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.

If you've got an interview coming up on Java soon and need to get up to speed, this is the ideal course for you. You'll refresh your memory on everything from the basics to more advanced functionality you’re likely to be asked about. Even if you're using Java every day, you may not have been exposed to parts of it in some time, so it's always useful to make sure you're updated.

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

<div class="card">
   # Question # 1
    <div class="qtn">
        <p>What is an interface in Java</p>
    </div>
    <div class="ans">
        <p><span class="hi-yl-b">An interface can be thought of as a <i>contract</i> an implementing object has with its consumers</span>. An interface will define the methods or API exposed by the implementing object. The consumers will rely on the method defined by the interface to interact with the implementing object. An interface allows us to hide the implementation details from the consumer. The object implementing the interface can change the functionality or be switched out for a completely different object that also implements the same interface. The consumer is only knowledgeable about the interface and not the concrete classes that implement it. <span class="hi-yl"><i>This makes the code more flexible and maintainable.</i></span></p>
## Interface Example
<b>

```java
public interface PersonActions {

    void sayHello();

}
```
</b>
<p> Methods in an interface that are not declared as default or static are implicitly abstract.</p>
</div>
</div>

<div class="card">
   # Question # 2
    <div class="qtn">
        <p>What is the difference between an interface and an abstract class in Java?</p>
    </div>
    <div class="ans">
        <p>Following are the main differences between an abstract class and an interface</p>
<ul>
<li><p>An abstract class can have final, static, or class member variables whereas an <span class="hi-yl-i">interface can only have variables that are final and static by default.</span></p>
</li>
<li><p>An abstract class can have static, abstract, or non-abstract methods. An <span class="hi-yl-i">interface can have static, abstract, or default methods.</span></p></li>
<li><p>Members of an abstract class can have varying visibility of private, protected, or public. Whereas, in an <span class="hi-yl-i">interface all methods and constants are public.</span></p>
</li>
<li><p>A child class can define abstract methods with the same or less restrictive accessibility, whereas a class implementing an interface must define the interface methods with the exact same accessibility which is public.</p>
</li>
<li><p>When inheriting an abstract class, a concrete child class must define the abstract methods. An abstract class can extend another abstract class and abstract methods from the parent class don't have to be defined.</p>
<p>In contrast, an interface extending another interface is not responsible for implementing methods from the parent interface. This is because interfaces cannot define any implementation.</p>
</li>
<li><p>A class can only extend another class, but it can implement multiple interfaces. <span class="hi-yl">Similarly, an interface can extend multiple interfaces. An interface never <i>implements</i> a class or an interface.</span></p></li>
</ul>
</div>
</div>

<div>
   # Question # 1
    <div>
        <p>What is an interface in Java</p>
    </div>
    <div>
        <p><span>An interface can be thought of as a <i>contract</i> an implementing object has with its consumers</span>. An interface will define the methods or API exposed by the implementing object. The consumers will rely on the method defined by the interface to interact with the implementing object. An interface allows us to hide the implementation details from the consumer. The object implementing the interface can change the functionality or be switched out for a completely different object that also implements the same interface. The consumer is only knowledgeable about the interface and not the concrete classes that implement it. <span><i>This makes the code more flexible and maintainable.</i></span></p>
## Interface Example
<b>

```java
public interface PersonActions {

    void sayHello();

}
```
</b>
<p> Methods in an interface that are not declared as default or static are implicitly abstract.</p>
</div>
</div>

<div>
   # Question # 2
    <div>
        <p>What is the difference between an interface and an abstract class in Java?</p>
    </div>
    <div>
        <p>Following are the main differences between an abstract class and an interface</p>
<ul>
<li><p>An abstract class can have final, static, or class member variables whereas an <span>interface can only have variables that are final and static by default.</span></p>
</li>
<li><p>An abstract class can have static, abstract, or non-abstract methods. An <span>interface can have static, abstract, or default methods.</span></p></li>
<li><p>Members of an abstract class can have varying visibility of private, protected, or public. Whereas, in an <span>interface all methods and constants are public.</span></p>
</li>
<li><p>A child class can define abstract methods with the same or less restrictive accessibility, whereas a class implementing an interface must define the interface methods with the exact same accessibility which is public.</p>
</li>
<li><p>When inheriting an abstract class, a concrete child class must define the abstract methods. An abstract class can extend another abstract class and abstract methods from the parent class don't have to be defined.</p>
<p>In contrast, an interface extending another interface is not responsible for implementing methods from the parent interface. This is because interfaces cannot define any implementation.</p>
</li>
<li><p>A class can only extend another class, but it can implement multiple interfaces. <span>Similarly, an interface can extend multiple interfaces. An interface never <i>implements</i> a class or an interface.</span></p></li>
</ul>
</div>
</div>

This lesson discusses interfaces in Java.

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

Interfaces

Question # 1

Interface Example

Question # 2