Java developer Junior job interview FAQ: part 4

In this fourth part of our popular series, we continue with frequently asked questions that may arise in the first round of job interviews for the position of Java Junior Developer. If you haven’t read the previous episodes, you can find them here:

When would you prefer a static method over a classic method in Java?

A static method in Java is a method that belongs to a class and not to a specific instance of that class. This means that we can call static methods directly from a class, without the need to create an object of that class.

The main differences of static methods compared to classical methods are:

  • Declaration: static methods are declared using the keyword static before the return type of the method.
  • Calling: we don’t need to create a class object to call a static method. We call it directly using the class name.
  • Attribute access: static methods can only access static attributes of the class, because they do not have access to the object instance.

Limitations of static methods

Since static methods do not operate on class instance attributes, we should therefore be aware that:
– a static method cannot directly refer to an instance attribute
– a static method cannot directly call an instance method
– the derived class cannot overload(override) static methods
– we cannot use keywords this and super in static methods

Static methods are used when the method does not need to change the state of the object or does not use instance variables. The same when we want to implement a method without creating an instance. It is thus most often used in auxiliary methods.

In which case would you use the final method in Java?

A final method, is the designation of a method that cannot be overridden (override) in derived classes. In this way, we achieve the prohibition of changing the implementation of this method.

The main differences of the final methods compared to the classical ones are:

  • Declaration: final methods are declared using the keyword final before the return type of the method.
  • Overlay: the final method retains its original implementation from the parent class and cannot be changed.

The use of final methods is useful when we are designing a class and want to ensure that a particular finished method should not be able to be further modified. The standard Java libraries are full of methods marked as final.

What is Singleton and what are its practical uses?

Singleton is a class design pattern that ensures that only one instance of a given class exists and provides external access to that instance. The goal of this design pattern is to prevent the creation of multiple instances of a class and to ensure that all references to that class point to a single existing instance.

The main properties of the Singleton class are:

  • Private constructor: the Singleton class has its constructor set to private, which prevents the creation of new instances outside the class itself.
  • Private static instance: Singleton stores its single instance as a private static variable of this class.
  • Public static method: Singleton provides apublic static method that provides access to the instance itself. This method checks if the instance already exists and either creates it or returns it.

The Singleton design pattern is useful when we need to ensure that only one instance of a class exists throughout the program environment, for example, when working with shared resources, database connections, or objects containing global settings.

What is the role of a destructor in Java?

In Java, there is no directly equivalent concept of a destructor as in some other languages such as C++. In Java, the garbagecollector takes care of the automatic memory freeing, so there is no need to explicitly define destructors.

When creating objects in Java, there is no need to consider memory freeing or destruction. When an object becomes unusable and its reference is lost, the garbage collector automatically detects such unused objects and frees the memory they occupy.

In Java, it is possible to implement a finalize() method that is called just before an object is removed from memory by the garbage collector. However, this method is not the same as the destructor in other languages and has limited impact on memory management and resource freeing. Using finalize() is not recommended because it can have unpredictable behavior.

Describe exception handling and explain why it is important

Exception handling is a concept that allows developers to treat and manage exceptional situations or errors that may occur during the execution of a program. Exceptions can be caused by a variety of factors, such as faulty input, file system problems, or inconsistent program behavior.

Main exception handling blocks

The main exception handling blocks are used to capture and handle exceptions. We recognize the following:

  • try: Contains code where exceptions may occur. This block must always be present when processing exceptions.
  • catch: a block that handles the caught exception. Catch blocks can be multiple for different types of exceptions.
  • finally: a block of code that is executed regardless of whether an exception has occurred or not. This block is used to ensure that certain code is always executed, for example, freeing resources.
  • throw: used to explicitly raise an exception within the code. It is placed inside the try block.

Throwing exceptions: exceptions can be explicitly thrown using the throw statement. This command is used to indicate that an exception has occurred.

Exception handling is an important part of ensuring that the program does not crash unexpectedly and providing information about any errors that may occur.

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About the author

Jozef Wagner

Java Developer Senior

Viac ako 10 rokov programujem v Jave, momentálne pracujem v msg life Slovakia ako Java programátor senior a pomáham zákazníkom implementovať ich požiadavky do poistného softvéru Life Factory. Vo voľnom čase si rád oddýchnem v lese, prípadne si zahrám nejakú dobrú počítačovú hru.

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