Core Java - Interview Questions and Answers for 'Default' | Search Interview Question

Core Java - Interview Questions and Answers for 'Default' - 40 question(s) found - Order By Rating

Q1. Difference between Default VPC and Custom VPC.

Ans. Default VPC is provided by AWS whereas custom VPC needs to be created.

All subnets in default VPC have internet access whereas it's specified by user in Custom VPC.

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Q2. What is the default size of list ? How it is growable ?

Core Java

Ans. Default size is 10.
Regard how size will increase.

Inside add() method there is one method called ensureCapacity() which will check list has enough size available.

This ensureCapacity() method if found size of list need to increase then it will call grow() method which will create new list of extra size and copy existing list data to new one and return.

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Q3. Which Java version supports Interface Default methods ?

Core Java

Ans. Java 1.8 or Java 8

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Q4. Can we have a private default method ?

Core Java

Ans. No. The whole idea of default method is to provide a default definition in case the implementing class intend to provide definition for only some of the methods. Making any of the interface method private would restrict it from being implemented by the implementing class. So default method is an option to provide implementation and not restricting a new definition.

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Q5. What could be the reason for allowing private methods in java 9 ?

Core Java

Ans. Java 8 allowed for method implementation using default methods in interfaces. As those default methods could contain complex logic and might need organizing the logic into multiple methods, they have allowed for private methods.

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Q6. What are the advantage of Abstract classes over interfaces with respect to Java 7 ? and What changed in Java 8 to help facilitate that in Java 8 ?

Core Java

Ans. Abstract Classes provide default implementations of methods that are inherited by the classes that extend them, which was not the case for Interfaces. This changed in Java 8, where default implementations are provided for methods.

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Q7. Why can't we have diamond problem with interfaces ?

Core Java

Ans. Interfaces don't have member elements and method definitions that could cause diamond problem. With Java 8, Interfaces have default method definitions. This could have created diamond problem but Java introduced a compile time check for "duplicate default methods" in case same method is derived from multiple interfaces and no definition is overridden by the class.

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Q8. What will we get if we try to print local variable that hasn't been initialized ?

Core Java

Ans. It will print the garbage value as compiler doesn't provide a default value to local variables.

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Q9. Why Java doesn't provide a default constructor if we define an argument constructor ?

Core Java

Ans. Java provides a default constructor ( no argument ) if no constructor is provided by the programmer. Java makes a check during pre compilation if there is any constructor provided by the programmer. If any constructor is provided, Java understands that programmer has provided all the mechanism for initialization and construction of the object and may not even intend to create objects by calling a no argument constructor. For example - One may like to have objects created with self defined state otherwise restrict its creation.

We make the constructor private if we want to make a class singleton as we would like to restrict creation of new objects using new operator and default constructor. Similar could be the situation wherein we would like to construct objects in a particular manner only.

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Q10. What access the variable gets if we don't provide any access specifier ?

Core Java

Ans. It gets the default access i.e package protected.

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Q11. Is it mandatory to have a default constructor ?

Core Java

Ans. No.

Default constructor is provided by compiler if no constructor is provided ( argument or no argument ) by us.

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Q12. How to Change Default Boot Kernel in Linux ?

Unix

Ans. https://www.redhat.com/archives/rhl-list/2005-April/msg04167.html

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Q13. Does default methods introduce multiple inheritance and the Diamond problem in Java 8 ?

Core Java

Ans. Default methods results in multiple inheritance of behavior and not of state. In case we try to implement multiple interfaces with default method having same name and signature, and don't override it in implementation class, it will throw an error.

For example -

interface MyInterface {
public void default myMethod(){
}
}

interface MyInterface2 {
public void default myMethod(){
}
}

class MyClass implements MyInterface,MyInterface2 {
}

This code will compilation error "Duplicate Default Method"

if we specify the definition of myMethod() in myClass, compiler won't complain and there is no conflict and MyClass can use overridden definition. But if we don't override myMethod() in MyClass, Java would be in conflict as to what definition should be carried to MyClass and hence throws compilation error.

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Q14. What was the driving force to introduce default methods in Interfaces wef from Java 8 ?

