What are the 10 most common Java interview questions? (And how to answer them)

As a widely used programming language, Java and its unique features are well known to recruiters. This is why questions about various aspects of Java are often asked in the early stages of an interview. We’ve compiled a list of ten frequently asked questions for Java developers and have included answers to each one as well.

This article provides an overview of the potential questions you may be asked about Java in a developer job interview. Depending on the level of professional experience required for the role, some questions may be more detailed. Sometimes in interviews for senior roles, scenarios are presented that contain deliberate errors or are clearly misleading in order to test the technical knowledge of the person interviewing for the position. This is why it’s important to be well prepared for your interview.

Question 1: What are Java’s special features and what advantages does the programming language offer?

Java is a powerful and widely used language, which has made it a popular programming language to learn. Several unique features make Java stand out, particularly in how it’s used. A key advantage of Java is that its code can run on various operating systems without any modifications.

This flexibility is enabled by the Java Virtual Machine, which ensures development and application aren’t restricted to a specific platform, allowing the code to be used across multiple platforms. This ensures programs run flexibly and efficiently on various devices.

Another advantage of Java is the automatic memory management provided by the Garbage Collector, which simplifies memory resource management and reduces potential errors. Additionally, Java’s extensive standard libraries of many ready-made functions accelerates the development of applications.

As an object-oriented programming language, Java, like other languages in this category, excels because software components can be modeled more easily, and the code can be reused repeatedly.

Question 2: How does Java deal with multiple inheritance?

In principle, Java doesn’t support multiple inheritance of classes. This means classes can only inherit from a single class. Multiple inheritance can negatively impact code in object-oriented programming, a well-known issue being the diamond problem.

Although multiple inheritance isn’t possible for classes in Java, this can be solved using interfaces. Since a class can implement several interfaces in Java, it can inherit functionalities from different sources. This assignment ensures that the functions are clearly defined and problems with multiple inheritance cannot occur.

Question 3: What’s the difference between an abstract class and an interface in Java?

Both abstract classes and interfaces are used in Java to define abstract types, which can then be implemented by other classes. There are significant differences, particularly when it comes to inheritance rules:

• Inheritance: In Java, a class can only inherit from an abstract class. Abstract classes in turn cannot perform multiple inheritance, meaning that several interfaces must be implemented for this.
• Class relationship: Since a class can only inherit from an abstract class, the use of abstract classes is well suited for “is a” relationships, while interfaces are better suited for “can” relationships.
• Concreteness: An abstract class can contain abstract (unimplemented) and concrete (implemented) methods and can also have instance variables. In contrast, interfaces can only define abstract methods and constants. All methods in an interface are implicitly abstract and public.
• Functionality: Abstract classes rely on sharing a common implementation (and implementing multiple interfaces). Interfaces, on the other hand, are designed to declare specific functions, which are then implemented in different classes.

Question 4: What’s the difference between instance variables and local variables?

The main difference between instance variables and local variables lies in their scope and lifetime. Instance variables are mainly properties of an object within a class. Local variables represent temporarily created values in a specific scope.

Instance variables

• Instance variables are variables declared at the class level, outside of methods, constructors and blocks.
• Each object of a class has its own copy of an instance variable.
• Instance variables are accessed through the instance of a class. The values can differ for each object of the class.

Local variables

• Local variables are declared within a method, a constructor or a block. Their validity is limited to this defined area.
• These variables must be explicitly initialized before use and exist only for the duration of the code block’s execution.
• They are not visible outside the code block where they were declared.

Question 5: What do the terms JVM, JDK and JRE mean and how do they differ from each other?

Although these terms may appear similar when abbreviated, their underlying tasks and scope within Java are fundamentally different.

Java Virtual Machine (JVM)

• The Java Virtual Machine (JVM) is a virtual machine that serves as the interface between the Java program and the underlying hardware or operating system, executing Java bytecode.
• The JVM is crucial because, as a runtime environment, it can execute the same bytecode on different operating systems, provided it is available on the respective platforms. This contributes significantly to the portability of Java.
• This portability is enabled by the Java compiler translating Java source code into bytecode, which the JVM then interprets.

