Preguntas de la entrevista de Java

Programación

1. Introducción

Este artículo contiene respuestas para algunas de las preguntas de entrevistas de trabajo más importantes sobre el núcleo de Java. Las respuestas a algunas de ellas pueden no ser obvias, por lo que este artículo ayudará a aclarar las cosas.

2. Preguntas sobre el lenguaje Core-Java para principiantes

Q1. ¿Los datos se pasan por referencia o por valor en Java?

Aunque la respuesta a esta pregunta es bastante simple, esta pregunta puede resultar confusa para los principiantes. Primero, aclaremos de qué se trata la pregunta:

  1. Pasar por valor: significa que pasamos una copia de un objeto como parámetro a un método.
  2. Pasar por referencia: significa que pasamos una referencia a un objeto como parámetro a un método.

Para responder a la pregunta tenemos que analizar dos casos. Representan dos tipos de datos que podemos pasar a un método: un primitivo y un objeto.

Cuando pasamos primitivas a un método, su valor se copia en una nueva variable. Cuando se trata de objetos, el valor de la referencia se copia en una nueva variable. Entonces, podemos decir que Java es un lenguaje estrictamente de paso por valor .

Podemos aprender más sobre eso en uno de nuestros artículos: Pass-By-Value as a Parameter Passing Mechanism en Java.

Q2. ¿Cuál es la diferencia entre importaciones y importaciones estáticas?

Podemos usar importaciones regulares para importar una clase específica o todas las clases definidas en un paquete diferente:

import java.util.ArrayList; //specific class import java.util.*; //all classes in util package

También podemos usarlos para importar clases anidadas públicas de una clase adjunta:

import com.baeldung.A.*

Sin embargo, debemos tener en cuenta que la importación anterior no importa la clase A en sí.

También hay importaciones estáticas que nos permiten importar miembros estáticos o clases anidadas:

import static java.util.Collections.EMPTY_LIST;

El efecto es que podemos usar la variable estática EMPTY_LIST sin anteponer el nombre de la clase completamente calificado, es decir, como si estuviera declarado en la clase actual.

Q3. ¿Qué modificadores de acceso están disponibles en Java y cuál es su propósito?

Hay cuatro modificadores de acceso en Java :

  1. privado
  2. predeterminado (paquete)
  3. protegido
  4. público

El modificador privado asegura que los miembros de la clase no serán accesibles fuera de la clase. Se puede aplicar a métodos, propiedades, constructores, clases anidadas, pero no a las propias clases de nivel superior.

A diferencia del modificador privado , podemos aplicar el modificador predeterminado a todos los tipos de miembros de la clase y a la clase misma. Podemos aplicar la visibilidad predeterminada al no agregar ningún modificador de acceso. Si usamos la visibilidad predeterminada, nuestra clase o sus miembros serán accesibles solo dentro del paquete de nuestra clase. Debemos tener en cuenta que el modificador de acceso predeterminado no tiene nada en común con la palabra clave predeterminada .

De manera similar al modificador predeterminado , todas las clases dentro de un paquete pueden acceder a miembros protegidos . Además, el modificador protegido permite que las subclases accedan a los miembros protegidos de una superclase, incluso si no están dentro del mismo paquete. No podemos aplicar este modificador de acceso a las clases, solo a los miembros de la clase.

El modificador público se puede usar junto con la palabra clave de clase y todos los miembros de la clase. Hace que las clases y los miembros de la clase sean accesibles en todos los paquetes y para todas las clases.

Podemos obtener más información en el artículo Modificadores de acceso de Java.

Q4. ¿Qué otros modificadores están disponibles en Java y cuál es su propósito?

Hay otros cinco modificadores disponibles en Java:

  • estático
  • final
  • resumen
  • sincronizado
  • volátil

Estos no controlan la visibilidad.

Primero que nada, podemos aplicar la palabra clave estática a campos y métodos. Los campos o métodos estáticos son miembros de la clase, mientras que los no estáticos son miembros del objeto . Los miembros de la clase no necesitan que se invoque ninguna instancia. Se llaman con el nombre de la clase en lugar del nombre de referencia del objeto. Este artículo ofrece más detalles sobre la palabra clave estática .

Entonces, tenemos la palabra clave final . Podemos usarlo con campos, métodos y clases. Cuando se utiliza final en un campo, significa que la referencia del campo no se puede cambiar. Por lo que no se puede reasignar a otro objeto. Cuando se aplica final a una clase o un método, nos asegura que esa clase o método no se puede extender ni anular. La palabra clave final se explica con más detalle en este artículo.

