Java Interview Questions

Front 1

Advantages of Java

Back 1

Unique advantage of being write once, and run almost anywhere.

Advantage of being platform independent, because it compiles to bytecode which can run in any platform that has a JVM.

Hint 1

There are two main advantages of Java.

Front 2

Difference between JDK, JRE, & JVM.

Back 2

Java Development Kit has everything you need to develop AND run a Java program.

Java Runtime Environment has everything you need to RUN the program.

Java Virtual Machine is what you compiles bytecode runs on.

Hint 2

They all either run a, or run on something. Know the difference.

Front 3

Difference between class & object

Back 3

Classes are blueprints for objects.

Hint 3

Classes are the essential.

Front 4

Is Java pass by reference or value, and what does pass by value mean?

Back 4

Java is pass by value. Meaning, when you pass an int x to doStuff (int y), you will pass the actual value of x, not set y = x.

Hint 4

It's more direct.

Front 5

What are primitive types in Java?

Back 5

byte, int, short, long, char, boolean, float, double

Hint 5

8 types

Front 6

Types of data types in Java.

Back 6

Primitive data types, and non-Primitive/ class reference data types.

Hint 6

So easy a caveman could understand it.

Front 7

Difference between == & .equals () in Java

Back 7

== is an operator used to compare the content of primitive values. It is not recommended to compare object content because it will compare the object location in memory.

.equals () is a method used to compare 2 objects (such as strings).

Hint 7

They are both used to compare things.

Front 8

How are reference variable different from primitive variables?

Back 8

Primitive variables are data types, reference variables are pointers.

Hint 8

One does something the other refers something.

Front 9

Have you used any version control systems, and which ones have you used?

Back 9

Git. A component of software configuration management, version control AKA revision control or source control, is the management of changes to documents, computer programs, large web sites, and other collections of information.

Hint 9

Git it.

Front 10

What is Git?

Back 10

Version control system for tracking changes in computer files and coordinating work on those files among multiple people.

As a distributed revision control system it is aimed at speed, data integrity, and support for distributed, non-linear workflows.

Front 11

Types of variables in Java

Back 11

Primitive variables and reference variables

Front 12

Explain immutability

Back 12

An object that cannot be changed/modified after it has been created.

Front 13

Why are Strings immutable in Java?

Back 13

Security. If strings weren't immutable a connection or file would be changed leading to serious security threats. Mutable Strings could also lead to security problems too, as the parameters are Strings. Synchronization and concurrency makes Strings automatically thread safe. Caching, compiler optimizes string objects, needing only one string object. Class loading, string is used as arguments for class loading, if mutable it could result in wrong class being loaded.

Front 14

What is .this keyword in Java?

Back 14

A keyword differentiating the local variable from an instance variable of the same name.

Front 15

What is string tokenizer?

Back 15

The string tokenizer can break strings into different sections.

Front 16

Difference between extends and implements

Back 16

Extends is used for extending classes to superclasses. Implements is used to extend an interface.

Front 17

What is autoboxing?

Back 17

When the compiler automatically converts a primitive variable to its corresponding class version when needed (to us certain class specific methods).

Front 18

What are wrapper objects?

Back 18

Wrapper classes are used to convert primitives into objects.

Front 19

Explain super keyword in Java.

Back 19

A keyword to call a subclasses parent class.

Front 20

Method overloading vs. Overriding

Back 20

Overriding - when extending from a superclass, a subclass can override it's parents method with custom implementation, this happens at runtime.

Overloading - when a class has multiple methods with the same name but different arrangements/number of parameters. This allows you to pass multiple different arrangments/ number of parameters to the same method and still run it. This happens at compile time.

Front 21

Abstract class vs. Interface and when would you use it?

Back 21

They are functionally the same thing.

Interface - when each subclass has unique implementation of the methods.

Abstract class - when you have mostly common behavior between subclasses

Front 22

What does static mean?

Back 22

Static things are stateless, they do not change for different instances.

Front 23

What is static block?

Back 23

A block if code that is excited before any other line of code, including the main method. Static blocks are executed first to last.

Front 24

What is finally block?

Back 24

In exception handling of code, a finally block of code is code that will run if an exception was caught or not.

Front 25

Difference between error and exception.

Back 25

An error cannot be recovered from, an exception can be. An error would be something like memory running out. An exception would be that a file that is requested cannot be found.

Front 26

What is recursion?

Back 26

Iteration will typically use Loops to solve a problem, while a recursive program will call itself again and again until the condition is satisfied.

Front 27

What is dynamic binding?

Back 27

Binding which happens at runtime is called Dynamic binding. For example, method overriding is considered Dynamic binding because at runtime it would run the overwritten method.

Front 28

Can an interface be instantiated in Java?

Back 28

No.

Front 29

Is multiple inheritance allowed in Java?

Back 29

No.

Front 30

Explain Diamond problem and how does Java solve it?

Back 30

The diamond problem is when a class extends from 2 parent classes that both extend from another class. In the case that both mediary classes have different implementation of an inherited method. The lowest subclass would not know which method to perform when calling it.

There is no multiple inheritance in Java i e a class can't extend more then one class, yeah there is a concept of interface. A class can implement multiple interfaces, but whenever an interface is implemented it is mandatory for a class to always Implement all its methods. Thus the class will have definition of all the parent methods which avoids problems like deadly diamond in Java.

Front 31

What is synchronized keyword and why is it used?

Back 31

In the case of multi-threading in Java, the synchronized keyword ensures that the same method can I be called by two threads at the same time.

Front 32

Difference between public, private, and protected access modifiers.

Back 32

Public - Anyone can access.

Default - within package.

Protected - within package and also subclasses in other packages.

Private - only accessible within the class.

Front 33

Checked vs unchecked exceptions

Back 33

Checked exceptions are caught by the compiler, unchecked or not. Unchecked exceptions can still be dealt with.

Front 34

Throws keyword in Java.

Back 34

The throws keyword informs the compiler that the method may throw an exception.

Front 35

What is encapsulation?

Back 35

Encapsulation in Java is a mechanism of wrapping the data (variables) and code acting on the data (methods) together as a single unit. In encapsulation, the variables of a class will be hidden from other classes, and can be accessed only through the methods of their current class. Therefore, it is also known as data hiding.

Front 36

What is serializable in Java?

Back 36

Simply speaking, serialization is a process of converting an object into stream of bytes so that it can be transferred over a network or stored in a persistent storage.