Java Development Kit (JDK)

• The Java Development Kit (JDK) is the complete development package, which bundles various tools that assist in the development of Java applications. It includes tools for creating, compiling and debugging applications.
• The JDK contains the Java compiler, the Java Virtual Machine (JVM), Java debugger, and Java profiler.
• In addition to these tools, the JDK includes a large number of predefined classes and interfaces for frequently used functions that are available in the Java API or the Java class library.

Java Runtime Environment (JRE)

• The Java Runtime Environment (JRE) provides a reduced environment where Java applications can be executed.
• The JRE contains the Java Virtual Machine (JVM) and the Java API, which are both necessary for starting and running applications.
• Development tools, such as the Java compiler, are not included in the JRE, so it is typically installed by end users.

Question 6: What are collection classes in Java and what are they used for?

In Java, the term collection class usually refers to classes that are part of the Java Collections Framework. This framework provides a standardized way to store, organize and manipulate groups of objects. It consists of various interfaces and concrete implementations of data structures.

Collection classes are used for various purposes:

• Data organization: They enable the efficient organization of data in lists, sets or maps.
• Data manipulation: They provide methods for adding, removing and searching elements.
• Generic programming: The use of generic types in collection classes enables the creation of reusable and type-safe code.
• Algorithms: The framework also contains algorithms that operate on data structures (e.g., sorting or searching).

Question 7: What’s the difference between == and equals() in Java?

== and equals() are two different mechanisms or operators that are used to compare objects:

• The == operator compares the references of objects, not their content values. When used with objects, == checks whether the two references point to the same object (i.e., they refer to the same memory area). In contrast, with primitive data types (e.g., char, int or byte), the operator matches the values.
• With equals(), you can compare the content in objects to determine if the objects are the same, even if they are in different memory locations. By default, equals() behaves like ==, inheriting the reference comparison logic from the Object class. However, it usually needs to be overridden in user-defined classes to enable meaningful content comparison.

Question 8: What are constructors used for?

Constructors are special methods within a class used to instantiate and initialize objects. The four main tasks of constructors are:

• Object initialization: Constructors are primarily used to put an object into a valid and initialized state as soon as it is created. Attributes are initialized, and the necessary resources are allocated.
• Parameter passing: Constructors can accept parameters to create different instances of the class with varying properties, allowing objects with specific attribute values to be created.
• Code management: Using constructors improves code readability, as object initialization can be performed directly in the constructor. This also simplifies code maintenance, since changes within the initialization logic only need to be made in one place.
• Inheritance: Constructors play an important role in inheritance hierarchy. A derived class typically calls the constructor of the base class to handle its initialization before performing its own initializations.

Question 9: Java uses a number of different string types. What are they and how do they differ from each other?

In Java, the string type is represented by the class java.lang.String. This class is the primary way to represent character strings. It also provides various options for string manipulation and processing:

• StringBuilder: This class is used to efficiently create modifiable strings in Java. Unlike the immutable String class, it allows changes without generating a new instance.
• String literal: This refers to character strings that are written in double quotation marks, for example "Nice to see you!". Identical string literals share one instance in the string pool to optimize memory usage.
• String objects: These can be created as a new instance by using the new keyword, for example as new string ("Nice to see you!"). The instance is created independently of the content.
• StringBuffer: Like StringBuilder, this class creates modifiable strings. The main difference is that StringBuffer is thread-safe, whereas StringBuilder is not.

Question 10: What distinguishes throw from throws?

In Java, the keywords throw and throws are both used to handle exceptions. Despite this similarity, they serve different purposes and are applied in different contexts:

• throw is used to throw an exception manually. Developers can use this to generate an exception and pass it to the calling program.
• In contrast, throws indicates that a method is capable of throwing a specific exception. It is used in the method declaration to specify which exceptions may not be handled by the method but instead passed on to the caller for handling, allowing the code to react accordingly.