Coffee break #173. What is the difference between HashMap and Hashtable. How to find and fix NullPointerException in Java

What is the difference between HashMap and Hashtable

Source: Medium Today you will learn the answer to one of the most popular Java interview questions: what is the difference between HashMap and Hashtable. A Hashtable is a data structure for storing key pairs and their values, based on hashing and an implementation of the Map interface. HashMap is also a data structure for storing keys and values, based on hashing and an implementation of the Map interface. HashMap allows you to quickly get a value by key. For example, the user's name and phone number. There are significant differences between HashMap and Hashtable in Java . These include thread safety, synchronization, and Map implementation consistency. It is important to consider these aspects when choosing the type of data structure to use in your Java program: A HashMap is a non-synchronized , unordered map of key-value pairs. It allows null values ​​and uses a hash code as an equality check, while Hashtable is a synchronized ordered map of key-value pairs. It does not allow null values ​​and uses the equals() method to check for equality. A HashMap has a default capacity of 16, and a Hashtable has a default initial capacity of 11. The values ​​of a HashMap object are iterated over using an iterator, and Hashtable is the only class other than a vector that uses an enumerator to iterate over the values ​​of a Hashtable object. Remember that the differences between HashMap and Hashtable is one of the most popular questions in Java interviews.

How to find and fix NullPointerException in Java

Source: Theflashreads After reading this article, you will be better able to find and fix a common error in Java code - NullPointerException.

What is a NullPointerException?

java.lang.NullPointerException is a runtime exception in Java. It occurs when an attempt is made to access a variable that does not point to or refer to any object, that is, it is null. Because NullPointerException is a run-time exception, it does not need to be caught and handled explicitly in application code.

Under what circumstances does a NullPointerException occur?

NullPointerException occurs when an attempt is made to access an uninitialized object. As already mentioned, this happens when the object reference goes nowhere and has a null value. Here are some of the most common NullPointerException exception scenarios :

• Access properties of a null object.
• Throwing null from a method that throws an exception.
• Incorrect configuration for dependency injection frameworks, for example: (Spring).
• Passing null parameters to a method.
• Calling methods on a null object.
• Using synchronized on a null object.
• Access the index element (for example, in an array) of a null object.

Example NullPointerException

In this example, we are trying to call the printLength method , which takes a string as a parameter and prints its length. If the value of the string that is passed as a parameter is null, a java.lang.NullPointerException will be thrown .

public class NullPointerExceptionExample {

    private static void printLength(String str) {

        System.out.println(str.length());

    }



    public static void main(String args[]) {

        String nullString = null;

        printLength(nullString);

    }

}





Exception in thread "main" java.lang.NullPointerException

    at NullPointerExceptionExample.printLength(NullPointerExceptionExample.java:3)

    at NullPointerExceptionExample.main(NullPointerExceptionExample.java:8)

How to avoid throwing NullPointerException

NullPointerException can be avoided by using the following checks and preventative methods:

• Ensure that an object is properly initialized by adding a null check before accessing its methods or properties.

• Use Apache Commons StringUtils for string operations: for example, StringUtils.isNotEmpty() to check whether a string is empty before using it further.

  import org.apache.commons.lang3.StringUtils;
  
  
  
  public class NullPointerExceptionExample {
  
      private static void printLength(String str) {
  
          if (StringUtils.isNotEmpty(str)) {
  
              System.out.println(str.length());
  
          } else {
  
              System.out.println("This time there is no NullPointerException");
  
          }
  
      }
  
  
  
      public static void main(String args[]) {
  
          String nullString = null;
  
          printLength(nullString);
  
      }
  
  }

• Use primitives rather than objects whenever possible, since they cannot have null references. For example, use int instead of Integer and boolean instead of Boolean .

• Make sure you double check your code. This will reduce the likelihood of a NullPointerException being thrown .