How HashMap Works in Java?

HashMap in Java
HashMap store Key/Value pairs just like HashTable. It is faster than HashTable and accept null values and it is not synchronized. These are the basic fundementals we know about HashMap. However, let us try to go through how Hashmap works in Java.

HashMap works on principle of hashing, we have put(key, value) and get(key) method for storing and retrieving Objects from HashMap. When we pass Key and Value object to put() method on Java HashMap, HashMap implementation calls hashCode() method on Key object and applies returned hashcode into its own hashing function to find a bucket location for storing Entry object, important point to mention is that HashMap in Java stores both key and value object as Map.Entry in bucket. Then, there is another important concept of Collision detection and Collision resolution in Java HashMap i.e What will happen when two objects have same hashcode?
According to the equals() and hashCode() contract, we know that for two objects to be equal, they need to have same hashcode. However, the oppositeis not true. i.e Two objects with same hashcode need not necessarily be equal. Here, when 2 objects with same hashcode is returned, this condition is called collision detection.

To resolve this collision, we should know that HashMap uses LinkedList to store object i.e the Map.Entry Object which comprises of Key and Value will be stored in a LinkedList. When an object is tried to be retrieved by calling the get(Object key) method, the linked list is traversed and if 2 objects are returned which has same hashCode, then it calls the key.equals() method to find the correct node in the linked list and return its associated value. So, it is very important that we maintain unique/unmodifiable keys.
Therefore, it is nice to remember and follow that using immutable, final object with proper equals() and hashcode() implementation would act as perfect Java HashMap keys and improve performance of Java HashMap by reducing collision. Immutability also allows caching there hashcode of different keys which makes overall retrieval process very fast and suggest that String and various wrapper classes e.g. Integer very good keys in Java HashMap.

Fail-Fast And Fail-Safe Iterators/ConcurrentModificationException

Fail-Fast And Fail-Safe Iterators/ConcurrentModificationException
Quite few times, while iterating over a List or collection and trying to perform some operations, we get ConcurrentModificationException. The idea behind getting to know the true reason for this is to know the type of iterator or to be precise, the type of collection that we are iterating on.
There are 2 types of Iterators:
1. Fail-Fast
2. Fail-Safe
The Fail-Fast iterators are ones which fails (And throws ConcurrentModificationException) when the collection's structure (i.e its size) changes upon iteration i.e whenever we add or remove an item from a collection or whenever a different thread adds/removes an item from a collection which is being interating currently, the ConcurrentModificationException is thrown. Collection Classes in java.util uses Fail-Fast iterators
Fail-Safe iterators are designed to overcome this. The classes in the package java.util.concurrent uses Fail-Safe iterators. i.e they do not throw ConcurrentModificationException when the collection structure i.e size changes while it is being in the middle of iteration
Looking at the example below, we see that Iterators on ArrayList from java. util package throws ConcurrentModificationException when during the iteration, we try to add another item to the ArrayList and hence change the structure of the collection. However, CopyOnWriteArrayList class in java.util.concurrent package uses Fail-Safe interators and hence, they do not throw the exception and are thread safe.

package com.sample.collectionsexamples;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;

public class FailFastAndFailSafeIteratorDemo {
 
   public static void main(String[] args) {
     List arrayList = new ArrayList();
     List copyOnWriteArrayList = new CopyOnWriteArrayList<>();
 arrayList.add(10);
 arrayList.add(20);
 arrayList.add(30);
 arrayList.add(40);
  
 copyOnWriteArrayList.add(10);
 copyOnWriteArrayList.add(20);
 copyOnWriteArrayList.add(30);
 copyOnWriteArrayList.add(40);
  
 iterateOperation("CopyOnWriteArrayList",copyOnWriteArrayList);
 iterateOperation("ArrayList", arrayList);
  
 }
 
 static void iterateOperation(String type, List numbers) {
   System.out.println("Iterating and adding item on: "+type);
       Iterator itr = numbers.iterator();
   while(itr.hasNext()) {
  // Add another integer 50 after 40
  // This code to add will fail for ArrayList 
                // throwing ConcurrentModificationException
  // But it works fine for CopyOnWriteArrayList
      if(itr.next() == 40) {
     numbers.add(50);
   }
 }
 System.out.println("Items in the list "+type+" = "+numbers);
} 

