The ultimate guide to Java interviews with 300+ real-world questions and answers. Get interview-ready in just a few hours with strategies developed by FAANG engineers.

It’s very common to have interviews testing your knowledge of a programming language you claim to be an expert in.

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This course features more than 300 of the most commonly asked core Java interview questions that you're likely to face as an experienced software engineer. You'll get to explore questions by topic, as well as see detailed answers for each one, and plenty of live code examples where relevant. Get started today.

The Java Interview Handbook: 300+ Interview Questions

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   # Question # 1
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        <p>Explain the concept of <code class="clazz">volatile</code>.</p>
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    <div class="ans">
<p>The <code class="clazz">volatile</code> concept is specific to Java. Its easier to understand <code class="clazz">volatile</code>, if you understand the problem it solves.<p>

<p>If you have a variable say a counter that is being worked on by a thread, it is possible the thread keeps a copy of the counter variable in the CPU cache and manipulates it rather than writing to the main memory. The JVM will decide when to update the main memory with the value of the counter, even though other threads may read the value of the counter from the main memory and may end up reading a stale value.</p>

<p><span class="hi-yl">If a variable is declared <code class="clazz">volatile</code> then whenever a thread writes or reads to the <code class="clazz">volatile</code> variable, the read and write always happen in the main memory. As a further guarantee, all the variables that are visible to the writing thread also get written-out to the main memory alongside the <code class="clazz">volatile</code> variable. Similarly, all the variables visible to the reading thread alongside the <code class="clazz">volatile</code> variable will have the latest values visible to the reading thread.</span></p>

<p>Volatile comes into play because of multiples levels of memory in hardware architecture required for performance enhancements. If there's a single thread that writes to the volatile variable and other threads only read the volatile variable then just using volatile is enough, however, if there's a possibility of multiple threads writing to the <code class="clazz">volatile</code> variable then <code class="clazz">synchronized</code> would be required to ensure atomic writes to the variable.</p>
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<div>
   # Question # 1
    <div>
        <p>Explain the concept of <code>volatile</code>.</p>
    </div>
    <div>
<p>The <code>volatile</code> concept is specific to Java. Its easier to understand <code>volatile</code>, if you understand the problem it solves.<p>

<p>If you have a variable say a counter that is being worked on by a thread, it is possible the thread keeps a copy of the counter variable in the CPU cache and manipulates it rather than writing to the main memory. The JVM will decide when to update the main memory with the value of the counter, even though other threads may read the value of the counter from the main memory and may end up reading a stale value.</p>

<p><span>If a variable is declared <code>volatile</code> then whenever a thread writes or reads to the <code>volatile</code> variable, the read and write always happen in the main memory. As a further guarantee, all the variables that are visible to the writing thread also get written-out to the main memory alongside the <code>volatile</code> variable. Similarly, all the variables visible to the reading thread alongside the <code>volatile</code> variable will have the latest values visible to the reading thread.</span></p>

<p>Volatile comes into play because of multiples levels of memory in hardware architecture required for performance enhancements. If there's a single thread that writes to the volatile variable and other threads only read the volatile variable then just using volatile is enough, however, if there's a possibility of multiple threads writing to the <code>volatile</code> variable then <code>synchronized</code> would be required to ensure atomic writes to the variable.</p>
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This lesson explains the volatile keyword.

Java Ecosystem

Methods

Classes

Interfaces

Inheritance

Lambda Expressions

Generics

Multi-Threading

Memory Management

Collections

Exceptions

Reflection

Serialization

Miscellaneous Topics

Java in Practice

Patterns

The End

Volatile

Question # 1