Java Interview Preparation

Internship

Analytical thinking

• Logical questions

Programming

• Anything around this topic depending on the candidate's education
• Basic Programming
  • What is a program? What structure does it have?
  • What is a variable?
  • What is a procedure/method?

Junior

Analytical thinking

• Logical questions
• Algorithms
  • Sort an array

OOP Principles

• Encapsulation
• Inheritance
• Abstraction
• Polymorphism
• overridding vs overloading

Java Fundamentals

• Main class
• Class Structure (Constructor, fields, methods)
• Access & Non-Access modifiers
• Statements (if, switch, for, while, do-while)
• Types (Primitives, Wrappers)

Strings

• what is a String in java
• immutability (== vs equals)
• String pool

Exceptions

• What is an Exception?
• Difference an Exception and an Error
• Checked vs Unchecked
  • RuntimeException
  • Exception
• Most common predefined exceptions: e.g. NullPointerException, FileNotFoundException
• try-catch-finally block
• throw new exception
• Method signature with throw exception
• Correct handling (higher in the application structure)
• Correct wrapping (don't lose the root cause in case of technical exceptions)
• Well-defined exception hierarchy (starting from base -> splitting in two: technical and business -> ending in concrete exception classes)

Collections

• Lists
  • ArrayList vs LinkedList
• Sets
  • TreeSet vs HashSet
• Maps
  • TreeMap vs HashMap
• equals() & hashCode()

SQL

Database & Table Structure

• Define a database
• Define a table with different column types (integer -> varchar -> blob -> clob)
• Define a primare key
• Define an index

Relationships

• Define Foreign Keys
• One-to-many
• Many-to-one
• Many-to-many

Queries

• WHERE clause

Update & Delete

• by using WHERE clause

Drop table

Frameworks

• Basic Knowledge about Spring

Tools

• Git

  • How does git work?
    • Remote & local repo, and commit-based tracking
  • What is a commit?
  • Have you ever used a branching strategy? why is this useful?
  • What are the main operations in git?
    • clone ? pull ? commit ? push ?
  • Have you ever used stash? (nice to have)

• Maven

  • What is maven used for?
    • Build tool, dependency management, ...
  • Do you know the default lifecycle in maven?
    • clean -> compile -> install (search for the complete answer)
  • How many repos are used by maven? And when?
    • central, local & private (if one defined)

• Jenkins (Nice to know)

  • What is jenkins used for?
    • Scheduling/Triggering build jobs
    • Useful for CI/CD
    • Automating builds/deployments

Higher Seniority Level

Java I/O

• What is a Stream?
• Difference between an InputStream and a Reader
• Difference between an OutputStream and a Writer
• Types of Streams
• Paths
• File Operations

Threads

• Thread vs Runnable
• Starting a Thread
• Syncing a thread
• synchronized blocks
• Synchronized collections
• What is a race condition?
• What is a deadlock?

Best practices & Design Patterns

• Naming Conventions
• SOLID principles
• Choose composition over inheritance
• Understand each type of relationship between classes and when to use them:
  • Inheritance
  • Aggregation
  • Asociation
  • Composition

Design Patterns

• any developer MUST know about the most of the design patterns and be able to use a few of them
• mandatory design patterns:
  • Singleton
  • Builder
  • Factory method
  • Strategy

SQL

• Normalization (Splitting tables in multiple tables with relations between them)

Queries

• inner, outer, left & right join
• order by
• group by

Frameworks & Concepts

• IoC & DI: Spring Framework
  • How does Spring IoC Container work?
  • What is a bean?
  • Difference between @Component, @Controller, @Service & @Repository
  • What is a @Configuration class and when should we use it?
    • What about @Bean annotation?
  • How do we inject an object in Spring?
  • What is application.yml file used for?
  • How does the main class look like in Spring Boot app?
    • What about the project structure?
• Persistance (JPA)
  • CrudRepository & JpaRepository
  • ORM: @Entity & @Table & @Column & @Id
• Security
  • Authentication & Authorization
  • Ways of authentication
    • Form-based
    • Basic Auth
    • Digest Auth (Nice to have)
    • OAuth2 (Nice to have)

Containerisation

• Docker
• Container orchestration
  • Docker Compose
  • Kubernetes
  • Openshift
  • Cloudfoundry

Senior

Cloud Solutions

• AWS
• Azure
• Google Cloud Platform
• Private Cloud

Transactions

• ACID
• Rollback/Commit