La siguiente palabra clave es abstracta . Éste puede describir clases y métodos. Cuando las clases son abstractas , no se pueden crear instancias de ellas. En cambio, están destinados a ser subclasificados. Cuando los métodos son abstractos , se dejan sin implementación y se pueden anular en subclases.

La palabra clave sincronizada puede ser la más avanzada. Podemos usarlo tanto con la instancia como con métodos estáticos y bloques de código. Cuando usamos esta palabra clave, hacemos que Java use un bloqueo de monitor para proporcionar sincronización en un fragmento de código dado. Puede encontrar más información sobre sincronizados en este artículo.

La última palabra clave que vamos a discutir es volátil . Solo podemos usarlo junto con campos de instancia. Declara que el valor del campo debe leerse y escribirse en la memoria principal, sin pasar por la memoria caché de la CPU. Todas las lecturas y escrituras de una variable volátil son atómicas. La palabra clave volátil se explica en detalle en este artículo.

Q5. ¿Cuál es la diferencia entre JDK, JRE y JVM?

JDK son las siglas de Java Development Kit , que es un conjunto de herramientas necesarias para que los desarrolladores escriban aplicaciones en Java. Hay tres tipos de entornos JDK:

  • Standard Edition: kit de desarrollo para crear aplicaciones de servidor o de escritorio portátiles
  • Enterprise Edition: una extensión de la Standard Edition con soporte para computación distribuida o servicios web
  • Micro Edition – development platform for embedded and mobile applications

There are plenty of tools included in the JDK which help programmers with writing, debugging or maintaining applications. The most popular ones are a compiler (javac), an interpreter (java), an archiver (jar) and a documentation generator (javadoc).

JRE is a Java Runtime Environment. It's a part of the JDK, but it contains the minimum functionality to run Java applications. It consists of a Java Virtual Machine, core classes, and supporting files. For example, it doesn't have any compiler.

JVM is the acronym for Java Virtual Machine, which is a virtual machine able to run programs compiled to bytecode. It's described by the JVM specification, as it's important to ensure interoperability between different implementations. The most important function of a JVM is to enable users to deploy the same Java application into different operating systems and environments without worrying about what lies underneath.

For more information, let's check the Difference Between JVM, JRE, and JDK article.

Q6. What Is the Difference Between Stack and Heap?

There are two parts of memory where all variables and objects are stored by the JVM. The first is the stack and the second is the heap.

The stack is a place where the JVM reserves blocks for local variables and additional data. The stack is a LIFO (last in first out) structure. It means that whenever a method is called, a new block is reserved for local variables and object references. Each new method invocation reserves the next block. When methods finish their execution, blocks are released in the reversed manner they were started.

Every new thread has its own stack.

We should be aware that the stack has much less memory space than the heap. And when a stack is full, the JVM will throw a StackOverflowError. It's likely to occur when there is a bad recursive call and the recursion goes too deep.

Every new object is created on the Java heap which is used for a dynamic allocation. There is a garbage collector which is responsible for erasing unused objects which are divided into young (nursery) and old spaces. Memory access to the heap is slower than access to the stack. The JVM throws an OutOfMemoryError when the heap is full.

We can find more details in the Stack Memory and Heap Space in Java article.

Q7. What Is the Difference Between the Comparable and Comparator Interfaces?

Sometimes when we write a new class, we would like to be able to compare objects of that class. It's especially helpful when we want to use sorted collections. There are two ways we can do this: with the Comparable interface or with the Comparator interface.

First, let's look at the Comparable interface:

public interface Comparable { int compareTo(T var1); }

We should implement that interface by the class whose objects we want to sort.

It has the compareTo() method and returns an integer. It can return three values: -1, 0, and 1 which means that this object is less than, equal to or greater than the compared object.

It's worth mentioning that the overridden compareT0() method should be consistent with the equals() method.

On the other hand, we can use the Comparator interface. It can be passed to the sort() methods of the Collection interface or when instantiating sorted collections. That's why it's mostly used to create a one-time sorting strategy.

What's more, it's also useful when we use a third-party class which doesn't implement the Comparable interface.

Like the compareTo() method, the overridden compare() methods should be consistent with the equals() method, but they may optionally allow comparison with nulls.

Let's visit the Comparator and Comparable in Java article for more information.

Q8. What Is the void Type and When Do We Use It?

Every time we write a method in Java, it must have a return type. If we want the method to return no value, we can use the void keyword.

We should also know that there is a Void class. It's a placeholder class that may be used, for example, when working with generics. The Void class can neither be instantiated nor extended.

