Core Java Inteview Questions.

Question: What is transient variable?
Answer: Transient variable can't be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can't be written to the stream instead when the class is retrieved from the ObjectStream the value of the variable becomes null.

Question: What do you understand by Synchronization?
Answer: Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption.

E.g. Synchronizing a function:
public synchronized void Method1 () {
   // Appropriate method-related code.
}
E.g. Synchronizing a block of code inside a function:public myFunction (){
  synchronized (this) {
  // Synchronized code here.
   }
}

Question: What is Collection API?
Answer: The Collection API is a set of classes and interfaces that support operation on collections of objects. These classes and interfaces are more flexible, more powerful, and more regular than the vectors, arrays, and hashtables if effectively replaces. Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap.Example of interfaces: Collection, Set, List and Map.

Question: What is similarities/difference between an Abstract class and Interface?
Answer:  Differences are as follows:

• Interfaces provide a form of multiple inheritance. A class can extend only one other class.
• Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc.
• A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class.
• Interfaces are slow as it requires extra indirection to to find corresponding method in in the actual class. Abstract classes are fast. 

Similarities:

• Neither Abstract classes or Interface can be instantiated.

Question: How to define an Abstract class?
Answer: A class containing abstract method is called Abstract class. An Abstract class can't be instantiated.
Example of Abstract class:
abstract class testAbstractClass {
  protected String myString;
  public String getMyString() {
  return myString;
  }
  public abstract string anyAbstractFunction();
}
 

Question: How to define an Interface?
Answer: In Java Interface defines the methods but does not implement them. Interface can include constants. A class that implements the interfaces is bound to implement all the methods defined in Interface.
Emaple of Interface:

public interface sampleInterface {
  public void functionOne();

  public long CONSTANT_ONE = 1000;
}

Question: Explain the user defined Exceptions?
Answer: User defined Exceptions are the separate Exception classes defined by the user for specific purposed. An user defined can created by simply sub-classing it to the Exception class. This allows custom exceptions to be generated (using throw) and caught in the same way as normal exceptions.
Example:
class myCustomException extends Exception {
   // The class simply has to exist to be an exception
} 

Question: Explain garbage collection?
Answer: Garbage collection is one of the most important feature of Java. Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. User program cann't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use. I Java on calling System.gc() andRuntime.gc(),  JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected.
 

Question: How you can force the garbage collection?
Answer: Garbage collection automatic process and can't be forced.  

Question: Describe the principles of OOPS.Answer: There are three main principals of oops which are called Polymorphism, Inheritance and Encapsulation.
 

Question: Explain the Encapsulation principle.
Answer: Encapsulation is a process of binding or wrapping the data and the codes that operates on the data into a single entity. This keeps the data safe from outside interface and misuse. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper.
 

Question: Explain the Inheritance principle.
Answer: Inheritance is the process by which one object acquires the properties of another object.
 

Question: Explain the Polymorphism principle.
Answer: The meaning of Polymorphism is something like one name many forms. Polymorphism enables one entity to be used as as general category for different types of actions. The specific action is determined by the exact nature of the situation. The concept of polymorphism can be explained as "one interface, multiple methods".
 

Question: Explain the different forms of Polymorphism.
Answer: From a practical programming viewpoint, polymorphism exists in three distinct forms in Java:

• Method overloading
• Method overriding through inheritance
• Method overriding through the Java interface

 
 

Question: What are Access Specifiers available in Java?
Answer: Access specifiers are keywords that determines the type of access to the member of a class. These are:

• Public
• Protected
• Private
• Defaults

 
 

Question: Describe the wrapper classes in Java.
Answer: Wrapper class is wrapper around a primitive data type. An instance of a wrapper class contains, or wraps, a primitive value of the corresponding type.

