Core Java Interview Questions:
1) What is Java?
2) What do you understand by Java virtual machine?
is a virtual machine that enables the computer to run the Java program. JVM acts like a run-time engine which calls the main method present in the Java code. JVM is the specification which must be implemented in the computer system. The Java code is compiled by JVM to be a Bytecode which is machine independent and close to the native code.
3) What is the difference between JDK, JRE, and JVM?
JVM
JRE
JDK
4) How many types of memory areas are allocated by JVM?
- Class(Method) Area: Class Area stores per-class structures such as the runtime constant pool, field, method data, and the code for methods.
- Heap: It is the runtime data area in which the memory is allocated to the objects
- Stack: Java Stack stores frames. It holds local variables and partial results, and plays a part in method invocation and return. Each thread has a private JVM stack, created at the same time as the thread. A new frame is created each time a method is invoked. A frame is destroyed when its method invocation completes.
- Program Counter Register: PC (program counter) register contains the address of the Java virtual machine instruction currently being executed.
- Native Method Stack: It contains all the native methods used in the application.
5) What are the various access specifiers in Java?
In Java, access specifiers are the keywords which are used to define the access scope of the method, class, or a variable. In Java, there are four access specifiers given below.
- Public The classes, methods, or variables which are defined as public, can be accessed by any class or method.
- Protected Protected can be accessed by the class of the same package, or by the sub-class of this class, or within the same class.
- Default Default are accessible within the package only. By default, all the classes, methods, and variables are of default scope.
- Private The private class, methods, or variables defined as private can be accessed within the class only.
6) What is the purpose of static methods and variables?
The methods or variables defined as static are shared among all the objects of the class. The static is the part of the class and not of the object. The static variables are stored in the class area, and we do not need to create the object to access such variables. Therefore, static is used in the case, where we need to define variables or methods which are common to all the objects of the class.
For example, In the class simulating the collection of the students in a college, the name of the college is the common attribute to all the students. Therefore, the college name will be defined as static.
7) What are the advantages of Packages in Java?
There are various advantages of defining packages in Java.
- Packages avoid the name clashes.
- The Package provides easier access control.
- We can also have the hidden classes that are not visible outside and used by the package.
- It is easier to locate the related classes.
8) What is Class in Java?
9) Difference between abstract class and interface
Abstract class and interface both are used to achieve abstraction where we can declare the abstract methods. Abstract class and interface both can't be instantiated.
But there are many differences between abstract class and interface that are given below.
| Abstract class | Interface |
|---|---|
| 1) Abstract class can have abstract and non-abstract methods. | Interface can have only abstract methods. Since Java 8, it can have default and static methods also. |
| 2) Abstract class doesn't support multiple inheritance. | Interface supports multiple inheritance. |
| 3) Abstract class can have final, non-final, static and non-static variables. | Interface has only static and final variables. |
| 4) Abstract class can provide the implementation of interface. | Interface can't provide the implementation of abstract class. |
| 5) The abstract keyword is used to declare abstract class. | The interface keyword is used to declare interface. |
| 6) An abstract class can extend another Java class and implement multiple Java interfaces. | An interface can extend another Java interface only. |
| 7) An abstract class can be extended using keyword "extends". | An interface can be implemented using keyword "implements". |
| 8) A Java abstract class can have class members like private, protected, etc. | Members of a Java interface are public by default. |
| 9)Example: public abstract class Shape{ public abstract void draw(); } | Example: public interface Drawable{ void draw(); } |
Simply, abstract class achieves partial abstraction (0 to 100%) whereas interface achieves fully abstraction (100%).
10) Java OOPs Concepts
- Inheritance
- Polymorphism
- Abstraction
- Encapsulation
Inheritance
When one object acquires all the properties and behaviors of a parent object, it is known as inheritance. It provides code reusability. It is used to achieve runtime polymorphism.
Polymorphism
If one task is performed in different ways, it is known as polymorphism. For example: to convince the customer differently, to draw something, for example, shape, triangle, rectangle, etc.
In Java, we use method overloading and method overriding to achieve polymorphism.
Another example can be to speak something; for example, a cat speaks meow, dog barks woof, etc.
Abstraction
Hiding internal details and showing functionality is known as abstraction. For example phone call, we don't know the internal processing.
In Java, we use abstract class and interface to achieve abstraction.
Encapsulation
Binding (or wrapping) code and data together into a single unit are known as encapsulation. For example, a capsule, it is wrapped with different medicines.
A java class is the example of encapsulation. Java bean is the fully encapsulated class because all the data members are private here.
11) Aggregation
Aggregation is a way to achieve Association. Aggregation represents the relationship where one object contains other objects as a part of its state. It represents the weak relationship between objects. It is also termed as a has-a relationship in Java. Like, inheritance represents the is-a relationship. It is another way to reuse objects.
12) Association
Association represents the relationship between the objects. Here, one object can be associated with one object or many objects. There can be four types of association between the objects:
- One to One
- One to Many
- Many to One, and
- Many to Many
Let's understand the relationship with real-time examples. For example, One country can have one prime minister (one to one), and a prime minister can have many ministers (one to many). Also, many MP's can have one prime minister (many to one), and many ministers can have many departments (many to many).
Association can be undirectional or bidirectional.
Final Keyword In Java
The final keyword in java is used to restrict the user. The java final keyword can be used in many context. Final can be:
- variable
- method
- class
The final keyword can be applied with the variables, a final variable that have no value it is called blank final variable or uninitialized final variable. It can be initialized in the constructor only. The blank final variable can be static also which will be initialized in the static block only. We will have detailed learning of these. Let's first learn the basics of final keyword.
Java final variable
If you make any variable as final, you cannot change the value of final variable(It will be constant).
Java final method
Java final class
If you make any class as final, you cannot extend it.
Comments
Post a Comment