Any aspiring Java programmer will need to have the fundamentals of the language well ingrained into their memory. After conducting many interviews for Java based software engineering candidates, I have decided to compile a list of the most commonly asked questions. These are in no particular order and have answers provided. Memorization and full understanding of these questions will greatly increase your chances of landing your next engineering position!
What is Java?
Java is a object oriented platform independent programming language that runs in billions of devices worldwide.
What are the features of Java?
- Java is platform independent, meaning it can be written and ran on any device or platform.
- It is object oriented which promotes code reuse and modularity.
- Java provides robust memory management and exception handling.
- It supports multi threading, allowing for concurrent execution of more than one thread.
- It is also high performance and provides just-in-time compilation.
What is the Java Virtual Machine (JVM)?
The JVM is an abstract computing machine that allows any device to run java programs by converting byte code into machine specific code that can run on any platform.
What is the difference between JDK, JRE, and JVM?
- The Java Development Kit (JDK) is a full featured software development kit required to develop java applications. It includes the JRE, an interpreter/loader, a compiler and other tools needed for Java development.
- The Java Runtime Environment (JRE) provides libraries, a Java Virtual Machine, Java class libraries and other components necessary to run Java applications.
- The Java Virtual Machine (JVM) interprets compile bytecode and converts it into machine code that can be executed on a host system. The JVM runs Java applications, the JRE provides a runtime environment and the JDK provides the tools needed to develop Java applications.
Explain the concept of the “platform independence” of Java.
Platform independence is the ability of a Java program to be executed on various types of devices, operating systems, and CPU architectures. This is achieved by providing a JVM that interprets bytecode and translates it to machine specific instructions.
What are the main principles of Object-Oriented Programming (OOP)?
The main principles of Object-Oriented programming are encapsulation, inheritance, polymorphism and abstraction.
What are classes and objects in Java?
A class is a blueprint or template for creating objects. It defines a datatype that bundles data and methods that operate on the data. An object is an instance of a class and defines the actual state of the class in memory.
What is inheritance in Java?
Inheritance is the ability of a subclass (or child class) to acquire the properties and behaviors of an existing class, known as the superclass (or parent class). It promotes code reuse and establishes a hierarchical relationship between classes.
What is polymorphism in Java?
Polymorphism is the ability of a single class to reference multiple underlying types. This allows you to write code code that operates on objects at a more abstract level, without needing to know their concrete types. Polymorphism is achieved through method overloading and method overriding. It enhances code flexibility and maintainability.
What is encapsulation in Java?
Encapsulation is the bundling of data and methods that operate on that data into a single unit known as a class. This unit is an object that provides a way to control access to its internal state (data). Encapsulation promotes data integrity, modular design, code reusability, and abstraction by hiding the internal details of the class and providing an interface to interact with objects.
What is abstraction in Java?
Abstraction is a concept that involves hiding complex implementation details and representing only the essential features of an application. It allows developers to focus on what an object does rather than how it does it. Abstraction is implemented using abstract classes and interfaces.
What are the different access modifiers in Java?
The four access modifiers of Java are public, protected, default, and private.
- Public access modifiers are the least restrictive allowing methods and variables to be accessed by any external class.
- Protected permits access within the same package and subclass.
- Private restricts access to the defining class only.
- Default access modifiers allow access to classes that exist in the same package.
What is the difference between a constructor and a method?
A constructor is a special type of methods used to initialize and instantiate objects. It has the same name as the class and no return type. A method performs a specific operation, has its own name, and can return a value. Constructors are called when an object is created, while methods are called to perform actions on objects.
What is method overloading?
Method overloading allows one class to have more than one method with the same name as long as the number or type of parameters are different. Method overloading is a type of compile time polymorphism.
What is method overriding?
Method overriding is the ability of a subclass to provide its own implementation of a method already defined in a super class. The overridden method in the subclass must have the same name and signature of the one existing in the superclass. Method overriding is an example of runtime polymorphism.
What is the super keyword in Java?
The super keyword allows a subclass to refer to its superclass. It is used to access a superclasses methods and constuctors.
What is the this keyword in Java?
The this keyword is a reference to the current object instance of a class. It is used to distinguish between instance variables and parameters having the same name.
What is the static modifier in Java?
- The
staticmodifier in Java is used to associate a method or variable with a class rather than with a specific instance of that class. It enables shared access to the method or variable among all instances of the class, and it can also be accessed directly using the class name.
What are restrictions on static methods?
- Static methods cannot access instance level data, only static variables.
- Static methods cannot be overridden.
- Static methods can only call other static methods.
What is stored on Heap and Stack memory in Java?
- Heap: All instantiated objects are stored on the heap in java.
- Stack: All reference variables, methods, and local variables are stored on the stack.
What is an interface in Java?
An interface is a programming construct that defines a contract or set of method signatures that a class must adhere to if it implements that interface. It serves as a blueprint for a class, specifying a set of behaviors or methods a class can implements.
What benefit do interfaces provide?
- Interfaces allow for the abstraction of behavior promoting code modularity and a separation of concerns. They enable polymorphism by allowing objects of different classes to implement the same interface and be treated interchangeably, enhancing code flexibility and reusability.
- Interfaces support multiple inheritance by implementing more than one interface. This adds flexibility by allowing developers to design complex class hierarchies and reuse code effectively.
- Interfaces serve as a contract that a certain set of methods will be available to a class. This ensures classes conform to a particular structure and is vital for maintaining large codebases and facilitating collaboration among developers.
What is the final modifier used for in Java?
The final keyword in Java is used to define an entity that cannot be changed once it is initialized. It can be applied to variables, methods, and classes. When used with variables, it makes the variable a constant, meaning its value cannot be modified once assigned. For methods, it prevents them from being overridden in subclasses. When applied to a class, it makes the class non-extendable, meaning no other class can inherit from it. This keyword is crucial for creating immutable objects and ensuring consistent behavior in inheritance hierarchies.
What is the difference between an abstract class and a interface?
An interface in Java is a reference type that can contain only abstract methods (until Java 8, which introduced default and static methods) and final variables (constants). Interfaces are used to define a contract that implementing classes must follow, promoting different types of inheritable behavior since a class can implement multiple interfaces. In contrast, an abstract class can have both abstract and concrete methods, as well as instance variables. Abstract classes are used when classes share a common base but also have differences that must be defined in subclasses. A class can extend only one abstract class, enforcing a single inheritance model. This distinction allows interfaces to provide a more flexible and decoupled design while abstract classes offer a more structured and hierarchically organized framework.
What is the difference between == operator and the equals() method?
The == operator is used to compare primitive java types or references for equality. Two objects can have their memory addresses compared using this operator. The equals() method is used to compare the contents of two objects. It may be overridden to determine whether two classes are equal based on specific fields. This distinction is crucial for correctly comparing objects based on their content rather than their memory addresses.
Summary
Study these questions thoroughly to be well prepared for your next technical interview. Knowledge of this information will not only increase your chances of landing a job, but will also provide a insight into how applications are working under the hood.