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 Q151. What are different types of variables a class can have ?Core Java
Ans. 1. Method local variables - These are declared and defined within a method ( instance or static methods ) and their scope is limited to the method itself. They are destructed once the execution of method completes. They are stored in stack memory.

2. Instance variables - These are declared as non static variables as part of the class.They are initialized as part of object creation ( constructor ) and are destructed by java's garbage collection mechanism and hence stored in heap.

3. Static variables - These are declared with the static keyword and are part of the class. They are initialized at the time of class loading and are destructed by java's garbage collection mechanism and hence stored in heap.

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  Q152. Why java doesn't support multiple Inheritance ?

or

Explain Java Diamond Problem.		Core Java
Ans. class A {
void test() {
System.out.println("test() method");
}
}

class B {
void test() {
System.out.println("test() method");
}
}

Suppose if Java allows multiple inheritance like this,

class C extends A, B {
}

A and B test() methods are inheriting to C class.

So which test() method C class will take? As A & B class test() methods are different , So here we would Facing Ambiguity.

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 Q153. Design an app to find 10 most visited urlsCore Java
 This question was recently asked at 'Wayfair'.This question is still unanswered. Can you please provide an answer.


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 Q154. Trim a string without using String library methodCore Java
 This question was recently asked at 'Ariba'.This question is still unanswered. Can you please provide an answer.


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Usually asked to entry level software developers.
  Q155. Write a program to swap two variables without using thirdCore Java
Ans. public static void main(String[] args) {
   int num1 = 1;   
   int num2 = 2;
   num1 = num1^num2;
   num2 = num1^num2;
   num1 = num1^num2;
   System.out.print("num1 = " + num1 +", num2 = "+num2);
}

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 Q156. Write a Program to print pattern like following

*
**
***
****
*****
*****
****
***
**
*		Core Java
Ans. public class BuggyBread {
   public static void main(String args[]) {
      for(int x=1;x<=5;x++){
         for(int y=1;y<=x;y++){
            System.out.print("*");
         }
         
         System.out.println("");
      }
      
      for(int x=5;x>=1;x--){
         for(int y=1;y<=x;y++){
            System.out.print("*");
         }
         
         System.out.println("");
      }   
   }
}

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 Q157. If an Abstract class has only abstract methods, What's the difference between such a class and an interface ?Core Java
Ans. Such a class still can have member elements which can be inherited and hence facilitate code reuse. Moreover Abstract class can have non final static elements whereas interfaces are only allowed to have static final elements.

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   Like         Discuss         Correct / Improve     abstract class   interfaces     Asked in 2 Companies      Basic

 Q158. Can we overload method as following ?

void method(int... x){};
void method(int[] x){}; 		Core Java
Ans. No. Because java internally treats var args as arrays and hence both method declarations will generate the same byte code and hence would result in ambiguity while determining call binding.

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   Like         Discuss         Correct / Improve     var args  methods  functions   method declaration  scjp  ocjp

 Q159. What does import java.util.* means in java ? Core Java
Ans. It means import all the classes and interfaces within java.util package and make them available to use within the current class or interface. This is shorthand wild card annotation for importing all classes within a particular package. This won't import the classes within the sub packages of java.util.

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 Q160. When we say final x = 10, is the reference final or the value ?Core Java
Ans. final keyword have meaning only to referenced and not the value. It means that the specified reference cannot be dereferenced. It doesn't control the value assigned to the memory that's being referenced. This is the reason that final object references doesn't mean that the object is immutable but means that the reference cannot be changed to point to new object.

In case of primitive types too, when we assign a reference to another, values are passed and not the object reference, and hence a new placeholder is created in memory with the same value. That is why final to that context means that you cannot change the assigned memory and there is no way we can have that memory place have another value.

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 Q161. What is the difference between uninitialized values for primitive types and Wrapper classes ? Core Java
Ans. When we create reference for primitive type, it's memory is allocated. So even if we don't assign any value to it, the default value is initialized.

int x; // default value 0 initialized

But for object references, it's different as references hold nothing till an object is assigned.

Object obj; // contains null

So all primitive types when declared , contains their respective default values on the basis of their type whereas all wrapper class References contains null irrespective of their types.

