Core Java

Why String is immutable in Java? (Security, String pool implementation)

1) Imagine StringPool facility without making string immutable , its not possible at all because in case of string pool one string object/literal e.g. "Test" has referenced by many reference variables , so if any one of them change the value others will be automatically gets affected i.e. lets say

String A = "Test"

String B = "Test"

Now String B called "Test".toUpperCase() which change the same object into "TEST" , so A will also be "TEST" which is not desirable.

2)String has been widely used as parameter for many Java classes e.g. for opening network connection, you can pass hostname and port number as string , you can pass database URL as string for opening database connection, you can open any file in Java by passing name of file as argument to File I/O classes.

In case, if String is not immutable, this would lead serious security threat, I mean some one can access to any file for which he has authorization, and then can change the file name either deliberately or accidentally and gain access of those file. Because of immutability, you don't need to worry about those kind of threats. This reason also gel with, Why String is final in Java, by making java.lang.String final, Java designer ensured that no one overrides any behavior of String class.

3)Since String is immutable it can safely shared between many threads ,which is very important for multithreaded programming and to avoid any synchronization issues in Java, Immutability also makes String instance thread-safe in Java, means you don't need to synchronize String operation externally. Another important point to note about String is memory leak caused by SubString, which is not a thread related issues but something to be aware of.

4) Another reason of Why String is immutable in Java is to allow String to cache its hashcode , being immutable String in Java caches its hashcode, and do not calculate every time we call hashcode method of String, which makes it very fast as hashmap key to be used in hashmap in Java. In short because String is immutable, no one can change its contents once created which guarantees hashCode of String to be same on multiple invocations.

Can abstract class have constructor in Java?

Yes, abstract class can have constructor in Java. You can either explicitly provide constructor to abstract class or if you don't, compiler will add default constructor of no argument in abstract class. This is true for all classes and it’s also applies on abstract class. For those who want to recall what is an abstract class in Java, it’s a class which cannot be instantiated with new() operator or any other ways. In order to use abstract class in Java, You need to extend it and provide a concrete class. Abstract class is commonly used to define base class for a type hierarchy with default implementation, which is applicable to all child classes.

Since abstract class cannot be instantiated, why abstract class need constructor?

Now if we say we cannot create instance of abstract class then why do Java adds constructor in abstract class. One of the reason which make sense is, when any class extend abstract class, constructor of sub class will invoke constructor of super class either implicitly or explicitly. This chaining of constructors is one of the reason abstract class can have constructors in Java.

What is difference between interface and abstract class in Java?

1) First and major difference between abstract class and interface is that, abstract class is a class while interface is a interface, means by extending abstract class you cannot extend another class because Java does not support multiple inheritance but you can implement multiple interfaces in Java.

2) Second difference between interface and abstract class in Java is that you cannot create non abstract method in interface, every method in interface is by default abstract, but you can create non abstract method in abstract class. Even a class which doesn't contain any abstract method can be abstract by using abstract keyword.

3) Third difference between abstract class and interface in Java is that abstract class are slightly faster than interface because interface involves a search before calling any overridden method in Java. This is not a significant difference in most of cases but if you are writing a time critical application than you may not want to leave any stone unturned.

4) Fourth difference between abstract class vs interface in Java is that, interface are better suited for Type declaration and abstract class is more suited for code reuse and evolution perspective.

5) Another notable difference between interface and abstract class is that when you add a new method in existing interface it breaks all its implementation and you need to provide an implementation in all clients which is not good. By using abstract class you can provide default implementation in super class.

When to use interface in Java?

Interface is best choice for Type declaration or defining contract between multiple parties. If multiple programmers are working in different module of project they still use each other’s API by defining interface and not waiting for actual implementation to be ready. This brings us lot of flexibility and speed in terms of coding and development. Use of Interface also ensures best practices like "programming for interfaces than implementation" and results in more flexible and maintainable code.

Which two methods are overridden by an Object, intended to be used as key in HashMap?

A: equals and hashCode

Difference between wait and sleep in Java?

A: Wait release lock, sleep keeps it

How do you make a class Immutable in Java?

A: Make it final, final fields without setter, state is only set in constructor, no leak of internal reference, copy data for mutable members.

public final class Contacts {

    private final String name;

    private final String mobile;

    public Contacts(String name, String mobile) {

        this.name = name;

        this.mobile = mobile;

    }

  

    public String getName(){

        return name;

    }

  

    public String getMobile(){

        return mobile;

    }

}

Which data type you should used to represent currency in Java?

Long or BigDecimal, if you say double, you need to convince them about rounding and how do you avoid floating point issues.

When to use abstract class and interface in Java?

Use interface for type declaration, use abstract class if evolution is concern, for few more points.

Difference between Overloading and Overriding in Java?

Former take place at compile time, later happens at runtime, only virtual method can be overridden, static, final and private method can't be overridden in Java.

What kind of reference types are exists in Java? Differences?

Strong reference, Weak references, Soft reference and Phantom reference. Except strong, all other reference allows object to be garbage collected. For example, if an object hash only weak reference, than it's eligible for GC, if program needs space

Does Java array is instance of Object?

