Java Interview Questions and Answers covered in this post:
Explain the Term Association, Composition, Aggregation? What is HAS-A
and IS-A Relationship?
-
Association:
Creating the relationship between two objects is referred to as
Association.
Ø
Association can be One-to-One, One-to-Many, Many-to-One,
Many-to-Many.
Ø
Aggregation and Composition are two forms of Association.
Ø
One-to-One = One Student can have one Class.
Ø
One-to-Many = One Student can have multiple Subjects.
Ø
Many-to-One = Many developers can have one Project
Manager.
Ø
Many-to-Many = Multiple Employees can have Multiple
Tasks.
Ø
Example of Association. We have created an association
between Company and Employees.
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package
simplifiedjava.crackedInterview;
public
class
Company {
private
String companyName;
public
Company(String companyName) {
this.companyName
= companyName;
}
public
String getCompanyName() {
return
companyName;
}
}
package
simplifiedjava.crackedInterview;
public
class
Employee {
private
String empName;
public
Employee(String empName) {
this.empName
= empName;
}
public
String getEmpName() {
return
empName;
}
}
package
simplifiedjava.crackedInterview;
public
class
AssociationExample {
public
static
void
main(String[] args) {
Employee employee1
= new
Employee("Rusty");
Company company
= new
Company("Company");
System.out.println(employee1.getEmpName() + " is working for "+ company.getCompanyName());
}
}
OUTPUT : Rusty is working for Company. |
- Composition:
ü
The composition can be called the HAS-A relationship.
ü
When two objects are tightly coupled with each other or we
can say when one object is dependent on another object is called
Composition.
ü
E.g. Engine cannot survive without a car. If a car exists then an engine exists. If a car is destroyed automatically
engine gets destroyed.
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package simplifiedjava.crackedInterview; public class Engine {
private
String
engineName;
private
String
power; private String modelNo;
// getter and Setter methods.
}
package
simplifiedjava.crackedInterview;
public
class
Car {
private
String
carName;
private
String
carModel; private String manufacturer; private Engine engine; // Car HAS-A reference of Engine.
public
Car(String carName, String carModel, String manufacturer, Engine engine) {
super();
this.carName
= carName;
this.carModel
= carModel;
this.manufacturer
= manufacturer;
engine
= new
Engine();
}
// getter and Setter
} |
-
Aggregation:
o
Aggregation can be called the HAS-A relationship.
o
When two objects are loosely coupled or we can say when one
object is not dependent on another object is called Aggregation.
o
E.g. We have two objects. The first one is College and the
second one is Student. If college gets destroyed still Student objects can
survive independently.
|
package simplifiedjava.crackedInterview; public class Student {
private
int
id;
private
String name;
private
String standard;
// getters and Setters method.
} package simplifiedjava.crackedInterview;
import
java.util.ArrayList; import java.util.List;
public
class
College {
List<Student> studentList
= new
ArrayList<Student>();
// add and remove Students.
} |
What are the uses of this keyword? Where it is applicable and what
are the rules to use this keywords?
- this keyword is used to refer to the current object.
- this keyword can be used to the invoked current class
constructor.
- this keyword can be used to the invoked current method.
- this can be used to return the current class object.
- this can be used to pass as an argument in method as well
as constructor.
 
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What are the wrapper classes in java? Why do we require wrapper
classes?
- Wrapper class is a class whose objects are wraps or
contains primitive data types.
- Wrapper class hold primitive values and is represented as
an object.
- All primitives has its own wrapper class.
- E.g. int has Integer, float has Float, double has Double
etc.
- In the collection framework if we wanted to represent primitive as an object for manipulation then wrapper class can be used.
What is SCP(String Constant Pool)?
- String constant pool is a separate place in heap memory
where all the String objects are stored.
- If you create an object using a new keyword then the object
gets created on heap memory and keep the same string object into string
constant pool.
- If you create an object without a new keyword then the
object doesn’t create on heap memory that object will be created on string
constant pool.
- If an object exists then the reference variable pointing to
an existing object instead of creating a new one.
What is a re-throwing exception in java?
- Exception throws from try block and catches it in the catch
block. A caught exception that can be thrown from the catch block is
called re-throwing the exception.
- Re-thrown exception must handle somewhere in the program
otherwise your program will be terminated abruptly.
- Please refer to the below example.
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package simplifiedjava.crackedInterview; public class NestedTryCatchDemo {
public static void
main(String[] args) {
try { int no = 10 / 0;
}catch(Exception e) {
throw new
NullPointerException();
}
}
}
Output:
Exception in thread "main" java.lang.NullPointerException
at simplifiedjava.crackedInterview.NestedTryCatchDemo.main(NestedTryCatchDemo.java:10) |
What happen if you don't override run() method on Thread class?
- If you don’t override the run() method it will execute the
run method of the Thread class that has empty implementation. The compiler
won’t throw any exception.
What is blocking queue?
- A Queue that additionally supports operations that wait for
the queue to become non-empty when retrieving an element, and wait for
space to become available in the queue when storing an element.
- A BlockingQueue does not accept null elements.
- Implementations throw NullPointerException on attempts to
add, put or offer a null.
- A BlockingQueue may be capacity bounded.
- At any given time it may have a remaining Capacity beyond
which no additional elements can be put without blocking. A BlockingQueue
without any intrinsic capacity constraints always reports a remaining
capacity of Integer.MAX_VALUE.
- BlockingQueue implementations are designed to be used
primarily for producer-consumer queues, but additionally support the
Collection interface.
- BlockingQueue implementations are thread-safe.
Which collection classes implements RandomAccess interface? What are
the benefits of RandomAccess interface?
- ArrayList and Vector are implemented Random Access
Interface.
- The presence of a marker interface is that it indicates (
or expects ) specific behaviour from the implementing class.
- So, in our case, ArrayList implements the RandomAccess
marker interface.
- So,
the expectation from the ArrayList class, is that it should produce
RandomAccess behaviour to the clients of the ArrayList class when the
client wants to access some element at some index.
Can you list out some functional interface used in java 6?
- FunctionalInterface must have only one abstract method.
- We have used 4 functional interfaces.
1. Runnable: Runnable interface has run() method.
2. Callable: Callable interface has call() method.
3. ActionListener: ActionListener has actionPerformed()
method.
4. Comparable: Comparable has compareTo() method.
What is Constructor Reference? How will you implements Constructor
reference?
- Constructor References in Java 8 can be created using the
Class Name and the keyword new keyword.
- Syntaxes for Constructor reference.
1. Integer::new;
2. String::new;
3. ArrayList::new;
4. Employee::new;
-
Please refer to the below example.
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package simplifiedjava.crackedInterview.java8;
@FunctionalInterface
public interface
StudentInterface {
public
Student get(String name); } package simplifiedjava.crackedInterview.java8; public class Student { private String name;
public
Student(String name) {
super();
this.name
= name;
System.out.println("Student Name is : "+ name); }
public static void
main(String[] args) {
StudentInterface stuInt
= Student :: new;
stuInt.get("Shruti");
}
}
Output: Student Name is : Shruti |
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