Core Java Theory Part 1

web development, php, java

Core Java Theory Part 1

Have a look into the most important questions asked in interviews on Core Java Theory.

Object-Oriented Programming (OOP)

Java is a computer programming language that is concurrent, class-based, and object-oriented. The advantages of object-oriented software development are shown below:

• Modular development of code, which leads to easy maintenance and modification.

• Reusability of code.

• Improved reliability and flexibility of code.

• Increased understanding of code.

Object-oriented programming contains many significant features, such as encapsulation, inheritance, polymorphism and abstraction. We analyze each feature separately in the following sections.

1.1 Encapsulation

Encapsulation provides objects with the ability to hide their internal characteristics and behaviour. Each object provides a number of methods, which can be accessed by other objects and change their internal data. In Java, there are three access modifiers: public, private and protected. Each modifier imposes different access rights to other classes, either in the same or in external packages. Some of the advantages of using encapsulation are listed below:

• The internal state of every objected is protected by hiding its attributes.

• It increases usability and maintenance of code because the behaviour of an object can be independently changed or extended.

• It improves modularity by preventing objects to interact with each other, in an undesired way.

1.2 Polymorphism

It is the ability of programming languages to present the same interface for differing underlying data types. A polymorphic type is a type whose operations can also be applied to values of some other type.

1.3 Inheritance

It provides an object with the ability to acquire the fields and methods of another class, called a base class. Inheritance provides re-usability of code and can be used to add additional features to an existing class, without modifying it.

1.4 Abstraction

It is the process of separating ideas from specific instances and thus, develop classes in terms of their own functionality, instead of their implementation details. Java supports the creation and existence of abstract classes that expose interfaces, without including the actual implementation of all methods. The abstraction technique aims to separate the implementation details of a class from its behavior.

1.5 Differences between Abstraction and Encapsulation

Abstraction and encapsulation are complementary concepts. On the one hand, abstraction focuses on the behaviour of an object. On the other hand, encapsulation focuses on the implementation of an object’s behaviour. Encapsulation is usually achieved by hiding information about the internal state of an object and thus, can be seen as a strategy used in order to provide abstraction.

General Questions about Java

2.1 Explain is JVM? Why is Java called the Platform Independent Programming Language?

A Java virtual machine (JVM) is a process virtual machine that can execute Java bytecode. Each Java source file is compiled into a bytecode file, which is executed by the JVM. Java was designed to allow application programs to be built that could be run on any platform, without having to be rewritten or recompiled by the programmer for each separate platform. A Java virtual machine makes this possible because it is aware of the specific instruction lengths and other particularities of the underlying hardware platform.

2.2 State Difference between JDK and JRE?

The Java Runtime Environment (JRE) is basically the Java Virtual Machine (JVM) where your Java programs are being executed. It also includes browser plugins for applet execution. The Java Development Kit (JDK) is the full-featured Software Development Kit for Java, including the JRE, the compilers and tools (like JavaDoc, and Java Debugger), in order for a user to develop, compile and execute Java applications.

2.3 What does the “static” keyword mean? Can you override private or static method in Java?

The static keyword denotes that a member variable or method can be accessed, without requiring an instantiation of the class to which it belongs. A user cannot override static methods in Java, because method overriding is based upon dynamic binding at runtime and static methods are statically binded at compile time. A static method is not associated with any instance of a class so the concept is not applicable.

2.4 Can you access a non-static variable in static context?

A static variable in Java belongs to its class and its value remains the same for all its instances. A static variable is initialized when the class is loaded by the JVM. If your code tries to access a non-static variable, without any instance, the compiler will complain, because those variables are not created yet and they are not associated with any instance.

2.5 What are the Data Types supported by Java? What are Autoboxing and Unboxing?

The eight primitive data types supported by the Java programming language are:

• byte

• short

• int

• long

• float

• double

• boolean

• char

Autoboxing is the automatic conversion made by the Java compiler between the primitive types and their corresponding object wrapper classes. For example, the compiler converts an int to an Integer, a double to a Double, and so on. If the conversion goes the other way, this operation is called unboxing.

2.6 Explain Function Overriding and Overloading in Java?

Method overloading in Java occurs when two or more methods in the same class have the exact same name, but different parameters. On the other hand, method overriding is defined as the case when a child class redefines the same method as a parent class. 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.

