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Interview Questions and Answers for 'Default' - 37 question(s) found - Order By Newest

Very Frequently asked across all type of companies and across all levels.
  Q1. Difference between Public, Private, Default and Protected ?Core Java
Anonymous
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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 Q2. What are the default or implicitly assigned values for data types in java ?
Anonymous
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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 Q3. Why do member variables have default values whereas local variables don't have any default value ?
Core Java
Anonymous
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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 Q4. 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
Admin
info@buggybread.com
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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 Q5. Why can't we have diamond problem with interfaces ?Core Java2017-07-30 08:15:33

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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Recently asked in Infoview Technologies.
 Q6. What is a Default Constructor ?Core Java
Anonymous
Ans. The no argument constructor provided by Java Compiler if no constructor is specified.

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 Q7. Will Compiler creates a default no argument constructor if we specify only multi argument constructor ?
Anonymous
Ans. No, Compiler will create default constructor only if we don't specify any constructor.

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 Q8. Difference between Class#getInstance() and new operator ?
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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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 Q9. Can we create an object if a Class doesn't have any constructor ( not even the default provided by constructor ) ?
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Ans. Yes , using Class.getInstance.

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 Q10. Does every class needs to have one non parameterized constructor ?Core Java
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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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 Q11. Variable of the boolean type is automatically initialized as?Core Java
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Ans. The default value of the boolean type is false.

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 Q12. Why Java provides default constructor ?Core Java
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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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 Q13. In a case where there are no instance variables what does the default constructor initialize?Core Java
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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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 Q14. 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);
}
Admin
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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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 Q15. If there is a conflict between Base Class Method definition and Interface Default method definition, Which definition is Picked ?
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Ans. Base Class Definition.

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Rarely asked as default methods have been introduced with Java 8.
 Q16. Can we have a default method without a Body ?Core Java
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Ans. No. Compiler will give error.

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 Q17. Does java allow implementation of multiple interfaces having Default methods with Same name and Signature ?Core Java
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Ans. No. Compilation error.

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  Q18. What are Default Methods ?Core Java
Admin
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Ans. With Java 8, We can provide method definitions in the Interfaces that gets carried down the classes implementing that interface in case they are not overridden by the Class. Keyword "default" is used to mark the default method.

  Sample Code for interface default

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Rarely asked as default methods have been introduced with Java 8.
 Q19. Can we have a default method definition in the interface without specifying the keyword "default" ? Core Java
Admin
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Ans. No. Compiler complains that its an abstract method and hence shouldn't have the body.

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 Q20. Can a class implement two Interfaces having default method with same name and signature ?

public interface DefaultMethodInterface {
default public void defaultMethod(){
System.out.println("DefaultMethodInterface");
}
}

public interface DefaultMethodInterface2 {
default public void defaultMethod(){
System.out.println("DefaultMethodInterface2");
}
}

public class HelloJava8 implements DefaultMethodInterface,DefaultMethodInterface2 {
public static void main(String[] args){
DefaultMethodInterface defMethIn = new HelloJava8();
defMethIn.defaultMethod();
}
}
Admin
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Ans. No. Compiler gives error saying "Duplicate Default Methods"

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 Q21. What If we make the method as abstract in another Interface ?

public interface DefaultMethodInterface {
default public void defaultMethod(){
System.out.println("DefaultMethodInterface");
}
}

public interface DefaultMethodInterface2 {
public void defaultMethod(){
System.out.println("DefaultMethodInterface2");
}
}

public class HelloJava8 implements DefaultMethodInterface,DefaultMethodInterface2 {
public static void main(String[] args){
DefaultMethodInterface defMethIn = new HelloJava8();
defMethIn.defaultMethod();
}
}
Admin
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Ans. Even then the Compiler will give error saying that there is a conflict.

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 Q22. What if we override the conflicting method in the Class ?

public interface DefaultMethodInterface {
default public void defaultMethod(){
System.out.println("DefaultMethodInterface");
}
}

public interface DefaultMethodInterface2 {
default public void defaultMethod(){
System.out.println("DefaultMethodInterface2");
}
}

public class HelloJava8 implements DefaultMethodInterface,DefaultMethodInterface2 {
public static void main(String[] args){
DefaultMethodInterface defMethIn = new HelloJava8();
defMethIn.defaultMethod();
}

public void defaultMethod(){
System.out.println("HelloJava8");
}
}
Admin
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Ans. There won't be any error and upon execution the overriding class method will be executed.

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 Q23. What will happen if there is a default method conflict as mentioned above and we have specified the same signature method in the base class instead of overriding in the existing class ?
Admin
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Ans. There won't be any problem as the Base class method will have precedence over the Interface Default methods.

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 Q24. If a method definition has been specified in Class , its Base Class , and the interface which the class is implementing, Which definition will be picked if we try to access it using Interface Reference and Class object ?
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Ans. Class method definition is overriding both the definitions and hence will be picked.

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 Q25. If a method definition has been specified in the Base Class and the interface which the class is implementing, Which definition will be picked if we try to access it using Interface Reference and Class object ?
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Ans. Base Class Definition will have precedence over the Interface Default method definition.

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 Q26. Can we have default method with same name and signature in the derived Interface as the static method in base Interface and vice versa ?
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Ans. Yes , we can do that as static methods are not accessible using references and hence cannot lead to conflict. We cannot do inverse as Default methods cannot be overridden with the static methods in derived interface.

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Not frequently asked as it was introduced with Java 8.
 Q27. What is the @FunctionalInterface annotation ?Core Java
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Ans. This is an informative annotation that specify that the interface is a functional interface. A Function Interface has only one abstract method and many default methods. Compiler generates an error if the interface specified with the annotation doesn't abide by the specifications for functional interface.

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 Q28. What was the driving force to introduce default methods in Interfaces wef from Java 8 ?Core Java2016-07-14 14:36:49

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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 Q29. Does default methods introduce multiple inheritance and the Diamond problem in Java 8 ?Core Java2016-07-20 17:16:28

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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 Q30. How to Change Default Boot Kernel in Linux ?Unix2016-11-11 11:31:08

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

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 Q31. Is it mandatory to have a default constructor ?Core Java2017-01-11 15:54:39

Ans. No.

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

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 Q32. What access the variable gets if we don't provide any access specifier ?Core Java2017-02-12 09:33:06

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

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 Q33. Why Java doesn't provide a default constructor if we define an argument constructor ?Core Java2017-03-10 09:58:43

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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 Q34. What will we get if we try to print local variable that hasn't been initialized ?Core Java2017-04-04 14:40:31

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

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 Q35. 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 Java2017-08-17 19:28:33

 This question is still unanswered. Can you please provide an answer.


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 Q36. What will be the output of following code ?

public class BuggyBread {
   
   private int x;
   private int y;
   
   BuggyBread(int x,int y){};
   
   public static void main(String[] args){
      BuggyBread buggybread = new BuggyBread();
      System.out.println(buggybread.x);
   }
}
Core Java
a. 0
b. null
c. compilation error due to uninitialized element
d. compilation error due to constructor

Ans.d. compilation error due to constructor

 Q37. What will be the output of following ?

public class BuggyBread {

   private int x;
   private Integer y;

   BuggyBread(int x,int y){};

   public static void main(String[] args){
      BuggyBread buggybread = new BuggyBread(1,2);
      System.out.println(buggybread.x);
      System.out.println(buggybread.y);
   }
}
Core Java
a. 0 0
b. 0 null
c. null 0
d. null null

Ans.b. 0 null



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