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Java is a language that has stood the test of time. It’s used to build programs that run on everything from phones to bank systems. If you’re trying to get a job in tech, learning Java is a smart choice. But learning the language is just the first step—acing the interview is the next.
Employers use Java interviews to test how well you know the language. They want to see if you understand key ideas like inheritance, abstraction, interfaces, loops, and collections. They might also test your problem-solving skills by asking you to write code or fix bugs.
This page is full of Java interview questions that many companies ask. Each question comes with a simple explanation to help you understand the concept better. Going through these questions will help you prepare for both technical rounds and coding tests.
No matter your experience level, these questions can help you sharpen your skills. Take your time to study them, and don’t be afraid to go back to the basics. The more you practice, the more confident you’ll feel when it’s time to sit for the interview.
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gradient_start_position=”0″ gradient_end_position=”100″ gradient_type=”linear” radial_direction=”center center” linear_angle=”180″ background_color=”” background_image=”” background_position=”left top” background_repeat=”no-repeat” background_blend_mode=”none” render_logics=”” filter_type=”regular” filter_hue=”0″ filter_saturation=”100″ filter_brightness=”100″ filter_contrast=”100″ filter_invert=”0″ filter_sepia=”0″ filter_opacity=”100″ filter_blur=”0″ filter_hue_hover=”0″ filter_saturation_hover=”100″ filter_brightness_hover=”100″ filter_contrast_hover=”100″ filter_invert_hover=”0″ filter_sepia_hover=”0″ filter_opacity_hover=”100″ filter_blur_hover=”0″ animation_type=”” animation_direction=”left” animation_speed=”0.3″ animation_offset=”” last=”true” border_position=”all” first=”true”][fusion_accordion type=”accordions” boxed_mode=”” border_size=”1″ border_color=”” background_color=”” hover_color=”” divider_line=”” title_font_size=”” icon_size=”” icon_color=”” icon_boxed_mode=”” icon_box_color=”#4f4f4f” icon_alignment=”right” toggle_hover_accent_color=”#3ed9df” hide_on_mobile=”small-visibility,medium-visibility,large-visibility” class=”interview_list” id=””][fusion_toggle title=”Question 161: What is the diamond problem in Java?” open=”no” class=”” id=””]
Answer:
Java does not support multiple inheritance where one class can inherit properties from more than one class. It is known as the diamond problem.
[/fusion_toggle][fusion_toggle title=”Question 162: What is Aggregation in Java?” open=”no” class=”” id=””]
Answer:
Aggregation is a term that defines a one-way relationship between two objects. In Java, Aggregation depicts the HAS-A relationship, meaning that a class contains another class reference.
[/fusion_toggle][fusion_toggle title=”Question 163: What is Recursion in Java?” open=”no” class=”” id=””]
Answer:
Recursion is a technique of making a function call itself. It provides a way to break down complex problems into simple problems that can be solved quickly.
[/fusion_toggle][fusion_toggle title=”Question 164: What is Autoboxing & Unboxing in Java?” open=”no” class=”” id=””]
Answer:
Autoboxing is a process of converting primitive data types into an object of their corresponding wrapper class. Unboxing is the reverse of the autoboxing process.
[/fusion_toggle][fusion_toggle title=”Question 165: What is an iterator in Java?” open=”no” class=”” id=””]
Answer:
An Iterator is an object used to loop through collections, like HashSet & ArrayList. It is called an “iterator” because “iterating” is the technical term for looping. You must import it from java.util package to use an Iterator.
[/fusion_toggle][fusion_toggle title=”Question 166: What is a HashMap?” open=”no” class=”” id=””]
Answer:
HashMap means a collection framework functionality used for storing data into key and value pairs. It internally uses linked lists to support the storage functionality.
[/fusion_toggle][fusion_toggle title=”Question 167: What is a stack?” open=”no” class=”” id=””]
Answer:
A Stack is a (LIFO) Last In First Out data structure that supports two fundamental operations called pop & push. The push operation helps to add an element at the top of the stack, while the pop operation is useful for removing an element from the top.