2.7 What is a Constructor, Constructor Overloading in Java and Copy-Constructor.

A constructor gets invoked when a new object is created. Every class has a constructor. In case the programmer does not provide a constructor for a class, the Java compiler (Javac) creates a default constructor for that class. The constructor overloading is similar to method overloading in Java. Different constructors can be created for a single class. Each constructor must have its own unique parameter list. Finally, Java does support copy constructors like C++, but the difference lies in the fact that Java doesn’t create a default copy constructor if you don’t write your own.

2.8 Does Java support multiple inheritances?

No, Java does not support multiple inheritances. Each class is able to extend only in one class but is able to implement more than one interfaces.

2.9 What is the difference between an Interface and an Abstract class?

Java provides and supports the creation of both abstract classes and interfaces. Both implementations share some common characteristics, but they differ in the following features:

 All methods in an interface are implicitly abstract. On the other hand, an abstract class may contain both abstract and non-abstract methods.

  • A class may implement a number of Interfaces but can extend only one abstract class.
  • In order for a class to implement an interface, it must implement all its declared methods. However, a class may not implement all declared methods of an abstract class. Though, in this case, the subclass must also be declared as abstract.
  • Abstract classes can implement interfaces without even providing the implementation of interface methods.
  • Variables declared in a Java interface is by default final. An abstract class may contain non-final variables.
  • Members of a Java interface are public by default. A member of an abstract class can either be private, protected or public.
  • An interface is absolutely abstract and cannot be instantiated. An abstract class also cannot be instantiated but can be invoked if it contains the main method.

2.10 Explain pass by reference and pass by value?

When an object is passed by value, this means that a copy of the object is passed. Thus, even if changes are made to that object, it doesn’t affect the original value. When an object is passed by reference, this means that the actual object is not passed, rather a reference of the object is passed. Thus, any changes made by the external method, are also reflected in all places.

Java Threads

3.1 What is the difference between processes and threads?

A process is an execution of a program, while a Thread is a single execution sequence within a process. A process can contain multiple threads. A Thread is sometimes called a lightweight process.

3.2 State the different ways of creating a thread. Which one would you prefer and why?

There are three ways that can be used in order for a Thread to be created:

• A class may extend the Thread class.
• A class may implement the Runnable interface.
• An application can use the Executor framework, in order to create a thread pool.

The Runnable interface is preferred, as it does not require an object to inherit the Thread class. In case your application design requires multiple inheritance, only interfaces can help you. Also, the thread pool is very efficient and can be implemented and used very easily.

3.3 Explain the available thread states in a high-level.

During its execution, a thread can reside in one of the following states:

• Runnable: A thread becomes ready to run, but does not necessarily start running immediately.

• Running: The processor is actively executing the thread code.

• Waiting: A thread is in a blocked state waiting for some external processing to finish.

• Sleeping: The thread is forced to sleep.

• Blocked on I/O: Waiting for an I/O operation to complete.

• Blocked on Synchronization: Waiting to acquire a lock.

• Dead: The thread has finished its execution.

3.4 What is the difference between a synchronized method and a synchronized block?


In Java programming, each object has a lock. A thread can acquire the lock for an object by using the synchronized keyword. The synchronized keyword can be applied in a method level (coarse-grained lock) or block level of code (fine-grained lock).

3.5 How does thread synchronization occurs inside a monitor? What levels of synchronization can you apply?

The JVM uses locks in conjunction with monitors. A monitor is basically a guardian that watches over a sequence of synchronized code and ensuring that only one thread at a time executes a synchronized piece of code. Each monitor is associated with an object reference. The thread is not allowed to execute the code until it obtains the lock.

3.6 What’s a deadlock?

A condition that occurs when two processes are waiting for each other to complete, before proceeding. The result is that both processes wait endlessly.

3.7 How do you ensure that N threads can access N resources without deadlock?

A very simple way to avoid deadlock while using N threads is to impose an ordering on the locks and force each thread to follow that ordering. Thus, if all threads lock and unlock the mutexes in the same order, no deadlocks can arise.

Check Core Java Theory Part 2

Wikipedia Java

3 thoughts on “Core Java Theory Part 1”

Leave a Comment

Your email address will not be published.