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

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Q131. Can we call the garbage collector explicitly ?

Ans. Yes, We can call garbage collector directly but it doesn't guarantees that the gc will start executing immediately. This gc( ) method appears in both Runtime and System classes of java.lang package.

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Q132. Similarity and Difference between static block and static method ?

Core Java

Ans. Both belong to the class as a whole and not to the individual objects. Static methods are explicitly called for execution whereas Static block gets executed when the Class gets loaded by the JVM.

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Q133. How Java provide high Performance ?

Core Java

Ans. Java uses Just-In-Time compiler to enable high performance. Just-In-Time compiler is a program that turns Java bytecode into instructions that can be sent directly to the processor.

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Q134. What is constructor chaining and how is it achieved in Java?

Core Java

Ans. A child object constructor always first needs to construct its parent. In Java it is done via an implicit call to the no-args constructor as the first statement

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Q135. 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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Q136. What will be the output of following code ?

public static void main(String[] args){
String parent = null;
File file = new File(parent, "myfile.txt");
try {
file.createNewFile();
} catch (IOException e) {
e.printStackTrace();
}
}

Core Java

Ans. It will create the file myfile.txt in the current directory.

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Q137. What is comparator interface used for ?

Core Java

Ans. The purpose of comparator interface is to compare objects of the same class to identify the sorting order. Sorted Collection Classes ( TreeSet, TreeMap ) have been designed such to look for this method to identify the sorting order, that is why class need to implement Comparator interface to qualify its objects to be part of Sorted Collections.

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Q138. Why are Getter and Setter better than directly changing and retrieving fields ?

or

What is the significance of variable hiding in Java ?

or

Why it's preferred to keep variables private and allow their access through getter and setter methods ?

Core Java

Ans. 1. Methods can participate in runtime polymorphism whereas member variables cannot.

For example -

Vehicle vehicle = new Car();

car.getObjVariable();// will return variable defined in Car

whereas

car.variable

will return the variable from Car class irrespective of the object it holds.

2. Validations can be performed before setting the variables.

3. If the input format changes or some other value or calculated value needs to be returned , that can be absorbed by making change ( wrapping ) in the setter and getter. By this the call to method from outside or interface to the object will remain the same.

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Q139. Can we overload main method in Java ?

Core Java

Ans. Yes, but the overloaded main methods without single String[] argument doesn't get any special status by the JVM. They are just another methods that needs to be called explicitly.

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Q140. Can we override compareTo method for Enumerations ?

Core Java

Ans. No. compareTo method is declared final for the Enumerations and hence cannot be overriden. This has been intentionally done so that one cannot temper with the sorting order on the Enumeration which is the order in which Enum constants are declared.

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Very Frequently asked.

Q141. Explain throw, throws , try and catch in Java ?

Core Java

Ans. throw is used to re throw an exception.throws is used to declare that the method throws the respective exceptions.try block is used to identify if the respective block has thrown any exception.catch is used to catch the exception that has been thrown by the respective try block.

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Q142. What is ConcurrentModificationException ?

Core Java

Ans. This is the exception that is thrown when we try to modify the non concurrent collection class while iterating through it.

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Q143. What is the use of runnable interface if we can always create a new thread using Thread class ?

Core Java

Ans. It's a choice to be made whether to use first approach ( Thread class ) or second approach ( runnable interface ) by the programmer. The second facility has been given for cases where your class is already extending some parent class and hence cannot extend another class ( for Thread ) as Java doesn't allow multiple inheritance.

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Q144. Which of the following collections stores its elements in insertion Order ?

a. HashMap
b. TreeMap
c. LinkedHashMap
d. LinkedMap

Core Java

Ans. LinkedHashMap

Sample Code for LinkedHashMap

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Q145. In what order the elements of a HashSet are retrieved ?

a. Random Order
b. Insertion Order
c. Natural Sorting Order
d. Inverse Natural Sorting Order

Core Java

Ans. Random Order

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Q146. Which of the following is false about Constructors ?

a. Constructor can be overloaded
b. A no argument constructor is provided by the compiler if we declare only constructors with arguments.
c. Constructors shouldn't have any return types , not even void.
d. If super is not explicitly called, still super() is intrinsically added by the compiler.

Core Java

Ans. A no argument constructor is provided by the compiler if we declare only constructors with arguments.

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Q147. Which of the following doesn't extend Collection interface ?

a. Set
b. List
c. Map
d. Queue

Core Java

Ans. Map

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Q148. Which of the following Java feature promotes Code Re usability ?

a. Abstraction
b. Encapsulation
c. Inheritance
d. Polymorphism

Core Java

Ans. Inheritance

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Q149. enums are intrinsically ..

a. private
b. public
c. static
d. final

Core Java

Ans. static

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Q150. In which cases , moving methods to utility class could be useful ?

Core Java

Ans. It could be worthy to move a method to util class if the method needs to be shared, doesn't require polymorphic behavior and need not be overridden in special cases.

Don't belong to one group through is-a relationship ( You can share through parent class method )

Don't implement a specific interface ( java 8 default methods )

Doesn't involve complex computing as you will be loosing the benefit of object state with just static method.

Doesn't require polymorphic behavior as static methods don't participate in runtime polymorphism.

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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 urls

Core 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 method

Core 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 third

Core 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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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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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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