Following table lists the primitive types and the corresponding wrapper classes:

Primitive	Wrapper
boolean	java.lang.Boolean
byte	java.lang.Byte
char	java.lang.Character
double	java.lang.Double
float	java.lang.Float
int	java.lang.Integer
long	java.lang.Long
short	java.lang.Short
void	java.lang.Void

Question: what is the class variables ?
Answer: When we create a number of objects of the same class, then each object will share a common copy of variables. That means that there is only one copy per class, no matter how many objects are created from it. Class variables or static variables are declared with the static keyword in a class, but mind it that it should be declared outside outside a class. These variables are stored in static memory. Class variables are mostly used for constants, variable that never change its initial value. Static variables are always called by the class name. This variable is created when the program starts i.e. it is created before the instance is created of class by using new operator and gets destroyed when the programs stops. The scope of the class variable is same a instance variable. The class variable can be defined anywhere at class level with the keyword static. It initial value is same as instance variable. When the class variable is defined as int then it's initial value is by default zero, when declared boolean its default value is false and null for object references. Class variables are associated with the class, rather than with any object. 

Question: What is the difference between the instanceof and getclass, these two are same or not ?
Answer: instanceof is a operator, not a function while getClass is a method of java.lang.Object class. Consider a condition where we use
if(o.getClass().getName().equals("java.lang.Math")){ }
This method only checks if the classname we have passed is equal to java.lang.Math. The class java.lang.Math is loaded by the bootstrap ClassLoader. This class is an abstract class.This class loader is responsible for loading classes. Every Class object contains a reference to the ClassLoader that defines. getClass() method returns the runtime class of an object. It fetches the java instance of the given fully qualified type name. The code we have written is not necessary, because we should not compare getClass.getName(). The reason behind it is that if the two different class loaders load the same class but for the JVM, it will consider both classes as different classes so, we can't compare their names. It can only gives the implementing class but can't compare a interface, but instanceof operator can.
The instanceof operator compares an object to a specified type. We can use it to test if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface. We should try to use instanceof operator in place of getClass() method. Remember instanceof opeator and getClass are not same. Try this example, it will help you to better understand the difference between the two.
Interface one{
}

Class Two implements one {
}
Class Three implements one {
}

public class Test {
public static void main(String args[]) {
one test1 = new Two();
one test2 = new Three();
System.out.println(test1 instanceof one); //true
System.out.println(test2 instanceof one); //true
System.out.println(Test.getClass().equals(test2.getClass())); //false
}
} 

How Observer and Observable are used?

Subclass of Observable class maintain a list of observers. Whenever an Observable object is updated, it invokes the update() method of each of its observers to notify the observers that it has a changed state. An observer is any object that implements the interface Observer.

What is the difference between yield() and sleep()?

When a object invokes yield() it returns to ready state. But when an object invokes sleep() method enters to not ready state.

Can we call finalize() method ?

Yes.  Nobody will stop us to call any method , if it is accessible in our class. But a garbage collector cannot call an object's finalize method if that object is reachable.

Can we declare an anonymous class as both extending a class and implementing an interface?

No. An anonymous class can extend a class or implement an interface, but it cannot be declared to do both

1. What is immutable object? Can you write immutable object?

You need to make class final and all its member final so that once objects gets crated no one can modify its state. You can achieve same functionality by making member as non final but private and not modifying them except in constructor.
2. Does all property of immutable object needs to be final?
Not necessary as stated above you can achieve same functionality by making member as non final but private and not modifying them except in constructor.
3. What is the difference between creating String as new () and literal?
When we create string with new () it’s created in heap and not added into string pool while String created using literal are created in String pool itself which exists in Perm area of heap.
String s = new String("Test");
will put the object in String pool , it it does then why do one need String.intern() method which is used to put Strings into String pool explicitly. its only when you create String object as String literal e.g. String s = "Test" it put the String in pool.

What is serialization ?

Serialization is the process of saving the state of an object.

What is de-serialization?

De-serialization is the process of restoring the state of an object.

What is externalizable ?

It is an interface that extends Serializable. It is having two different methods writeExternal() and readExternal. This interface allows us to customize the output.

What are different type of exceptions in Java?

There are two types of exceptions in java. Checked exceptions and Unchecked exceptions. Any exception that is is derived from Throwable and Exception is called checked exception except RuntimeException and its sub classes. The compiler will check whether the exception is caught or not at compile time. We need to catch the checked exception or declare in the throws clause. Any exception that is derived from Error and RuntimeException is called unchecked exception. We don't need to explicitly catch a unchecked exception.

Can we catch an error in our java program ?

