Вопросы на собеседовании Java-разработчика на английском

Пример ответа · EN

The JVM executes bytecode and manages memory. The JRE is the runtime environment: the JVM plus the standard class libraries. The JDK adds development tools such as the javac compiler and the debugger. Since Java 11, Oracle no longer ships a separate JRE, so in practice teams install a JDK or build a custom runtime with jlink.

Пример ответа · EN

A final field can be assigned only once, a final method cannot be overridden, and a final class cannot be extended. For a reference field, final fixes the reference, not the object it points to, so a final list can still be modified. Final fields also get special visibility guarantees after construction. I use final fields to build immutable classes.

Пример ответа · EN

An interface describes a capability that unrelated classes can share, and a class can implement many interfaces. An abstract class suits a family of closely related types that share state or constructor logic, but a class can extend only one. Since Java 8, interfaces can have default methods, so shared behaviour alone is no longer a reason. I start with an interface and add an abstract base class only when it removes real duplication.

Пример ответа · EN

Static initializers run once, when the class is initialized, and the parent class goes first. When an object is created, the constructor first calls the superclass constructor. Then the instance field initializers and instance initializer blocks of the class run in source order, and finally the rest of the constructor body. Calling an overridable method from a constructor is risky, because subclass fields are not initialized yet.

Пример ответа · EN

Overloading means several methods with the same name but different parameter lists; the compiler picks one at compile time. Overriding means a subclass provides its own implementation of an inherited instance method with the same signature; the JVM chooses it at runtime by the object's actual type. I add @Override so that the compiler catches a signature mismatch. Static methods are hidden, not overridden.

Пример ответа · EN

The contract says that equal objects must have equal hash codes. Hash-based collections such as HashMap and HashSet use the hash code to find a bucket and then call equals. If I override only equals, two equal objects can land in different buckets, so lookups fail. I base both methods on the same fields, or use a record, which generates them.

Пример ответа · EN

It calculates the key's hash code, mixes its bits and uses the result to choose a bucket in an internal array. Inside the bucket, it compares keys with equals. Since Java 8, a bucket with many collisions can turn into a balanced tree, which keeps lookups fast. When the map grows past its load factor, it resizes and redistributes the entries.

Пример ответа · EN

ArrayList stores elements in an array, so access by index is fast and iteration is cache-friendly. LinkedList makes insertion cheap only when you already hold a position, and each node costs extra memory. In practice, I use ArrayList almost everywhere and ArrayDeque for queues. I would choose LinkedList only after measuring a real benefit.

Пример ответа · EN

With a for-each loop over an ArrayList, removing an element changes the list's modification count, and the iterator throws ConcurrentModificationException on the next step. The check is fail-fast on a best-effort basis, so it detects bugs but is not a safety mechanism. I use Iterator.remove() or removeIf instead. For lists shared between threads, I choose a concurrent collection.

Пример ответа · EN

Immutability makes strings safe to share between threads, to keep in the string pool and to use as map keys. Every change creates a new String object. When I build a string in a loop, I use StringBuilder to avoid creating many temporary objects. For a simple concatenation in one expression, the compiler already optimises it, so plain + is fine.

Пример ответа · EN

A write to a volatile field becomes visible to other threads that read it later, and it also orders the actions around it through a happens-before relationship. It does not make compound actions atomic, so count++ on a volatile field can still lose updates. I use volatile for simple flags such as a stop signal. For counters, I use AtomicInteger or a lock.

Пример ответа · EN

A synchronized block takes the monitor of an object, so only one thread at a time can run code guarded by that monitor. Releasing the monitor also makes the changes visible to the next thread that takes it. I keep the critical section small and lock on a private object. Taking several locks in different orders can cause a deadlock.

Пример ответа · EN

An ExecutorService separates the tasks from the way they run. It reuses threads from a pool, limits how many run at once and returns a Future for results and errors. It also lets me shut work down in a controlled way. Creating a thread per task is harder to manage and can exhaust memory under load.

Пример ответа · EN

Virtual threads, final since Java 21, are lightweight threads managed by the JVM rather than the operating system. When one blocks on I/O, the JVM unmounts it from its carrier thread, so the carrier can run other work. That lets the simple thread-per-request style scale to many concurrent requests. They do not speed up CPU-heavy code, and there is no point in pooling them.

Пример ответа · EN

The collector starts from GC roots, such as local variables on thread stacks and static fields, and finds every object reachable from them. Unreachable objects can be reclaimed. Many HotSpot collectors are generational, because most objects die young. In practice, I watch for leaks caused by references that stay reachable, such as an ever-growing static cache.

Пример ответа · EN

Checked exceptions must be declared or handled, and the compiler enforces it. Unchecked exceptions extend RuntimeException or Error and do not need that. I use checked exceptions for expected situations that a caller can recover from, and unchecked ones for programming errors such as invalid arguments. I never catch an exception just to ignore it.

Пример ответа · EN

Any object that implements AutoCloseable can be declared in the try header. When the block ends, normally or with an exception, the resources are closed in reverse order of declaration. If closing also throws, that exception is added as suppressed to the main one, so no error is lost. It replaces long finally blocks.