Q9. What Are the Methods of the Object Class and What Do They Do?

It's important to know what methods the Object class contains and how they work. It's also very helpful when we want to override those methods:

  • clone() – returns a copy of this object
  • equals() – returns true when this object is equal to the object passed as a parameter
  • finalize() – the garbage collector calls this method while it's cleaning the memory
  • getClass() – returns the runtime class of this object
  • hashCode() – returns a hash code of this object. We should be aware that it should be consistent with the equals() method
  • notify() – sends a notification to a single thread waiting for the object's monitor
  • notifyAll() – sends a notification to all threads waiting for the object's monitor
  • toString() – returns a string representation of this object
  • wait() – there are three overloaded versions of this method. It forces the current thread to wait the specified amount of time until another thread calls notify() or notifyAll() on this object.

Q10. What Is an Enum and How We Can Use It?

Enum is a type of class that allows developers to specify a set of predefined constant values. To create such a class we have to use the enum keyword. Let's imagine an enum of days of the week:

public enum Day { SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY }

To iterate over all constants we can use the static values() method. What's more, enums enable us to define members such as properties and methods like regular classes.

Although it's a special type of class, we can't subclass it. An enum can, however, implement an interface.

Another interesting advantage of Enums is that they are thread-safe and so they are popularly used as singletons.

We can find more about Enums in one of our guides.

Q11. What Is a JAR?

JAR is a shortcut for Java archive. It's an archive file packaged using the ZIP file format. We can use it to include the class files and auxiliary resources that are necessary for applications. It has many features:

  • Security – we can digitally sign JAR files
  • Compression – while using a JAR, we can compress files for efficient storage
  • Portability – we can use the same JAR file across multiple platforms
  • Versioning – JAR files can hold metadata about the files they contain
  • Sealing – we can seal a package within a JAR file. This means that all classes from one package must be included in the same JAR file
  • Extensions – we can use the JAR file format to package modules or extensions for existing software

Q12. What Is aNullPointerException?

The NullPointerException is probably the most common exception in the Java world. It's an unchecked exception and thus extends RuntimeException. We shouldn't try to handle it.

This exception is thrown when we try to access a variable or call a method of a null reference, like when:

  • invoking a method of a null reference
  • setting or getting a field of a null reference
  • checking the length of a null array reference
  • setting or getting an item of a null array reference
  • throwing null

Q13. What Are Two Types of Casting in Java? Which Exception May Be Thrown While Casting? How Can We Avoid It?

We can distinguish two types of casting in Java. We can do upcasting which is casting an object to a supertype or downcasting which is casting an object to a subtype.

Upcasting is very simple, as we always can do that. For example, we can upcast a String instance to the Object type:

Object str = "string";

Alternatively, we can downcast a variable. It's not as safe as upcasting as it involves a type check. If we incorrectly cast an object, the JVM will throw a ClassCastExcpetion at runtime. Fortunately, we can use the instanceof keyword to prevent invalid casting:

Object o = "string"; String str = (String) o; // it's ok Object o2 = new Object(); String str2 = (String) o2; // ClassCastException will be thrown if (o2 instanceof String) { // returns false String str3 = (String) o2; }

We can learn more about type casting in this article.

3. Core-Java Language Questions for Advanced Programmers

Q1. Why Is String an Immutable Class?

We should know that String objects are treated differently than other objects by the JVM. One difference is that String objects are immutable. It means that we can't change them once we have created them. There are several reasons why they behave that way:

  1. They are stored in the string pool which is a special part of the heap memory. It's responsible for saving a lot of space.
  2. The immutability of the String class guarantees that its hash code won't change. Due to that fact, Strings can be effectively used as keys in hashing collections. We can be sure that we won't overwrite any data because of a change in hash codes.
  3. They can be used safely across several threads. No thread can change the value of a String object, so we get thread safety for free.
  4. Strings are immutable to avoid serious security issues. Sensitive data such as passwords could be changed by an unreliable source or another thread.

We can learn more about the immutability of Strings in this article.

Q2. What Is the Difference Between Dynamic Binding and Static Binding?

Binding in Java is a process of associating a method call with the proper method body. We can distinguish two types of binding in Java: static and dynamic.

The main difference between static binding and dynamic binding is that static binding occurs at compile time and dynamic binding at runtime.

Static binding uses class information for binding. It's responsible for resolving class members that are private or static and final methods and variables. Also, static binding binds overloaded methods.

Dynamic binding, on the other hand, uses object information to resolve bindings. That's why it's responsible for resolving virtual and overridden methods.

Q3. What Is JIT?

