Top 10 Java Interview Questions Every Fresher Should Know
Top 10 Java Interview Questions Every Fresher Should Know Work through these 10 questions out loud. Open any question for the full model answer, the trap to avoid, and the follow-up a panel usually asks.
Explain immutability and why String is immutable.
Immutable objects cannot change after build — thread-safe without synchronization. String immutability allows intern pool, caching hashCode, and security when used as...
Explain garbage collection basics in the JVM.
New objects allocate in Eden; minor GC copies survivors to survivor spaces, then old gen. When old gen fills, major/full GC runs — longer pauses. Tune -Xmx, choose G1 for...
What is Optional and when should you use it?
Use Optional as return type when absence is expected — findById returns Optional<User>. Chain with map/flatMap/orElseThrow. Do not use for fields, method parameters, or Jackson...
How does Stream API parallel stream work?
parallelStream() splits work across common pool threads. Good for CPU-heavy map on large in-memory collections. Bad for small lists, IO inside stream, or ordered stateful...
What is an ExecutorService and why use it?
Instead of new Thread per task, submit Runnable to fixed pool sized to CPU/IO profile. Handle Future results and exceptions. On shutdown hook call shutdown() and...
What are virtual threads in Java 21 and when should you use them?
Virtual threads excel when code blocks on IO — HTTP clients, JDBC, file reads — allowing one platform thread to serve thousands of concurrent requests. For CPU-heavy parallel...
volatile vs synchronized — when to use each?
volatile for single-writer flags (shutdown, initialized) where no read-modify-write. synchronized or java.util.concurrent locks when multiple threads update shared state. For...
Explain the difference between == and .equals() in Java.
For primitives, == compares values. For objects, == asks if two references point to the same instance. equals() compares content when overridden — String overrides it for...
What is the difference between ArrayList and LinkedList?
ArrayList: dynamic array, great for get(i) and iteration, amortized append. LinkedList: node chain, costly random access, only wins for frequent insert/remove in middle with...
What is the difference between Comparable and Comparator?
Comparable built into the type — Employee implements compareTo by id. Comparator external — sort by name or salary without changing Employee. Use...
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