Yes. We can . We can catch anything that is derived from Throwable. Since Error is a sub class of Throwable we can catch an error also.

What is thread priority?

Thread Priority is an integer value that identifies the relative order in which it should be executed with respect to others. The thread priority values ranging from 1- 10 and the default value is 5. But if a thread have higher priority doesn't means that it will execute first. The thread scheduling depends on the OS.

How many times may an object's finalize() method be invoked by the garbage collector?

Only once.

What is the difference between notify and notifyAll method ?

notify wakes up a single thread that is waiting for object's monitor. If any threads are waiting on this object, one of them is chosen to be awakened. The choice is arbitrary and occurs at the discretion of the implementation. notifyAll Wakes up all threads that are waiting on this object's monitor. A thread waits on an object's monitor by calling one of the wait methods.

What is the difference between String and StringBuffer class ?

Strings are immutable (constant), their values cannot be changed after they are created. StringBuffer supports mutable objects.

What is the difference between readers and streams?

Readers are character oriented where streams are byte oriented. The readers are having full support for Unicode data.

What are the different primitive data type in java ?

There are 8 primitive types in java. boolean , char, byte, short, int long, float, double.

What are the restrictions placed on overriding a method ?

The overridden method have the exact signature of the super class method, including the return type. The access specified cannot be less restrictive than the super class method. We cannot throw any new exceptions in overridden method.

What are the restrictions placed on overloading a method ?

Overloading methods must differ in their parameter list, or number of parameters.

What is casting ?

Casting means converting one type to another. There are mainly two types of casting. Casting between primitive types and casting between object references. Casting between primitive numeric types is used to convert larger data types to smaller data types. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.

What will happen if an exception is not caught ?

An uncaught exception results in the uncaughtException() method of the thread's ThreadGroup, which results in the termination of the program.

What are the different ways in which a thread can enter into waiting state?

There are three ways for a thread to enter into waiting state. By invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method.

What is hashCode?

The hashcode of a Java Object is simply a number (32-bit signed int) that allows an object to be managed by a hash-based data structure. A hashcode should be, equal for equal object (this is mandatory!) , fast to compute based on all or most of the internal state of an object, use all or most of the space of 32-bit integers in a fairly uniform way , and likely to be different even for objects that are very similar. If you are overriding hashCode you need to override equals method also.

What is an I/O filter?

An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.

What is the difference between RandomAccessFile and File?

The File class contains information the files and directories of the local file system. The RandomAccessFile class contains the methods needed to directly access data contained in any part of a file.

What is final ?

A final is a keyword in java. If final keyword is applied to a variable, then the variable will become a constant. If it applied to method, sub classes cannot override the method. If final keyword is applied to a class we cannot extend from that class.

What is the difference between Hashtable and HashMap ?

Hashtable is synchronized . but HashMap is not synchronized. Hashtable does not allow null values , but HashMap allows null values.

Why java is said to be pass-by-value ?

When assigning an object to a variable, we are actually assigning the memory address of that object to the variable. So the value passed is actually the memory location of the object. This results in object aliasing, meaning you can have many variables referring to the same object on the heap.

What is object cloning?

It is the process of duplicating an object so that two identical objects will exist in the memory at the same time.

What is object pooling?

Creating a large number of identical short lived objects is called object pooling. This helps to minimize the need of garbage collection and makes the memory use more effective.

What is JAR file ?

JAR stands for Java Archive. This is a file format that enables you to bundlemultiple files into a single archive file. A jar file will contains a manifest.mf file inside META-INF folder that describes the version and other features of jar file.

Why Java is not fully objective oriented ?

Due to the use of primitives in java, which are not objects.

What is a marker interface ?

An interface that contains no methods. Eg: Serializable, Cloneable, SingleThreadModel etc. It is used to just mark java classes that support certain capability.

What are tag interfaces?

Tag interface is an alternate name for marker interface.

What are the restrictions placed on static method ?

We cannot override static methods. We cannot access any object variables inside static method. Also the this reference also not available in static methods.

What is JIT?

JIT stands for Just In Time compiler. It compiles java byte code to native code.

What is java byte code?

Byte code is an sort of intermediate code. The byte code is processed by virtual machine.

