Most Important Core Java Interview Questions Answers

 1.) What is serialization?

Quite simply, object serialization provides a program the ability to read or write a whole object to and from a raw byte stream. It allows Java objects and primitives to be encoded into a byte stream suitable for streaming to some type of network or to a file system, or more generally, to a transmission medium or storage facility. A serializable object must implement the Serializable interface. We use Object Output Stream to write this object to a stream and Object Input Stream to read it from the stream.

2.) Is synchronized a modifier? identifier?? what is it??

It's a modifier. Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

3.) What is a singleton class? where is it used?

Singleton is a design pattern meant to provide one and only one instance of an object. Other objects can get a reference to this instance through a static method (class constructor is kept private). Why do we need one? Sometimes it is necessary, and often sufficient, to create a single instance of a given class. This has advantages in memory management, and for Java, in garbage collection. Moreover, restricting the number of instances may be necessary or desirable for technological or business reasons--for example, we may only want a single instance of a pool of database connections.

4.) Why java does not have multiple inheritance?

The Java design team strove to make Java:

• Simple, object-oriented, and familiar

• Robust and secure

• Architecture neutral and portable

• High performance

• Interpreted, threaded, and dynamic

The reasons for omitting multiple inheritance from the Java language mostly stem from the "simple, object-oriented, and familiar" goal. As a simple language, Java's creators wanted a language that most developers could grasp without extensive training. To that end, they worked to make the language as similar to C++ as possible (familiar) without carrying over C++'s unnecessary complexity (simple).

In the designers' opinion, multiple inheritance causes more problems and confusion than it solves. So they cut multiple inheritance from the language (just as they cut operator overloading). The designers' extensive C++ experience taught them that multiple inheritance just wasn't worth the headache.

5.)Why java is not 100% oops?

Many people say this because Java uses primitive types such as int, char, double. But then all the rest are objects. Confusing question.

6.) What is a resource bundle?

In its simplest form, a resource bundle is represented by a text file containing keys and the text value for each key.

7.) What is a transient variable?

The transient variables can't be serialized. For example, if a variable is declared as transient in a Serializable class and the class is written to an Object Stream, the value of the variable can't be written to the stream instead when the class is retrieved from the Object Stream the value of the variable becomes null.

What are Packages in core java and how to use them ?

  A Java package is a technique for organizing Java classes into namespaces similar to the modules of Modula, providing modular programming in Java. Java packages can be stored in compressed files called JAR files, allowing classes to be downloaded faster as groups rather than individually.

Packages are used in Java in order to prevent naming conflicts, to control access, to make searching/locating and usage of classes, interfaces, enumerations and annotations easier, etc.

A Package can be defined as a grouping of related types(classes, interfaces, enumerations and annotations ) providing access protection and namespace management.

Some of the existing packages in Java are::

·        java. lang - bundles the fundamental classes

·        java.io - classes for input, output functions are bundled in this package

Programmers can define their own packages to bundle groups of classes/interfaces, etc. It is a good practice to group related classes implemented by you so that a programmer can easily determine that the classes, interfaces, enumerations, annotations are related.

Since the package creates a new namespace there won't be any name conflicts with names in other packages. Using packages, it is easier to provide access control and it is also easier to locate the related classes.

Creating a package:

When creating a package, you should choose a name for the package and put a package statement with that name at the top of every source file that contains the classes, interfaces, enumerations, and annotation types that you want to include in the package.

The package statement should be the first line in the source file. There can be only one package statement in each source file, and it applies to all types in the file.

If a package statement is not used then the class, interfaces, enumerations, and annotation types will be put into an unnamed package.

 

What are Packages in java and how to use them?

 OOPS CONCEPT

Packages in Java are a mechanism to encapsulate a group of classes, interfaces, and sub-packages. Many implementations of Java use a hierarchical file system to manage source and class files. It is easy to organize class files into packages. All we need to do is put related class files in the same directory, give the directory a name that relates to the purpose of the classes, and add a line to the top of each class file that declares the package name, which is the same as the directory name where they reside.

In java, there are already many predefined packages that we use while programming.

For example: java.lang, java.io, java.util etc.

However one of the most useful features of java is that we can define our own packages

Advantages of using a package:

Before discussing how to use them Let see why we should use packages.

§  Reusability:  Reusability of code is one of the most important requirements in the software industry. Reusability saves time, effort and also ensures consistency. A class once developed can be reused by any number of programs wishing to incorporate the class in that particular program.

§  Easy to locate the files.

§  In a real-life situation, there may arise scenarios where we need to define files of the same name. This may lead to “name-space collisions”. Packages are a way of avoiding “name-space collisions”.

Types of packages:

1) User-defined package: The package we create is called a user-defined package.
2) Built-in package: The already defined package like java.io.*, java.lang.* etc are known as built-in packages.

Defining a Package:

This statement should be used at the beginning of the program to include that program in that particular package.

package  <package name>;

 

Most common Core Java Interview Questions

1) What is the difference between an Abstract class and an Interface?

1. Abstract classes may have some executable methods and methods left unimplemented. Interfaces contain no implementation code.

2. An class can implement any number of interfaces, but subclass at most one abstract class.

3. An abstract class can have nonabstract methods. All methods of an interface are abstract.

4. An abstract class can have instance variables. An interface cannot.

5. An abstract class can define the constructor. An interface cannot.

6. An abstract class can have any visibility: public, protected, private or none (package). An interface's visibility must be public or none (package).

7. An abstract class inherits from Object and includes methods such as clone() and equals().

2) What are checked and unchecked exceptions?

Java defines two kinds of exceptions :

• Checked exceptions: Exceptions that inherit from the Exception class are checked exceptions. Client code has to handle the checked exceptions thrown by the API, either in a catch clause or by forwarding it outward with the throws clause. Examples - SQLException, IOException.

• Unchecked exceptions: RuntimeException also extends from Exception. However, all of the exceptions that inherit from RuntimeException get special treatment. There is no requirement for the client code to deal with them, and hence they are called unchecked exceptions. Example Unchecked exceptions are NullPointerException, OutOfMemoryError, DivideByZeroException typically,programming errors.

3) What is the difference between C++ & Java?

Well as Bjarne Stroustrup says "..despite the syntactic similarities, C++ and Java are very different languages. In many ways, Java seems closer to Smalltalk than to C++..". 

Here are a few I discovered:

• Java is multithreaded

• Java has no pointers

• Java has automatic memory management (garbage collection)

• Java is platform-independent (Stroustrup may differ by saying "Java is a platform"

• Java has built-in support for comment documentation

• Java has no operator overloading

• Java doesn’t provide multiple inheritance.

• There are no destructors in Java

4) What are the statements in JAVA?

Statements are equivalent to sentences in natural languages. A statement forms a complete unit of execution. The following types of expressions can be made into a statement by terminating the expression with a semicolon

• Assignment expressions

• Any use of ++ or --

• Method calls

• Object creation expressions

These kinds of statements are called expression statements. In addition to these kinds of expression statements, there are two other kinds of statements. A declaration statement declares a variable. A control flow statement regulates the order in which statements get executed. The for loop and the if statement are both examples of control flow statements.

5) What is the JAR file?

JavaARchive files are a big glob of Java classes, images, audio, etc., compressed to make one simple, smaller file to ease Applet downloading. Normally when a browser encounters an applet, it goes and downloads all the files, images, audio, used by the Applet separately. This can lead to slower downloads.