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 Q162. What is the use of Object class ?Core Java
Ans. 1. Default definition for some of the methods , like equals, hashcode etc that gets carried to all objects even if you don't define anything. But that default definition is kind of like assigning 0 to integer, it just provide a safe state , nothing much for comparison.

2. Places where you have no idea about what object you may receive and just want to perform out of 8 basic methods of object class, something like printing their string representation ( defined by their toString method ), or equality ( using their equals method )

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 Q163. How Java 9 would help resolve NoClassDefFoundError through it's modularity ?Core Java
Ans. Java 9 would provide an error at compile time if there are different modules with the same package mapped to the same class loader and hence would provide a compile time check for this issue.

NoClassDefFoundError exists because of ambiguity at realtime because of multiple versions of same package / class exist. As Java 9 would provide a compile time check for it through clearly defined dependencies and exports and a check on duplicate packages being loaded by single class loader, it will fix the problem.

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   Like         Discuss         Correct / Improve     java 9  java 9 modules

 Q164. Differences between class not found exception and noclassdef found error ?Core Java
Ans. ClassNotFoundException is an exception that occurs when we try to load a class at run time using Class.forName() or loadClass() methods and mentioned classes are not found in the classpath.

NoClassDefFoundError is an error that occurs when a particular class is present at compile time, but was missing at run time.

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 Q165. serialization and deserialization of singleton classCore Java
Ans. The problem with serialized singleton class is that whenever we deserialize it, it will create a new instance of the class. To overcome this scenario all we need to do is to provide the implementation of readResolve() method.

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 Q166. Can I import same package/class twice? Will the JVM load the package twice at runtime?Core Java
Ans. One can import the same package or same class multiple times. Neither compiler nor JVM complains wil complain about it. And the JVM will internally load the class only once no matter how many times you import the same class.

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 Q167. Which access specifier can be used with Class ?Core Java
Ans. For top level class we can only use "public" and "default". We can use private with inner class.

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 Q168. Different ways of implementing Threads in Java ?Core Java
Ans. Threads in Java can be implement either by Extending Thread class or implementing runnable interface.

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 Q169. What is a Final Method ?Core Java
Ans. A Method that cannot be overriden in the sub class.

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Very frequently asked. Usually asked with questions related to String. Frequently asked at CTS / Cognizant
  Q170. What is an Immutable Object ?Core Java
Ans. Object that can't be changed after instantiation.

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 Q171. Does Declaring an object "final" makes it immutable ?Core Java
Ans. Only declaring variables as final makes them immutable. Making objects final means that the object handler cannot be used to target some other object but the object is still mutable.

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 Q172. Can we override static methods ? Why ?Core Java
Ans. No.

Static methods belong to the class and not the objects. They belong to the class and hence doesn't fit properly for the polymorphic behavior.

A static method is not associated with any instance of a class so the concept of overriding for runtime polymorphism using static methods is not applicable.

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 Q173. Can we reduce the visibility of the inherited or overridden method ?Core Java
Ans. No.

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 Q174. Name few Java Exceptions ?Core Java
Ans. IndexOutofBoundException ,
NoClassDefFoundException ,
OutOfMemoryException ,
IllegalArgumentException,
NullPointerException,
IllegalStateException

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 Q175. Difference between boolean and Boolean ?Core Java
Ans. boolean is a primitive type whereas Boolean is a class.

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 Q176. Difference between Serialization and Deserialization ?Core Java
Ans. Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.

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 Q177. What is an Enum type ?Core Java
Ans. An enum type is a special data type that enables for a variable to be a set of predefined constants

  Sample Code for enum

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 Q178. What are Wrapper Classes ? What are Primitive Wrapper Classes ?
Core Java
Ans. A wrapper class is any class which "wraps" or "encapsulates" the functionality of another class or component. A Wrapper Class that wraps or encapsulates the primitive data type is called Primitive Wrapper Class.

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 Q179. What is "Import" used for ?Core Java
Ans. Enables the programmer to abbreviate the names of classes defined in a package.

  Sample Code for import

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 Q180. Different types of memory used by JVM ?Core Java
Ans. Class , Heap , Stack , Register , Native Method Stack.

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