What is deadlock?

Deadlock is a situation when two threads are waiting on each other to release a resource. Each thread waiting for a resource which is held by the other waiting thread.

What is the Locale class?

A Locale object represents a specific geographical, political, or cultural region

What is anonymous class ?

A An anonymous class is a type of inner class that don't have any name.

What is the difference between URL and URLConnection?

A URL represents the location of a resource, and a URLConnection represents a link for accessing or communicating with the resource at the location.

What is classpath?

Classpath is the path where Java looks for loading class at run time and compile time.

What is path?

It is an the location where the OS will look for finding out the executable files and commands.

Can we compile a java program without main?

Yes, we can. In order to compile a java program, we don't require any main method. But to execute a java program we must have a main in it (unless it is an applet or servlet). Because main is the starting point of a java program.

How parameters are passed to methods in java program ?

All java method parameters in java are passed by value only. Obviously primitives are passed by value. In case of objects a copy of the reference is passed and so all the changes made in the method will persist.

What will happen if a thread cannot acquire a lock on an object?

It enters to the waiting state until lock becomes available.

What will happen if you are invoking a thread's interrupt method while the thread is waiting or sleeping?

When the task enters to the running state, it will throw an InterruptedException.

What is the difference between creating a thread by extending Thread class and by implementing Runnable interface? Which one should prefer?

When creating a thread by extending the Thread class, it is not mandatory to override the run method (If we are not overriding the run method , it is useless), because Thread class have already given a default implementation for run method. But if we are implementing Runnable , it is mandatory to override the run method. The preferred way to create a thread is by implementing Runnable interface, because it give loose coupling.

What is coupling?

Coupling is the dependency between different components of a system

What is volatile variable?

A volatile variable is not allowed to have a local copy of a variable that is different from the value currently held in "main" memory. Volatile modifier requests the JVM to always access the shared copy of the variable so the its most current value is always read.

Why java does not support multiple inheritance?

Because the multiple inheritance causes the redundancy. Also we cannot solve diamond problem.

What is diamond problem?

The diamond problem is an ambiguity that can occur when a class multiply inherits from two classes that both descend from a common super class

Why ArrayList is faster than Vector?

Because Vector is synchronized. Synchronization reduces the performance.

What is the security mechnaism used in java?

Java uses sand box security model.

What is sandbox?

A sandbox is a security mechanism for safely running programs. The sandbox typically provides a tightly-controlled set of resources for guest programs to run in, such as scratch space on disk and memory.

What is phantom memory?

Phantom memory is the memory that does not exist in reality.

What is reflection?

Reflection is the process of finding out the different features of a class dynamically.

What is the difference between throw and throws clause?

throw is used to throw an exception manually, where as throws is used in the case of checked exceptions, to tell the compiler that we haven't handled the exception, so that the exception will be handled by the calling function.

What is the difference between Comparable and Comparator ?

The Comparable is for natural ordering and Comparator is for custom ordering. But we can override the compareTo method of comparable interface to give a custom ordering.

What is the difference between List, Set and Map?

A Set is a collection that has no duplicate elements. A List is a collection that has an order associated with its elements. A map is a way of storing key/value pairs. The way of storing a Map is similar to two-column table.

What is the difference between Exception and Error ?

Error is unrecoverable.

Can an Interface be final? - No

1. Can an Interface have an inner class? - Yes.

           public interface abc {
   static int i=0; void dd();
                   class a1
                   {  a1()
                           {   int j;
                                   System.out.println("inside");
                           };
                           public static void main(String a1[])
                           {    System.out.println("in interfia");
                           }
                   }
           }

What is Abstraction?

Abstraction refers to the act of representing essential features without including the background details or explanations.

What is Encapsulation?

Encapsulation is a technique used for hiding the properties and behaviors of an object and allowing outside access only as appropriate. It prevents other objects from directly altering or accessing the properties or methods of the encapsulated object.

What is the difference between abstraction and encapsulation?

• Abstraction focuses on the outside view of an object (i.e. the interface) Encapsulation (information hiding) prevents clients from seeing it’s inside view, where the behavior of the abstraction is implemented.
• Abstraction solves the problem in the design side while Encapsulation is the Implementation.
• Encapsulation is the deliverables of Abstraction. Encapsulation barely talks about grouping up your abstraction to suit the developer needs.
  

