120+ Advanced Java Interview Questions and Answers

Answer:

Java is not a complete object-oriented language as it supports primitive data types & everything isn’t an object in it.

Answer:

Following are the key differences between default and parameterized constructors:

• A constructor that takes zero or no arguments is called a default constructor. A constructor that takes one or multiple arguments is called a parameterized constructor.
• In case no explicit constructor is defined in a class, the compiler inserts a default no-arg constructor post compilation. When defining a parameterized constructor, the programmer needs to explicitly define a default no-arg constructor, if required.
• When constructing new objects with the default constructor, there is no need to pass any parameters. When constructing new objects using argument constructors, you need to pass at least one or more parameters.
• A default constructor enables object initialization with the same data. A parameterized constructor helps to create distinct objects with different data.

Answer:

Java has two types of modifiers, namely: access modifiers & non-access modifiers. The access modifiers define the scope or accessibility of a method, field, class, or constructor. One can change the access level of a field, constructor, method, or class using the access modifier. The access modifiers further come in four types:

• Private Access Modifier: The access level of a private modifier is only within a class. Thus, it cannot be accessed outside a class.
• Default Access Modifier: A default modifier’s access level is only within the package; hence it isn’t accessible outside the package. If you don’t specify any access level, it will be a default.
• Protected Access Modifier: A protected modifier’s access level is within & outside the package through a child class. If you don’t make a child class, the protected modifier cannot be accessed outside the package.
• Public Access Modifier: A public modifier is accessible everywhere, be it within or outside the class & within or outside the package.

Answer:

An instance variable in Java is defined in a class but outside a method, constructor, or block. Its declaration is similar to the declaration of a local variable of methods, but:

• The variable is defined inside the class yet outside the methods;
• A private access modifier precedes the variable;
• The variable always initializes when an object is created, either implicitly or explicitly, by the constructor.

Answer:

Let us differentiate between these two:

• A class is a blueprint through which you can create an instance or object. An object is an instance that enables developers to use variables & methods inside the class.
• A class is used to bind data & methods together in a single unit. An object acts like a variable of the class.
• The classes have logical existence. The objects exist physically.
• When a programmer creates a class, it doesn’t take any memory spaces. When a programmer creates an object, it takes the memory space.
• The class is declared only once. An object can be declared many times based on the requirement.

Answer:

The following are the advantages & disadvantages of Java Sockets:

Advantages of Java Sockets:

• Java sockets are easy to implement & flexible for general communications.
• A socket causes low network traffic, unlike CGI scripts & HTML forms that generate and transfer web pages for each new request.

Disadvantages of Java Sockets:

• Socket-based communications enable only to send packets of raw data amongst applications.
• Both the server-side & client-side have to provide mechanisms to make the data useful in any way.

Answer:

There exist two ways of creating a thread in Java, which are as follows:

• Extend the Thread class- Create a thread through a new class that extends the Thread class & creates an instance of that class.
• Implement the Runnable Interface- The easiest & most efficient way of creating a thread is to create a class that implements the runnable interface.

Answer:

The immutable class simply refers to an unchangeable class whose content cannot be changed once created. In Java, all the wrapper classes such as Integer, String, Character, Boolean, Byte, Short, BigDecimal, Long, Float, Double, & BigInteger are immutable.  We can also create our immutable class in Java.

Below are the requirements for creating an immutable class:

• The class needs to be declared as final so that it cannot create child classes.
• Declare the data members in the class as private to restrain direct access.
• Also, declare the class’s data members as final so that its value becomes unchangeable after object creation.
• A parameterized constructor must initialize all the fields performing a deep copy to prevent data members’ modification with an object reference.
• Perform the deep copy of objects in the getter methods for returning a copy instead of returning the actual object reference.
• No setters to remove the option of changing an instance variable’s value.

Answer:

Pass by value is a copy in memory of the parameter’s value passed in a copy of the actual parameter’s contents. Whereas pass by reference or pass by address is a copy of the address of the actual parameter

Answer:

A Map is an interface that maps keys to the values; it cannot have duplicate keys as each key can map no more than one value. Java HashMap is a class that efficiently implements the Map interface.

Answer:

Following are the difference between a constructor and a method:

• A constructor creates and initializes an object. A method in Java executes certain statements.
• Constructors get implicitly invoked by the system. Methods get explicitly invoked during program code.
• A constructor doesn’t have any return type. A method does have a return type.
• A constructor initializes a nonexistent object. A method can only invoke the existing object.
• Constructors have the same name as that of a class. A method name has its own name & cannot have the same as that of a class.
• A subclass cannot inherit a constructor. A subclass can inherit a method.

Answer:

Major differences between multitasking & multithreading are as follow:

• Multitasking enables users to perform multiple tasks by CPU. In multithreading, several threads are created from a process that increases the computer power.
• Multitasking involves CPU switching between the tasks. Multithreading also involves CPU switching but between the threads.
• In multitasking, the processes share separate memory. In multithreading, the processes share the same memory.
• In multitasking, the CPU is used to execute many tasks at a time. In multithreading, the CPU is used to execute many threads at a time.
• The processes don’t share the same resources in multitasking. While, in multithreading, the processes share the same resources.
• The termination of a process takes more time in multitasking. The termination of a thread takes less time in multithreading.

Answer:

Key differences between multiprocessing & multithreading are:

• In Multiprocessing, multiple CPUs or processors are added to increase the computing power of a system. While, In Multithreading, multiple threads are created to execute in a parallel fashion to increase the throughput of a system.
• Multiprocessing is classified into types, symmetric & asymmetric. Whereas, there is no classification for multithreading.
• In multiprocessing, process creation is time-consuming. On the other hand, Thread creation is easy & time savvy in multithreading.

Answer:

A class, when declared with an abstract keyword, is called an abstract class. It may or may not have abstract methods without a body & cannot be instantiated.

Answer:

If you remove the static modifier from the main method, the program will compile but throw an error “NoSuchMethodError” at runtime.

Answer:

There are five types of inner classes in Java:

• A nested class is accessed like a package.
• A top-level inner class can implicitly access static variables only.
• Member inner class is like other member methods & variables.
• Local classes are similar to local variables. They are specific to a block of code & their scope is within the block only.
• Anonymous classes have no name & no constructor.

Answer:

Yes, the imports get checked for semantic validity during compile-time. Code containing import lines like java.lang.ABCD  will not compile. It will further throw an error saying, cannot resolve

• symbol: class ABCD
• location: package io
• import:io.ABCD;

Answer:

The variable declaration informs the compiler about the data type & a variable’s size, while the variable definition allocates the memory to a variable. It is possible to declare variables several times into the program. However, it can only be defined once in a program. The variable declaration is for the assignment of properties & identification to a variable. Whereas variable definition is for assignments of storage space to a variable.

Answer:

Any variable that does not assign a value, whether it is a local variable, static variable, or instance variable, will have a default value of null.

Answer:

No, a top-level class cannot be protected or private. It can either have public or no modifier. If the top-level class does not have a modifier, it should have a default access. If a top-level class is declared private, then the compiler will complain; the modifier private is not allowed here. It means that a top-level class cannot be private & protected at the same time.