Yes, and this is stark difference from array in C/C++, though it doesn't have any method, it has an attribute called length, which denotes size of array

Does List<Number> can hold Integers?  (Yes)

Can we pass ArrayList<Number> to a method which accepts List<Number> in Java? (Yes)

Can we pass ArrayList<Integer> to a method which accepts List<Number>? (No) How to fix that? (use wildcards e.g. List<? extends Number>

What is volatile variable in Java?

Guarantees happens before relationship, variable's value is read by main memory

Difference between CountDownLatch and CyclicBarrier in Java?

Former cannot be reused once count reaches zero, while later can be reused even after barrier is broken

Does BlockingQueue is thread-safe?

Yes, take() and put() method of this class guarantees thread-safety, no need to externally synchronize this class for adding and retrieving objects

Why wait and notify method should be called in loop?

To prevent doing task, if condition is not true and thread is awake due to false alarms, checking conditions in loop ensures that processing is only done when business logic allows

What is difference between "abc".equals(unknown string) and unknown.equals("abc")?

Former is safe from NullPointerException

What is marker or tag interface in Java?

An interface, which presence means an instruction for JVM or compiler e.g. Serializable, from Java 5 onwards Annotation is better suited for this job.

Difference between Serializable and Externalizable interface in Java?

Later provides more control over serialization process, and allow you to define custom binary format for your object, later is also suited for performance sensitive application

Can Enum types implement interface in Java? (Yes)

Can Enum extend class in Java?

No, because Java allows a class to only extend one class and enum by default extends java.lang.Enum,

How to prevent your class from being subclassed?

Make it final or make constructor private

Can we override Static method in Java? Compilation error?

No, it can only be hidden, no compilation error

Which design pattern have you used recently?

Give any example except Singleton and MVC e.g. Decorator, Strategy or Factory pattern)

                                                                                      

Difference between StringBuffer and StringBuilder in Java?

Former is synchronized, and slow, while later is not synchronized and fast.

How to check if a thread holds lock on a particular object in Java?

Thread.holdslock() is the best way to figure out whether thread holds a lock on a particular object or not

Why ArrayList is declared with List interface List<String> myList = new ArrayList<String>();

Because if you decide to change it later it's easier:

List<String> myList = new ArrayList<String>();

List<String> myList = new LinkedList<String>();

It's more important in method signatures, when changing what you return can force everyone using your function to change their code.

Why should we use BigDecimal in monetary calculation?

public class BigDecimalExample {

    public static void main(String args[]) throws IOException {

     

      //floating point calculation

      double amount1 = 2.15;

      double amount2 = 1.10;

      System.out.println("difference between 2.15 and 1.0 using double is: " + (amount1 - amount2));

   

      //Use BigDecimal for financial calculation

      BigDecimal amount3 = new BigDecimal("2.15");

      BigDecimal amount4 = new BigDecimal("1.10") ;

      System.out.println("difference between 2.15 and 1.0 using BigDecimal is: " + (amount3.subtract(amount4)));     

    }    

}

Output:

difference between 2.15 and 1.0 using double is: 1.0499999999999998

difference between 2.15 and 1.0 using BigDecmial is: 1.05

Don’t use float and double on monetary calculation.

Use BigDecimal, long or int for monetary calculation.

Use BigDecimal with String constructor and avoid double one.

Don’t use floating point result for comparing loop conditions.

Which two method you need to implement for key Object in HashMap ?

In order to use any object as Key in HashMap, it must implements equals and hashcode methods in Java.

How HashMap works in Java

HashMap works on the principle of Hashing.  To understand Hashing, we should understand the three terms first   i.e  Hash Function , Hash Value and Bucket.

What is Hash Function, Hash Value  and Bucket ?

hashCode() function  which returns an integer value is the Hash function. The important point to note that, this method is present in Object class

This is the code for the hash function(also known as hashCode method) in Object Class:

    public native int hashCode();

The most important point to note from the above line:  hashCode method return int value.

So the Hash value is the int value returned by the hash function.

What is bucket?

A bucket is used to store key value pairs. A bucket can have multiple key-value pairs. In hash map, bucket used simple LinkedList to store objects.

After understanding the terms we are ready to move next step, How hash map works in java or How get() works internally in java.

Code inside Java API (HashMap class internal implementation) for HashMap get(Obejct key) method

1.  Public  V get(Object key)

   {

2.     if (key ==null)

3.     //Some code

   

4.     int hash = hash(key.hashCode());

   

5.     // if key found in hash table then  return value

6.     //    else return null

   }

Hash map works on the principle of hashing

HashMap get(Key k) method calls hashCode method on the key object and applies returned hashValue to its own static hash function to find a bucket location(backing array) where keys and values are stored in form of a nested class called Entry (Map.Entry) . So you have concluded that from the previous line that Both key and value is stored in the bucket as a form of  Entry object . So thinking that Only value is stored  in the bucket is not correct and will not give a good impression on the interviewer .

