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Java & Spring Interview Questions [Most Asked]

This compilation covers critical interview topics across Core Java, Collections, Hibernate, and the Spring Framework as discussed in the Code Decode tutorial.

1. ArrayList vs. LinkedList​

FeatureArrayListLinkedList
Internal Data StructureResizable ArrayDoubly Linked List
ManipulationSlow (requires shifting elements)Fast (only pointer changes)
Search/AccessFast (Random Access)Slow (Sequential traversal)
Best Use CaseStoring and accessing dataFrequent insertion/deletion

2. Lazy Loading in Hibernate​

Lazy loading is a design pattern used to postpone the initialization of an object until it is actually needed.

  • Purpose: It improves performance by avoiding the retrieval of large amounts of data from the database if that data isn't being used.
  • Example: If you fetch an Employee object, Hibernate won't fetch their Address until you explicitly call employee.getAddress().

3. Hibernate Caching: First Level vs. Second Level​

  • First Level Cache: Enabled by default. It is associated with the Session object. Data is available only within the scope of a single session.
  • Second Level Cache: Must be enabled explicitly (e.g., using EHCache). It is associated with the SessionFactory. Data is available across all sessions in the application.

4. JVM Garbage Collection Generations​

JVM memory is divided into different generations to optimize Garbage Collection (GC):

  • Young Generation: Where new objects are created. Most objects die here quickly. It is further divided into Eden space, Survivor S0, and Survivor S1.
  • Old (Tenured) Generation: Objects that survive several GC cycles in the Young Generation are moved here.
  • Permanent Generation (Metaspace in Java 8+): Stores metadata about classes and methods.

5. What is Serialization?​

Serialization is the process of converting an object's state into a byte stream, so it can be saved to a file or sent over a network. The reverse process is called Deserialization. To make a class serializable, it must implement the java.io.Serializable marker interface.

6. Spring IoC and Dependency Injection (DI)​

  • Inversion of Control (IoC): A principle where the control of object creation and lifecycle is transferred from the developer to the Spring Container.
  • Dependency Injection (DI): The pattern used to implement IoC. Instead of an object creating its dependencies, the container "injects" them (via Constructor, Setter, or Field injection).

7. What is WeakHashMap?​

A WeakHashMap is a specialized Map where keys are stored as Weak References. If a key is no longer in use elsewhere in the program, the Garbage Collector can discard it, even if it is still a key in the WeakHashMap. This is useful for preventing memory leaks in cache implementations.

8. Functional Interfaces (Java 8)​

A Functional Interface is an interface that contains exactly one abstract method.

  • It can contain any number of default or static methods.
  • It is marked with the @FunctionalInterface annotation.
  • These interfaces are used as the assignment target for Lambda Expressions.

9. ConcurrentHashMap vs. SynchronizedMap​

  • SynchronizedMap: Locks the entire map for both read and write operations.
  • ConcurrentHashMap: Uses Lock Stripping. It allows multiple threads to read concurrently and locks only specific segments (buckets) for write operations, making it much faster in multi-threaded environments.

Source: Code Decode - Java Interview Questions for Fresher & Experienced