}

Java 7 new features

1. Java 7 Switch String Support
Until the previous version, Switch statements work either with primitive types or enumerated types. Java 7 introduced another type that we can use in Switch statements: the String type. i.e Prior to Java 7, it supported variables and expressions which resolved into int, byte, short or char only (OR) it supported Enums. But now, Java 7 has added supporting String types for Switch statement Case values and with this feature, it will surely save a lot of complex if-else structures and improve the overall code readability. Here is a self explanatory example of Switch with Strings in action. We simply accept the String input and print the corresponding matching case

package com.java7.features;

import java.util.Scanner;

public class Java7SwitchDemo {
    public static void main(String[] args) {
 System.out.println(" Sachin\n Dhoni\n Kumble\n ");
 Scanner sc = new Scanner(System.in);
 String cricketer = sc.nextLine();
 System.out.println("\n");
 switch (cricketer.toUpperCase()) {
    case "SACHIN":
         System.out.println("You like batting");
         break;
    case "DHONI":
         System.out.println("You like Wicket keeping");
  break;
    case "KUMBLE":
         System.out.println("You like bowling");
                break;
    default:
  System.out.println("You dont like cricket");
 }
  
   }
}

2. Java 7 Try with resources
The try-with-resources statement is a try statement that declares one or more resources. A resource is an object that must be closed after the program is finished with it. The try-with-resources statement ensures that each resource is closed at the end of the statement. Any object that implements java.lang.AutoCloseable, which includes all objects which implement java.io.Closeable, can be used as a resource. In the below example, the resources declared in the try-with-resources statement are: Connection, Statement and ResultSet from java.sql package. The declaration statement appears within parentheses immediately after the try keyword. The class Connection, Statement and ResultSet, in Java SE 7 and later, implements the interface java.lang.AutoCloseable. Because these instances are declared in a try-with-resource statement, it will be closed regardless of whether the try statement completes normally or abruptly (as a result of the method throwing an SQLException). Prior to Java SE 7, you have to use a finally block to ensure that a resource is closed regardless of whether the try statement completes normally or abruptly.

package com.java7.features;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class TryWithResources {
   public static void main(String[] args) throws SQLException {
      try(Connection con = DriverManager.getConnection("jdbc:oracle:thin:@localhost:1521:orcl","scott","tiger");
 Statement stmt = con.createStatement();
 ResultSet rs = stmt.executeQuery("select * from dept")) 
 {
      while(rs.next()) {
      System.out.println("Employee Name : "+rs.getString("deptno"));
      System.out.println("Salary : "+rs.getString("dnmae"));
    }
       };
    }
}

Note: It should be noted that the resources will be closed in the reverse order of their declaration i.e in the above example, first the ResultSet will be closed followed by Statement interface and then finally, the Connection will get closed.

3. Java 7 Multi catch block
In the pre Java 7 world, you would handle exceptions by writing separate catch blocks for different types of exceptions that can occur in the try block. In case of related exception, the derived class exception are listed above the base class exception, e.g. FileNotFoundException should be listed above IOException since FileNotFoundException is derived from the IOException. Java 7 multi catch block provides you the flexibility to combine such catch blocks, provided you have similar code to handle these exceptions and Java 7 provides this by the help of the pipe (|) operator which seperates the different exceptions. Lets have a look at the below example where we handle all the exceptions thrown by the try block in a single catch block, but still specifying the type of exceptions we are handling:

package com.java7.features;

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
 
public class Java7MultiCatch {
  public static void main(String[] args) {
     try {
       BufferedReader br = new BufferedReader(new FileReader(
          "c:/test.txt"));
       Class.forName("oracle.jdbc.driver.OracleDriver");
       Connection con = DriverManager.getConnection("jdbc:oracle:thin:@127.0.0.1:1565:myDB");
        } catch (IOException | ClassNotFoundException | SQLException e) {
            System.out.println("Multi catch block here!");
            e.printStackTrace();
        }
 
    }
}

Note: It should be noted that when we use multi catch block, we cannot place the exceptions in a hierarchy seperated by the pipe (|) operator like catch(FileNotFoundException | IOException e) because then the compiler will generate an error that the 'subclass exception (i.e FileNotFoundException) is already caught by an alternative superclass exception (i.e IOException)' which seems to be valid where there is a single exception's stack trace is something we will be dealing with.

Java 7 also introduced new support for File IO operations i.e it introduced Java NIO 2.0 API where the most important concepts are : Path Class and the File Change Notifications using Watch service. I will post it as a seperate entry in the blog soon. Till then, Enjoy reading more about the exciting features of Java 7 :)