Java Interview Questions

Question 1

Which memory areas does the JVM provide and what’s the difference between them?

Answer 1

Heap Area: represents the runtime data, where arrays and classes are allocated. It is created on the startup, doesn’t need to be continuous.

Question 2

Where a primitive is stored in memory?

Answer 2

Stack area

Question 3

How can you override a static method?

Answer 3

They can’t be overriden

Question 4

Difference between StringBuffer and StringBuilder?

Answer 4

StringBuffer is not thread safe

Question 5

What is JIT for?

Answer 5

JIT gets a block of code that is executed frequently and compiles it to native machine language to speed up process

Question 6

Where are complex objects stored?

Answer 6

The heap

Question 7

What do the stack store?

Answer 7

local variables like int,float

Question 8

What is Metaspace? What types of variables is stored in the metaspace?

Answer 8

Metaspace is only for Java 8+.
static primitives are stored in metaspace.
static objects have the reference stored in the metaspace

Question 9

Where is the String Pool stored?

Answer 9

In the Heap

Question 10

When is an object eligible for GC?

Answer 10

any elements on the heap that are not reachable

Question 11

Where is the Young and Old gen stored?

Answer 11

The heap

Question 12

What’s the difference between Young and Old generation?

Answer 12

The Young gen. is way smaller. That’s why the GC on that part of the memory only takes a fraction of second. If it survives, it gets moved to the Old Gen.
GC on the Old gen are way slower, but they happen less frequently

Question 13

What are the survivor spaces in the Young gen?

Answer 13

Eden: When it gets full, the GC happens

S0/S1: When GC happens, objects get moved from one to another, either one them will be empty

Question 14

Explain JDK, JRE, JVM

Answer 14

JDK: Java development kit, it is the tool necessary to compile, document and package Java Programs. It contains JRE + development tools

JRE: Java Runtime Environment, refers to a runtime env. in which Java bytecode can be executed. It’s an implementation of JVM which physically exists.

JVM: Java virtual Machine, it’s an abstract machine. It is a specification that provides a runtime environment in which java bytecode can be executed

Question 15

What is the difference between ArrayList and Vector?

Answer 15

ArrayList is not syncronized.

Question 16

What is the difference between Stack and Heap?

Answer 16

Memory: Stack is only used for a single thread execution. Heap is used by all parts of the application

Access: stack memory can’t be accessed by other threads. Heap memory are globally accessed

Memory management: Stack Follows LIFO manner to free memory. Heap is based on the generation associated with each object

Lifetime: Stack, until the end of execution of a thread. Heap memory lives from the start of the application till the end.

Usage: Stack only contains local primitives and reference variables to objects on the heap.
Heap, whenever an object is created, it’s stored in the heap.

Question 17

What are the main concepts of OOP?

Answer 17

Inheritance, Encapsulation, Abstraction and Polymorphism

Question 18

What is the difference between compilation time polymorphism and runtime?

Answer 18

Compilation time polymorphism would be method overloading, where Runtime polymorphism is done by interfaces and abstractions

Question 19

In Java, what is association?

Answer 19

Relationship where objects have their own lifecycle and there is no owner. For example, teacher and students

Question 20

What do you mean by aggregation in java?

Answer 20

Is a specialized form of association where all objects have their own lifecycle but there is an owner and a child object cannot belong to another parent object. Ex, Department and Teacher, a single teacher cannot belong to multiple departments, but if you remove the department, you don’t have to remove the the teacher

Question 21

What is a composition in Java?

Answer 21

Is a specialized form of Aggregation and we can call this “death” relationship. Is a strong type of aggregation, child objects does not have their own lifecycle, and if the parent is removed, the child are as well. For ex. Houses and rooms

Question 22

What are the different types of GC in Java?

Answer 22

• Parallel
• Serial
• CMS
• G1 GC

Question 23

Is java Pass-by-reference or Pass-by-value?

Answer 23

Java is always pass-by-value.
However, when we pass the value of an object, we are actually passing its REFERENCE. Thats why its confusing.

Lets say you have:

Dog myDog;

myDog is not the OBJECT, it’s the pointer(reference)