What is Inheritance?

• Inheritance is the process by which objects of one class acquire the properties of objects of another class.
• A class that is inherited is called a superclass.
• The class that does the inheriting is called a subclass.
• Inheritance is done by using the keyword extends.
• The two most common reasons to use inheritance are:
  • To promote code reuse
  • To use polymorphism
    

What is Polymorphism?

Polymorphism is briefly described as "one interface, many implementations." Polymorphism is a characteristic of being able to assign a different meaning or usage to something in different contexts - specifically, to allow an entity such as a variable, a function, or an object to have more than one form.

How does Java implement polymorphism?

(Inheritance, Overloading and Overriding are used to achieve Polymorphism in java).
Polymorphism manifests itself in Java in the form of multiple methods having the same name.

• In some cases, multiple methods have the same name, but different formal argument lists (overloaded methods).
• In other cases, multiple methods have the same name, same return type, and same formal argument list (overridden methods).
  

Explain the different forms of Polymorphism.

There are two types of polymorphism one is Compile time polymorphism and the other is run time polymorphism. Compile time polymorphism is method overloading. Runtime time polymorphism is done using inheritance and interface.
Note: From a practical programming viewpoint, polymorphism manifests itself in three distinct forms in Java:

• Method overloading
• Method overriding through inheritance
• Method overriding through the Java interface

What is runtime polymorphism or dynamic method dispatch?

In Java, runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a superclass. The determination of the method to be called is based on the object being referred to by the reference variable.

What is Dynamic Binding?

Binding refers to the linking of a procedure call to the code to be executed in response to the call. Dynamic binding (also known as late binding) means that the code associated with a given procedure call is not known until the time of the call at run-time. It is associated with polymorphism and inheritance.

What is method overloading?

Method Overloading means to have two or more methods with same name in the same class with different arguments. The benefit of method overloading is that it allows you to implement methods that support the same semantic operation but differ by argument number or type.
Note:

• Overloaded methods MUST change the argument list
• Overloaded methods CAN change the return type
• Overloaded methods CAN change the access modifier
• Overloaded methods CAN declare new or broader checked exceptions
• A method can be overloaded in the same class or in a subclass
  

What is method overriding?

Method overriding occurs when sub class declares a method that has the same type arguments as a method declared by one of its superclass. The key benefit of overriding is the ability to define behavior that’s specific to a particular subclass type.
Note:

• The overriding method cannot have a more restrictive access modifier than the method being overridden (Ex: You can’t override a method marked public and make it protected).
• You cannot override a method marked final
• You cannot override a method marked static
  

What are the differences between method overloading and method overriding?

	Overloaded Method	Overridden Method
Arguments	Must change	Must not change
Return type	Can change	Can’t change except for covariant returns
Exceptions	Can change	Can reduce or eliminate. Must not throw new or broader checked exceptions
Access	Can change	Must not make more restrictive (can be less restrictive)
Invocation	Reference type determines which overloaded version is selected. Happens at compile time.	Object type determines which method is selected. Happens at runtime.

What is super?

super is a keyword which is used to access the method or member variables from the superclass. If a method hides one of the member variables in its superclass, the method can refer to the hidden variable through the use of the super keyword. In the same way, if a method overrides one of the methods in its superclass, the method can invoke the overridden method through the use of the super keyword. Note:

• You can only go back one level.
• In the constructor, if you use super(), it must be the very first code, and you cannot access any this.xxx variables or methods to compute its parameters.

How do you prevent a method from being overridden?

To prevent a specific method from being overridden in a subclass, use the final modifier on the method declaration, which means "this is the final implementation of this method", the end of its inheritance hierarchy.

                               public final void exampleMethod() {
                          //  Method statements
                          }

What are the differences between Interface and Abstract class?