JIT stands for “just in time”. It's a component of the JRE that runs in the runtime and increases the performance of the application. Specifically, it's a compiler which runs just after the program's start.

This is different from the regular Java compiler which compiles the code long before the application is started. JIT can speed up the application in different ways.

For example, the JIT compiler is responsible for compiling bytecode into native instructions on the fly to improve performance. Also, it can optimize the code to the targeted CPU and operating system.

Additionally, it has access to many runtime statistics which may be used for recompilation for optimal performance. With this, it can also do some global code optimizations or rearrange code for better cache utilization.

Q4. What Is Reflection in Java?

Reflection is a very powerful mechanism in Java. Reflection is a mechanism of Java language which enables programmers to examine or modify the internal state of the program (properties, methods, classes etc.) at runtime. The java.lang.reflect package provides all required components for using reflection.

When using this feature, we can access all possible fields, methods, constructors that are included within a class definition. We can access them irrespective of their access modifier. It means that for example, we are able to access private members. To do that, we don't have to know their names. All we have to do is to use some static methods of Class.

It's worth to know that there is a possibility to restrict access via reflection. To do that we can use the Java security manager and the Java security policy file. They allow us to grant permissions to classes.

When working with modules since Java 9, we should know that by default, we aren't able to use reflection on classes imported from another module. To allow other classes to use reflection to access the private members of a package we have to grant the “Reflection” Permission.

This article goes into more depth about Java Reflection.

Q5. What Is a Classloader?

The classloader is one of the most important components in Java. It's a part of the JRE.

Simply put, the classloader is responsible for loading classes into the JVM. We can distinguish three types of classloaders:

  • Bootstrap classloader – it loads the core Java classes. They are located in the /jre/lib directory
  • Extension classloader – it loads classes located in /jre/lib/ext or in the path defined by the java.ext.dirs property
  • System classloader – it loads classes on the classpath of our application

A classloader loads classes “on demand”. It means that classes are loaded after they are called by the program. What's more, a classloader can load a class with a given name only once. However, if the same class is loaded by two different class loaders, then those classes fail in an equality check.

There is more information about classloaders in the Class Loaders in Java article.

Q6. What Is the Difference Between Static and Dynamic Class Loading?

Static class loading takes place when we have source classes available at compile time. We can make use of it by creating object instances with the new keyword.

Dynamic class loading refers to a situation when we can't provide a class definition at the compile time. Yet, we can do that at runtime. To create an instance of a class, we have to use the Class.forName() method:

Class.forName("oracle.jdbc.driver.OracleDriver")

Q7. What Is the Purpose of the Serializable Interface?

We can use the Serializable interface to enable serializability of a class, using Java's Serialization API. Serialization is a mechanism for saving the state of an object as a sequence of bytes while deserialization is a mechanism for restoring the state of an object from a sequence of bytes. The serialized output holds the object's state and some metadata about the object's type and types of its fields.

We should know that subtypes of serializable classes are also serializable. However, if we want to make a class serializable, but its supertype is non-serializable we have to do two things:

  • implement the Serializable interface
  • assure that a no argument constructor is present in the superclass

We can read more about Serialization in one of our articles.

Q8. Is There a Destructor in Java?

In Java, the garbage collector automatically deletes the unused objects to free up the memory. Developers have no need to mark the objects for deletion, which is error-prone. So it's sensible Java has no destructors available.

In case the objects hold open sockets, open files, or database connections, the garbage collector is not able to reclaim those resources. We can release the resources in close method and use try-finally syntax to call the method afterward before Java 7, such as the I/O classes FileInputStreamand FileOutputStream. As of Java 7, we can implement interface AutoCloseable and use try-with-resources statement to write shorter and cleaner code. But it's possible the API users forget to call the close method, so the finalize method and Cleaner class come into existence to act as the safety net. But please be cautioned they are not equivalent to the destructor.

It's not assured both the finalize method and Cleaner class will run promptly. They even get no chance to run before the JVM exits. Although we could call System.runFinalization to suggest that JVM run the finalize methods of any objects pending for finalization, it's still non-deterministic.

Moreover, the finalize method can cause performance issues, deadlocks etc. We can find more information by looking at one of our articles: A Guide to the finalize Method in Java.

A partir de Java 9 , se agrega la clase Cleaner para reemplazar el método de finalización debido a las desventajas que tiene. Como resultado, tenemos un mejor control sobre el hilo que realiza las acciones de limpieza.

Pero la especificación de Java señala que el comportamiento de los limpiadores durante System.exit es específico de la implementación y Java no ofrece garantías de si las acciones de limpieza se invocarán o no.