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Java Interview Questions & Answers

Java Technical Interview Questions

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Why must equals and hashCode agree for keys stored in HashMap?

HashMap uses a key’s hash code to locate a bucket and equality to identify the matching key. If two equal objects produce different hash codes, lookup may search the wrong bucket. Equal objects must t...

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How does try-with-resources improve exception-safe resource handling?

try-with-resources closes objects implementing AutoCloseable when the scope exits, including when the body throws. Resources close in reverse declaration order. If the body throws and closing also thr...

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When would you choose an interface over an abstract class in Java?

Use an interface to define a capability that unrelated classes can implement, such as a payment gateway contract. Use an abstract class when implementations genuinely share state, construction rules, ...

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Why does volatile not make counter++ thread-safe, and what would you use instead?

volatile provides visibility and ordering guarantees for accesses to the field, but increment is a read-modify-write sequence. Two threads can read the same value and both write the same incremented r...

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