Abstract Class	Interfaces
An abstract class can provide complete, default code and/or just the details that have to be overridden.	An interface cannot provide any code at all,just the signature.
In case of abstract class, a class may extend only one abstract class.	A Class may implement several interfaces.
An abstract class can have non-abstract methods.	All methods of an Interface are abstract.
An abstract class can have instance variables.	An Interface cannot have instance variables.
An abstract class can have any visibility: public, private, protected.	An Interface visibility must be public (or) none.
If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly.	If we add a new method to an Interface then we have to track down all the implementations of the interface and define implementation for the new method.
An abstract class can contain constructors .	An Interface cannot contain constructors .
Abstract classes are fast.	Interfaces are slow as it requires extra indirection to find corresponding method in the actual class.

When should I use abstract classes and when should I use interfaces?

Use Interfaces when…

• You see that something in your design will change frequently.
• If various implementations only share method signatures then it is better to use Interfaces.
• you need some classes to use some methods which you don't want to be included in the class, then you go for the interface, which makes it easy to just implement and make use of the methods defined in the interface.
  

Use Abstract Class when…

• If various implementations are of the same kind and use common behavior or status then abstract class is better to use.
• When you want to provide a generalized form of abstraction and leave the implementation task with the inheriting subclass.
• Abstract classes are an excellent way to create planned inheritance hierarchies. They're also a good choice for nonleaf classes in class hierarchies.

What is Constructor?

• A constructor is a special method whose task is to initialize the object of its class.
• It is special because its name is the same as the class name.
• They do not have return types, not even void and therefore they cannot return values.
• They cannot be inherited, though a derived class can call the base class constructor.
• Constructor is invoked whenever an object of its associated class is created.
  

What are the differences between Contructors and Methods?

	Constructors	Methods
Purpose	Create an instance of a class	Group Java statements
Modifiers	Cannot be abstract, final, native, static, or synchronized	Can be abstract, final, native, static, or synchronized
Return Type	No return type, not even void	void or a valid return type
Name	Same name as the class (first letter is capitalized by convention) -- usually a noun	Any name except the class. Method names begin with a lowercase letter by convention -- usually the name of an action
this	Refers to another constructor in the same class. If used, it must be the first line of the constructor	Refers to an instance of the owning class. Cannot be used by static methods.
super	Calls the constructor of the parent class. If used, must be the first line of the constructor	Calls an overridden method in the parent class
Inheritance	Constructors are not inherited	Methods are inherited

How are this() and super() used with constructors?

• Constructors use this to refer to another constructor in the same class with a different parameter list.
• Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must use it in the first line; otherwise, the compiler will complain.
  

What are the differences between Class Methods and Instance Methods?

Class Methods	Instance Methods
Class methods are methods which are declared as static. The method can be called without creating an instance of the class	Instance methods on the other hand require an instance of the class to exist before they can be called, so an instance of a class needs to be created by using the new keyword. Instance methods operate on specific instances of classes.
Class methods can only operate on class members and not on instance members as class methods are unaware of instance members.	Instance methods of the class can also not be called from within a class method unless they are being called on an instance of that class.
Class methods are methods which are declared as static. The method can be called without creating an  instance of the class.	Instance methods are not declared as static.

What are Access Specifiers available in Java?

Java offers four access specifiers, listed below in decreasing accessibility:

• Public- public classes, methods, and fields can be accessed from everywhere.
• Protected- protected methods and fields can only be accessed within the same class to which the methods and fields belong, within its subclasses, and within classes of the same package.
• Default(no specifier)- If you do not set access to specific level, then such a class, method, or field will be accessible from inside the same package to which the class, method, or field belongs, but not from outside this package.
• Private- private methods and fields can only be accessed within the same class to which the methods and fields belong. private methods and fields are not visible within subclasses and are not inherited by subclasses.
  

Situation	public	protected	default	private
Accessible to class  from same package?	yes	yes	yes	no
Accessible to class  from different package?	yes	no, unless it is a subclass	no	no

What is final modifier?

The final modifier keyword makes that the programmer cannot change the value anymore. The actual meaning depends on whether it is applied to a class, a variable, or a method.

• final Classes- A final class cannot have subclasses.
• final Variables- A final variable cannot be changed once it is initialized.
• final Methods- A final method cannot be overridden by subclasses.

What are the uses of final method?

