questionsforinterviews.com

In Java, Arrays are type safe, meaning of that if we are declaring an array of int, only integers can be assigned to it, If it is a string array only strings are allowed. But for collections till Java 5 there was no type safe mechanism available. To make the collections type safe is known as making it generic type.

For example:

List someList= new ArrayList();

Above list will allow only Animal objects to be assigned to someList List.

Final class is a class which can’t be sub classed. Final keyword ensures, the class implementation is never overridden. Let us take one example. In java.lang the class String is a final class, It can’t be sub classed. The meaning of this the class implementation of String class shouldn’t be altered and this is achieved through final key word.

Example :
Package qfi;
Public final class QuestionsforInterviews{
.
.
.
}

Abstract class is a java class which can’t be instantiated. The meaning of this is, an object can’t be created from an abstract class. The lone purpose to create an abstract class is to give an abstract skeleton for a class. Just imagine a class Car. It shouldn’t be instantiated as this is class with quite generic properties. But this can be sub classed to a model of a brand like BMW which will have concrete properties, like color, horse power, seating capacity and so on.

A class will be considered if the keyword abstract is used or any of the method inside the class is abstract.

Example:
Package qfi;
abstract class Car{
..
..
}

Or

Package qfi;
Class Car{
Public abstract void speed();
}

If a method is declared as synchronized, it can be accessed by only one thread at a time. Synchronized modifier can be used with any of the three modifiers. This type of methods are generally created to make sure that a transaction can only happen by a single thread for a single source. For example, if bank transaction like fund transfer, fund withdraw will happen by several threads at a time consistency can’t be maintained. To avoid such scenarios, the method can be declared as synchronized.

Example: public synchronized void questionForInterviews{
..
..
}

In java language, this is always possible to create multiple methods with the same name, which is known as method overloading. Here the methods are called overloaded methods. This is one of the example how Java implements polymorphism.

Over loaded methods may have same return type or different return type, but they must differ in parameter type or number of parameters.

Initializing all the instance variable manually in a class during instancing process is a tedious process. But unless we initialize the variables, it is not possible to use it for any operation. As initialization process is mandatory, JVM provides a mechanism to initialize all the variables in a class. that automatic initialization is performed through the use of constructors.

Though constructors and methods looks same the following points makes them distinguishable.
i. Constructors have no return type.
ii. Constructors, always have the name exactly same as that of the class.
iii. Unlike methods, constructors are not designed to perform any particular task other than initialization.
iv. Constructors are called only once during the instantiation process.