* Whenever we call get( Key k )  method on the HashMap object . First it checks that whether key is null or not .  Note that there can only be one null key in HashMap . 

If key is null , then Null keys always map to hash 0, thus index 0.

If key is not null then , it will call hashfunction on the key object , see line 4 in above method i.e. key.hashCode()  ,so after key.hashCode() returns hashValue , line 4 looks like

4. int hash = hash(hashValue)

 , and now ,it applies returned hashValue into its own hashing function .

We might wonder why we are calculating the hashvalue again using hash(hashValue). Answer is ,It defends against poor quality hash functions.

Now step 4 final  hashvalue is used to find the bucket location at which the Entry object is stored . Entry object stores in the bucket like this (hash,key,value,bucketindex) . 

Interviewer: What if  when two different keys have the same hashcode ?

Solution, equals() method comes to rescue.

What is Object Class?

Mother of all classes, in other words every other class in java is the subclass of Object class.

Object class is in the default package i.e. java.lang package. The Object class defines the basic state and behavior that all objects must have, such as the ability to compare oneself to another object, to convert to a string, to wait on a condition variable, to notify other objects that a condition variable has changed, and to return the object's class. There are 11 methods in Object class .

 Access                         ReturnType                         Methods                                          

Modifier

-----------------------------------------------------------------------------------

Public                             boolean                            *  equals(Object obj)

Checks whether the obj object is equal to the object on which the equals method is called.

Public                             int                                *  hashCode()

hashCode() is used  for the HashTable. It returns the hash value of the object.

Protected                       void                                  *  finalize()

Public                            Class<?>                           *  getClass()

It returns the runtime class object.

Protected                       Object                               *  clone()

It creates and  returns the copy of the object .   

Public                            void                                   *  notify()

It will wake up the thread waiting for the objects monitor.

Public                            void                                   *  notifyAll()

It will wakes up all the thread that are waiting for the objects monitor .

Public                            String                                 *  toString()

It will return the string representation of the object .

Public                           void                                     *  wait()

This method causes the current thread to place itself in the wait set for this object and then to relinquish any and all synchronization claims on this object. It releases the lock.

Public                           void                                     *  wait(long timeout)   

Public                           void                                     *  wait (long timeout , int nano )

What is immutable object? Can you write immutable object?

Immutable classes are Java classes whose objects can not be modified once created. Any modification in Immutable object result in new object. For example is String is immutable in Java. Mostly Immutable are also final in Java, in order to prevent sub class from overriding methods in Java which can compromise Immutability. You can achieve same functionality by making member as non final but private and not modifying them except in constructor.

What is the difference between creating String as new() and literal?

When we create string with new() Operator, 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 PermGen area of heap.

String s = new String("Test");

does not  put the object in String pool , we need to call String.intern() method which is used to put  them into String pool explicitly. its only when you create String object as String literal e.g. String s = "Test" Java automatically put that into String pool.

How do you handle error condition  while writing stored procedure or accessing stored procedure from java?

Stored procedure should return error code if some operation fails but if stored procedure itself fail than catching SQLException is only choice.

What is difference between Executor.submit() and Executer.execute() method

There is a difference when looking at exception handling. If your tasks throws an exception and if it was submitted with execute this exception will go to the uncaught exception handler (when you don't have provided one explicitly, the default one will just print the stack trace to System.err). If you submitted the task with submit any thrown exception, checked exception or not, is then part of the task's return status. For a task that was submitted with submit and that terminates with an exception, the Future.get will re-throw this exception, wrapped in an ExecutionException.

When do you override hashcode and equals() ?

Whenever necessary especially if you want to do equality check or want to use your object as key in HashMap.

What will be the problem if you don't override hashcode() method ?

You will not be able to recover your object from hash Map if that is used as key in HashMap.

Is it better to synchronize critical section of getInstance() method or whole getInstance() method ?

Answer is critical section because if we lock whole method than every time some one call this method will have to wait even though we are not creating any object

Does not overriding hashcode() method has any performance implication ?

This is a good question and open to all , as per my knowledge a poor hashcode function will result in frequent collision in HashMap which eventually increase time for adding an object into Hash Map.

What’s wrong using HashMap in multithreaded environment? When get() method go to infinite loop ?

Concurrent access and re-sizing

What will happen if you call return statement or System.exit on try or catch block ? will finally block execute?

Finally block will execute even if you put return statement in try block or catch block but finally block won't run if you call System.exit form try or catch.

What will happen if we put a key object in a HashMap which is already ther?

If you put the same key again than it will replace the old mapping because HashMap doesn't allow duplicate keys.

If a method throws NullPointerException in super class, can we override it with a method which throws RuntimeException?

You can very well throw super class of RuntimeException in overridden method but you cannot do same if it is checked Exception.

How do you ensure that N thread can access N resources without deadlock?

Key point here is order, if you acquire resources in a particular order and release resources in reverse order you can prevent deadlock.

What is difference between CyclicBarrier and CountDownLatch in Java?

Main difference between both of them is that you can reuse CyclicBarrier even if Barrier is broken but you cannot reuse CountDownLatch in Java

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