There are two reasons for marking a method as final:

• Disallowing subclasses to change the meaning of the method.
• Increasing efficiency by allowing the compiler to turn calls to the method into inline Java code.

What is an Iterator ?

• The Iterator interface is used to step through the elements of a Collection.
• Iterators let you process each element of a Collection.
• Iterators are a generic way to go through all the elements of a Collection no matter how it is organized.
• Iterator is an Interface implemented a different way for every Collection.

How do you traverse through a collection using its Iterator?

To use an iterator to traverse through the contents of a collection, follow these steps:

• Obtain an iterator to the start of the collection by calling the collection’s iterator() method.
• Set up a loop that makes a call to hasNext(). Have the loop iterate as long as hasNext() returns true.
• Within the loop, obtain each element by calling next().
  

How do you remove elements during Iteration?

Iterator also has a method remove() when remove is called, the current element in the iteration is deleted.

What is the difference between Enumeration and Iterator?

Enumeration	Iterator
Enumeration doesn't have a remove() method	Iterator has a remove() method
Enumeration acts as Read-only interface, because it has the methods only to traverse and fetch the objects	Can be abstract, final, native, static, or synchronized

Note: So Enumeration is used whenever we want to make Collection objects as Read-only.

Difference between ArrayList and Vector ?

ArrayList	Vector
ArrayList is NOT synchronized by default.	Vector List is synchronized by default.
ArrayList can use only Iterator to access the elements.	Vector list can use Iterator and Enumeration Interface to access the elements.
The ArrayList increases its array size by 50 percent if it runs out of room.	A Vector defaults to doubling the size of its array if it runs out of room
ArrayList has no default size.	While vector has a default size of 10.

How to obtain Array from an ArrayList ?

Array can be obtained from an ArrayList using toArray() method on ArrayList.

        List arrayList = new ArrayList();
        arrayList.add(…
        Object  a[] = arrayList.toArray();

How do you decide when to use ArrayList and When to use LinkedList?

If you need to support random access, without inserting or removing elements from any place other than the end, then ArrayList offers the optimal collection. If, however, you need to frequently add and remove elements from the middle of the list and only access the list elements sequentially, then LinkedList offers the better implementation.

Why insertion and deletion in ArrayList is slow compared to LinkedList ?

• ArrayList internally uses and array to store the elements, when that array gets filled by inserting elements a new array of roughly 1.5 times the size of the original array is created and all the data of old array is copied to new array.
• During deletion, all elements present in the array after the deleted elements have to be moved one step back to fill the space created by deletion. In linked list data is stored in nodes that have reference to the previous node and the next node so adding element is simple as creating the node an updating the next pointer on the last node and the previous pointer on the new node. Deletion in linked list is fast because it involves only updating the next pointer in the node before the deleted node and updating the previous pointer in the node after the deleted node.

Why are Iterators returned by ArrayList called Fail Fast ?

Because, if list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

How do you decide when to use ArrayList and When to use LinkedList?

If you need to support random access, without inserting or removing elements from any place other than the end, then ArrayList offers the optimal collection. If, however, you need to frequently add and remove elements from the middle of the list and only access the list elements sequentially, then LinkedList offers the better implementation.

What is the Set interface ?

• The Set interface provides methods for accessing the elements of a finite mathematical set
• Sets do not allow duplicate elements
• Contains no methods other than those inherited from Collection
• It adds the restriction that duplicate elements are prohibited
• Two Set objects are equal if they contain the same elements
  

What are the main Implementations of the Set interface ?

The main implementations of the List interface are as follows:

• HashSet
• TreeSet
• LinkedHashSet
• EnumSet
  

What is a HashSet ?

• A HashSet is an unsorted, unordered Set.
• It uses the hashcode of the object being inserted (so the more efficient your hashcode() implementation the better access performance you’ll get).
• Use this class when you want a collection with no duplicates and you don’t care about order when you iterate through it.
  

What is a TreeSet ?

TreeSet is a Set implementation that keeps the elements in sorted order. The elements are sorted according to the natural order of elements or by the comparator provided at creation time.

What is an EnumSet ?

An EnumSet is a specialized set for use with enum types, all of the elements in the EnumSet type that is specified, explicitly or implicitly, when the set is created.