Core Java

Ans. Earlier any class implementing an interface was supposed to implement all methods declared in an interface. There was no place for optionally implementing all or subset of methods.Though we have abstract classes wherein we could have provided such a mechanism by declaring some methods as abstract while providing definition for some. But as Abstract classes have a body and are comparatively heavier than interfaces and interfaces associate closely to the concept of providing interfacing than abstract classes, Java might have though of providing optional implementation for default methods. This way same interface can be reused in variety of ways rather than making copies of an interface to suit different needs.

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Q15. Difference between Public, Private, Default and Protected ?

Core Java

Ans. Private - Not accessible outside object scope.

Public - Accessible from anywhere.

Default - Accessible from anywhere within same package.

Protected - Accessible from object and the sub class objects.

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Q16. What are the default or implicitly assigned values for data types in java ?

Ans. boolean ---> false
byte ----> 0
short ----> 0
int -----> 0
long ------> 0l
char -----> /u0000
float ------> 0.0f
double ----> 0.0d
any object reference ----> null

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Q17. Why do member variables have default values whereas local variables don't have any default value ?

Core Java

Ans. member variable are loaded into heap, so they are initialized with default values when an instance of a class is created. In case of local variables, they are stored in stack until they are being used.

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Q18. What is a Default Constructor ?

Core Java

Ans. The no argument constructor provided by Java Compiler if no constructor is specified.

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Q19. Will Compiler creates a default no argument constructor if we specify only multi argument constructor ?

Ans. No, Compiler will create default constructor only if we don't specify any constructor.

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Q20. Difference between Class#getInstance() and new operator ?

Ans. Class.getInstance doesn't call the constructor whereas if we create an object using new operator , we need to have a matching constructor or copiler should provide a default constructor.

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Q21. Can we create an object if a Class doesn't have any constructor ( not even the default provided by constructor ) ?

Ans. Yes , using Class.getInstance.

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Q22. Does every class needs to have one non parameterized constructor ?

Core Java

Ans. No. Every Class only needs to have one constructor - With parameters or without parameters. Compiler provides a default non parameterized constructor if no constructors is defined.

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Q23. Variable of the boolean type is automatically initialized as?

Core Java

Ans. The default value of the boolean type is false.

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Q24. Why Java provides default constructor ?

Core Java

Ans. At the beginning of an object's life, the Java virtual machine (JVM) allocates memory on the heap to accommodate the object's instance variables. When that memory is first allocated, however, the data it contains is unpredictable. If the memory were used as is, the behavior of the object would also be unpredictable. To guard against such a scenario, Java makes certain that memory is initialized, at least to predictable default values before it is used by any code.

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Q25. In a case where there are no instance variables what does the default constructor initialize?

Core Java

Ans. Java expects the superclass ( Object Class ) constructor to be called while creation of any object. So super constructor is called in case there are no instance variables to initialize.

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Q26. What will be the output of following code ?

public static void main(String[] args)
{
int x = 10;
int y;
if (x < 100) y = x / 0;
if (x >= 100) y = x * 0;
System.out.println("The value of y is: " + y);
}

Ans. The code will not compile raising an error that the local variable y might not have been initialized. Unlike member variables, local variables are not automatically initialized to the default values for their declared type.

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Q27. What will be the output of following code ?
class BuggyBread2 {
   private static int counter = 0;
   void BuggyBread2() {
      counter = 5;
   }

   BuggyBread2(int x){
      counter = x;
   }

   public static void main(String[] args) {
      BuggyBread2 bg = new BuggyBread2();
      System.out.println(counter);
   }
}

Core Java

Ans. Compile time error as it won't find the constructor matching BuggyBread2().
Compiler won't provide default no argument constructor as programmer has already defined one constructor.
Compiler will treat user defined BuggyBread2() as a method, as return type ( void ) has been specified for that.

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Q28. If there is a conflict between Base Class Method definition and Interface Default method definition, Which definition is Picked ?

Ans. Base Class Definition.

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Rarely asked as default methods have been introduced with Java 8.

Q29. Can we have a default method without a Body ?

Core Java

Ans. No. Compiler will give error.

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Q30. Does java allow implementation of multiple interfaces having Default methods with Same name and Signature ?

Core Java

Ans. No. Compilation error.

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