[/fusion_toggle][fusion_toggle title=”Question 168: What is a TreeMap?” open=”no” class=”” id=””]
Answer:
TreeMap in Java is a navigable map interpretation built around the concepts of red & black trees. Its keys are always sorted in ascending order.
[/fusion_toggle][fusion_toggle title=”Question 169: Who executes the bytecode in Java?” open=”no” class=”” id=””]
Answer:
Java Virtual Machine or JVM executes the bytecode generated by the compiler.
[/fusion_toggle][fusion_toggle title=”Question 170: How many types of constructors does Java support?” open=”no” class=”” id=””]
Answer:
Java supports the following types of constructors:
- Default Constructor or No-argument constructor
- Parameterized Constructor
- Copy Constructor
[/fusion_toggle][fusion_toggle title=”Question 171: What are the benefits of making a class singleton?” open=”no” class=”” id=””]
Answer:
A singleton class in Java is a class that can have one instance of it in an application. If a new instance is created for the same class, it points to the first instance. Thus, have the same values for all properties & attributes. A singleton class helps to create global points of access to objects. The singleton class primary usage is caching, logging, and device drivers that are all entities for universal access.
[/fusion_toggle][fusion_toggle title=”Question 172: How is polymorphism achieved in Java?” open=”no” class=”” id=””]
Answer:
There are two types of polymorphisms that can be performed through the following ways:
- Static binding or compile-time polymorphism– Method overloading
- Dynamic binding or runtime polymorphism – Method overriding
[/fusion_toggle][fusion_toggle title=”Question 173: Can the main method be declared as private in Java?” open=”no” class=”” id=””]
Answer:
Yes, it is possible to declare the main method as private in Java.
[/fusion_toggle][fusion_toggle title=”Question 174: What is an object in Java?” open=”no” class=”” id=””]
Answer:
An object is an entity that has a state & behavior. It is an instance of a class & an object is a real-world entity. You can create an object to access the defined members in the class.
[/fusion_toggle][fusion_toggle title=”Question 175: What is thread-safe in Java?” open=”no” class=”” id=””]
Answer:
When a thread is working on an object & preventing other threads from working on the same object, it is called thread-safe. In Java, there is a collection of thread-safe classes like Hashtable, Properties, Vector, Stack, etc.
[/fusion_toggle][fusion_toggle title=”Question 176: What is Java Main Method public static void main?” open=”no” class=”” id=””]
Answer:
When you start Java programming, the first method you will encounter is the public static void main(String [] args). The starting point of any Java program where it starts its execution is known as the main() method. It is an essential method of Java. JVM looks for the main method to start running a Java application.
[/fusion_toggle][fusion_toggle title=”Question 177: What is the difference between Declaration & Definition in Java?” open=”no” class=”” id=””]
Answer:
Declaration includes informing the compiler about the variable’s properties, but Definition is all about implementation & memory location. Declaration of a variable or function can occur multiple times, but a variable or function can only be defined once.
[/fusion_toggle][fusion_toggle title=”Question 178: What is Method Overriding?” open=”no” class=”” id=””]
Answer:
Declaring a method in child class that already exists in the parent class is known as Method Overriding. To simply put, overriding means to override the functionality of an existing method.
[/fusion_toggle][fusion_toggle title=”Question 179: What is the sleep() & wait() methods in Java?” open=”no” class=”” id=””]
Answer:
The sleep() is a method in Java used to pause the process for a few seconds or for the time we want it to be paused. In the case of the wait() method, the thread goes in the waiting mode & doesn’t come back automatically till we call notifyAll() or notify(). The major difference between wait() & sleep() methods is the former monitor or releases the lock when waiting, while the latter doesn’t monitor or releases the lock while waiting. The wait() method helps in inter-thread communication, while the sleep() method enables us to introduce pause on execution.
[/fusion_toggle][fusion_toggle title=”Question 180: When to use a transient keyword with a variable in Java?” open=”no” class=”” id=””]
Answer:
Transient keyword in Java is useful to mark a variable not to be serialized when persisted to streams of bytes. It plays an essential role in meeting the security constraints in Java. Transient keyword ignores a variable’s original value & saves the default value of that data type.
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