Difference between HashSet and TreeSet ?

HashSet	TreeSet
HashSet is under set interface i.e. it  does not guarantee for either sorted order or sequence order.	TreeSet is under set i.e. it provides elements in a sorted  order (acceding order).
We can add any type of elements to hash set.	We can add only similar types of elements to tree set.

What is a Map ?

• A map is an object that stores associations between keys and values (key/value pairs).
• Given a key, you can find its value. Both keys  and  values are objects.
• The keys must be unique, but the values may be duplicated.
• Some maps can accept a null key and null values, others cannot.
  

What are the main Implementations of the Map interface ?

The main implementations of the List interface are as follows:

• HashMap
• HashTable
• TreeMap
• EnumMap
  

What is a TreeMap ?

TreeMap actually implements the SortedMap interface which extends the Map interface. In a TreeMap the data will be sorted in ascending order of keys according to the natural order for the key's class, or by the comparator provided at creation time. TreeMap is based on the Red-Black tree data structure.

How do you decide when to use HashMap and when to use TreeMap ?

For inserting, deleting, and locating elements in a Map, the HashMap offers the best alternative. If, however, you need to traverse the keys in a sorted order, then TreeMap is your better alternative. Depending upon the size of your collection, it may be faster to add elements to a HashMap, then convert the map to a TreeMap for sorted key traversal.

Difference between HashMap and Hashtable ?

HashMap	Hashtable
HashMap lets you have null values as well as one null key.	HashTable  does not allows null values as key and value.
The iterator in the HashMap is fail-safe (If you change the map while iterating, you’ll know).	The enumerator for the Hashtable is not fail-safe.
HashMap is unsynchronized.	Hashtable is synchronized.

Note: Only one NULL is allowed as a key in HashMap. HashMap does not allow multiple keys to be NULL. Nevertheless, it can have multiple NULL values.

How does a Hashtable internally maintain the key-value pairs?

TreeMap actually implements the SortedMap interface which extends the Map interface. In a TreeMap the data will be sorted in ascending order of keys according to the natural order for the key's class, or by the comparator provided at creation time. TreeMap is based on the Red-Black tree data structure.

What Are the different Collection Views That Maps Provide?

Maps Provide Three Collection Views.

• Key Set - allow a map's contents to be viewed as a set of keys.
• Values Collection - allow a map's contents to be viewed as a set of values.
• Entry Set - allow a map's contents to be viewed as a set of key-value mappings.
  

What is a KeySet View ?

KeySet is a set returned by the keySet() method of the Map Interface, It is a set that contains all the keys present in the Map.

What is a Values Collection View ?

Values Collection View is a collection returned by the values() method of the Map Interface, It contains all the objects present as values in the map.

What is an EntrySet View ?

Entry Set view is a set that is returned by the entrySet() method in the map and contains Objects of type Map. Entry each of which has both Key and Value.

How do you sort an ArrayList (or any list) of user-defined objects ?

Create an implementation of the java.lang.Comparable interface that knows how to order your objects and pass it to java.util.Collections.sort(List, Comparator).

What is the Comparable interface ?

The Comparable interface is used to sort collections and arrays of objects using the Collections.sort() and java.utils.Arrays.sort() methods respectively. The objects of the class implementing the Comparable interface can be ordered.

The Comparable interface in the generic form is written as follows:

       interface Comparable<T>

where T is the name of the type parameter.

All classes implementing the Comparable interface must implement the compareTo() method that has the return type as an integer. The signature of thecompareTo() method is as follows:

      int i = object1.compareTo(object2)

• If object1 < object2: The value of i returned will be negative.
• If object1 > object2: The value of i returned will be positive.
• If object1 = object2: The value of i returned will be zero.
  

What are the differences between the Comparable and Comparator interfaces ?

Comparable	Comparato
It uses the compareTo() method. int objectOne.compareTo(objectTwo).	t uses the compare() method. int compare(ObjOne, ObjTwo)
It is necessary to modify the class whose instance is going to be sorted.	A separate class can be created in order to sort the instances.
Only one sort sequence can be created.	Many sort sequences can be created.
It is frequently used by the API classes.	It used by third-party classes to sort instances.

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