Core Java Interview Questions

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Q) What is polymorphism?

Ans) The ability to define a function in multiple forms is called Polymorphism. In java, c++ there are two types of polymorphism: compile time polymorphism (overloading) and runtime polymorphism (overriding).

Method overriding: happens when a child class implements the method with the same signature as a method in a parent class. When you override methods, JVM determines the proper methods to call at the program's runtime, not at the compile time.

Method overloading: happens when several methods have same names but different number or type of parameters. Overloading is determined at the compile time.

  • Overloading happens when:
  • Different method signature and different number or type of parameters.
  • Same method signature but different number of parameters.
  • Same method signature and same number of parameters but of different type
Example of Overloading
int add(int a,int b)
 float add(float a,int b)
 float add(int a ,float b)
 void add(float a)
 int add(int a)
 void add(int a) //error conflict with the  method int add(int a)
class BookDetails {
  String title;
  setBook(String title){}
}
class ScienceBook extends BookDetails {
  setBook(String title){} //overriding
  setBook(String title, String publisher,float price){} //overloading
}

Q) What is the difference between final, finally and finalize() in Java?

Ans) final - A final variable acts as a constant, a final class is immutable and a final method cannot be overriden in a child class.

finally - finally keyword is used with try-catch block for handling exceptions. The finally block is optional in try-catch block. The finally code block is always executed after try or catch block is completed. The finally block is mainly executed to close the resources or clean up objects used in the try block. For e.g. Closing a FileStream, I/O stream objects, Database connections, HTTP connections are generally closed in a finally block.

finalize() - This is the method of Object class.It is invoked before an object is discarded by the garbage collector, allowing it to clean up its state. Should not be used to release non-memory resources like file handles, sockets, database connections etc because Java has only a finite number of these resources and you do not know when the garbage collection is going to kick in to release these non-memory resources through the finalize() method.

Q4) What is the difference between HashMap and HashTable?

Ans) Both collections implements Map. Both collections store value as key-value pairs. The key differences between the two are:

  1. Hashmap is not synchronized in nature but hashtable is(thread-safe). This means that in a multithreaded application, only one thread can get access to a hashtable object and do an operation on it. Hashmap doesn't gurantee such behavior and is not used in multithreaded environment.
  2. Hashmap is traveresed using an iterator, hashtable can be traversed by enumerator or iterator.
  3. Iterator in hashmap is fail-fast, enumerator in hashtable is not fail-fast
  4. HashMap permits null values and only one null key, while Hashtable doesn't allow key or value as null.
  5. Since hashtable is synchornized, it is relatively slower in performance than hashmap

Q) What is the difference between abstract class and interface ?

Ans) A class is called abstract when it is declared with the keywordabstract. Abstract class contains at least one abstract method. It can also contain n numbers of concrete method. An interface can only contain abstract methods.

  • An interface can have only abstract methods. Abstract class can have concrete and abstract methods.
  • The abstract class can have public, private, protected or default variables and also constants. In interface the variable is by default public final. In nutshell the interface doesn't have any variables it only has constants.
  • A class can extend only one abstract class but a class can implement multiple interfaces. Abstract class doesn't support multiple inheritance whereas abstract class does.
  • If an interface is implemented its mandatory to implement all of its methods but if an abstract class is extended its mandatory to implement all abstract methods.
  • The problem with an interface is, if you want to add a new feature (method) in its contract, then you MUST implement the new method(s) in all of the classes which implement that interface. However, in the case of an abstract class, the method can be simply implemented in the abstract class and the same can be called by its subclass.

Q) What is the difference between equals() and == ?

Ans) == operator is used to compare the references of the objects.
public boolean equals(Object o) is the method provided by the Object class. The default implementation uses == operator to compare two objects which mean by default the objects address are compared. But since the method can be overriden like for String class. equals() method can be used to compare the values of two objects.

String str1 = "MyName"; 
String str2 = "MyName";
String str3 = new String(str2);

if (str1 == str2) {
  System.out.println("Objects are equal")
}else{
  System.out.println("Objects are not equal")
}
if(str1.equals(str2)) {
  System.out.println("Objects are equal")
} else {
  System.out.println("Objects are not equal")
}

Output:
Objects are not equal
Objects are equal
String str2 = "MyName";
String str3 = str2;
if (str2 == str3) {
System.out.println("Objects are equal")
}else{
System.out.println("Objects are not equal")
}
if (str3.equals(str2)) {
  System.out.println("Objects are equal")
} else {
  System.out.println("Objects are not equal")
}

Output:
Objects are equal
Objects are equal

Q) What is the use of synchronized keyword?

Ans) synchronized keyword can be applied to static/non-static methods or a block of code. Only one thread at a time can access synchronized methods and if there are multiple threads trying to access the same method other threads have to wait until current thread finishes the execution and release lock. Synchronized keyword provides a lock on the object and thus prevents race condition. E.g.

public void synchronized method(){} 
  public void synchronized staticmethod(){}
  public void myMethod(){
     synchronized (this){ 
        //synchronized keyword on block of code
     }
  }

Recommend Reading

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  2. Use of hashcode and equals
  3. Comparable and Comparator in java
  4. How to create Singleton class in Java?
  5. Difference between equals and ==?
  6. Data structures
  7. Cap Theorem in Details
  8. Why is it recommended to use Immutable objects in Java