and non-abstract methods (method with a body).

1.  This class can have public, private, protected, or constants and default variables.
2.  It needs to be extended and its method needs to be implemented. It cannot be instantiated.
3.  If a class has at least one abstract method, then the class must be declared abstract.

26. Describe annotations.

• Java annotation is a tag that symbolizes the metadata associated with class, interface, methods, fields, etc.
• Annotations do not directly influence operations.
• The additional information carried by annotations is utilized by Java compiler and JVM.

27. Java doesn't use pointers. Why?

Pointers are susceptible and slightly carelessness in their use which may result in memory problems, and hence Java basically manages their use.

28. Distinguish between static loading and dynamic class loading.

• • Static loading: In static loading, classes are loaded statically with the operator ‘new.’
  • Dynamic class loading: It is a technique for programmatically invoking the functions of a class loader at runtime. The syntax for this is as follows:

Class.forName (Test className);

29. Struts 1 classes are not thread-safe, whereas Struts 2 classes are thread-safe. Why?

Struts 1 actions are singleton. So, all threads operate on the single action object and hence make it thread-unsafe.

Struts 2 actions are not singleton, and a new action object copy is created each time a new action request is made and hence it is thread-safe.

30. Define JAXP and JAXB.

JAXP: It stands for Java API for XML Processing. This provides a common interface for creating and using DOM, SAX, and XSLT APIs in Java regardless of which vendor’s implementation is actually being used.

JAXB: It stands for Java API for XML Binding. This standard defines a system for a script out of Java objects as XML and for creating Java objects from XML structures.

 

31. Define an enumeration?

Usually, we call enumeration as an enum. An enumeration is an interface containing methods for accessing the original data structure from which the enumeration is obtained. It allows sequential access to all the elements stored in the collection.

32. How can we find the actual size of an object on the heap?

In Java, there is no way to find out the actual size of an object on the heap.

33. Which API is provided by Java for operations on a set of objects?

Java provides a Collection API that provides many useful methods, which can be applied to a set of objects. Some of the important classes provided by the Collection API include ArrayList, HashMap, TreeSet, and TreeMap.

34. What’s the base class of all exception classes?

Java.Lang.throwable: It is the superclass of all exception classes, and all exception classes are derived from this base class.

35. Why do we use a vector class?

A vector class provides the ability to execute a growable array of objects. A vector proves to be very useful if you don’t know the size of the array in advance or if we need one that can change the size over the lifetime of a program.

36. What is the difference between transient and volatile variables in Java?

Transient: In Java, it is used to specify whether a variable is not being serialized. Serialization is a process of saving an object’s state in Java. When we want to persist the object’s state by default, all instance variables in the object are stored. In some cases, we want to avoid persisting a few variables because we don’t have the necessity to transfer across the network. So, we declare those variables as transient.

If the variable is confirmed as transient, then it will not be persisted. The transient keyword is used with the instance variable that will not participate in the serialization process. We cannot use static with a transient variable as they are part of the instance variable.

Volatile: The volatile keyword is used with only one variable in Java, and it guarantees that the value of the volatile variable will always be read from the main memory and not from the thread’s local cache; it can be static.

37. Why Map interface does not extend the Collection interface in the Java Collections Framework?

The Map interface is not compatible with the Collection interface, because Map requires a key as well as a value, for example, if we want to add a key–value pair, we will use put(Object key, Object value).

There are two parameters required to add an element to HashMap object. In Collection interface, add(Object o) has only one parameter.

The other reasons are: Map supports valueSet, keySet, and other suitable methods that have just different views from the Collection interface.

38. Mention the uses of the synchronized block.

We use the synchronized block because:

•  It helps lock an object for every shared resource.
•  The scope of the synchronized block is smaller than the method.

39. What are the functions of hashCode() method?

The hashCode() method returns a hash code value (an integer number) and also the same integer number if the two keys (by calling the equals() method) are the same.
But sometimes, the two hash code numbers can have different or the same keys.

 

40. What is the default size of the load factor in the hashing-based collection?

Default size = 0.75

Default capacity = initial capacity * load factor

41. What are the differences between the JSP custom tags and Java beans?

• Custom tags can manipulate JSP content, but beans cannot.
• Composite operations can be reduced to a considerably simpler form with custom tags than with beans.
• Custom tags require reasonably a bit more work to set up than beans do.
• Custom tags are available only in JSP 1.1 and so on, but beans can be used in all JSP 1.x versions.

 

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