Coffee break #224. How to iterate through a Hashmap using a loop. Understanding Void and Null in Java with Examples

How to Iterate through a Hashmap Using a Loop

Source: FreeCodeCamp This tutorial takes a detailed look at how to iterate over a Hashmap using different types of loops in Java. Hashmap is a data structure used to store data in key-value pairs. It is widely used in many programming languages, including Java, Python and JavaScript. Iterating over a Hashmap is a common operation that developers often perform. In most cases, its steps are quite simple. You first initiate a Hashmap, then use an iterator to iterate over it, and finally display the result.

How to Iterate through a Hashmap in Java Using a For-each Loop

One of the easiest ways to iterate over a Hashmap is to use a for-each loop . Here's an example of how to do it:

HashMap<String, Integer> map = new HashMap<>();
map.put("A", 1);
map.put("B", 2);
map.put("C", 3);

for (Map.Entry<String, Integer> entry : map.entrySet()) {
    String key = entry.getKey();
    Integer value = entry.getValue();
    System.out.println("Key: " + key + ", Value: " + value);
}

In this example, we first create a new Hashmap and add some key-value pairs to it. We then use a for-each loop to iterate through the Hashmap , retrieving each key-value pair as a Map.Entry object . We then extract the key and value from each Map.Entry object and print them to the console.

How to iterate over a Hashmap in Java using a while loop with an iterator

Another way to iterate over a Hashmap is to use a while loop with an iterator. Here is an example of how to do this:

HashMap<String, Integer> map = new HashMap<>();
map.put("A", 1);
map.put("B", 2);
map.put("C", 3);

Iterator<Map.Entry<String, Integer>> iterator = map.entrySet().iterator();
while (iterator.hasNext()) {
    Map.Entry<String, Integer> entry = iterator.next();
    String key = entry.getKey();
    Integer value = entry.getValue();
    System.out.println("Key: " + key + ", Value: " + value);
}

Here we again create a new Hashmap and add some key-value pairs to it. We then create a new Iterator object using the entrySet() method , which returns a set of key-value pairs as Map.Entry objects . We then use a while loop with the hasNext() and next() methods to iterate through the set and extract each key-value pair. Finally, we extract the key and value from each Map.Entry object and print them to the console.

How to iterate over a Java Hashmap using a for loop with keySet()

In Java, the keySet() method is a method of the java.util.HashMap class that returns a set representation of the keys contained in the Hashmap . This means that it returns a set of all the keys in the Hashmap , which can be used to iterate over the keys or perform other operations on them. The fact that the keySet() method returns a set of unique elements without duplicates is because the keys in a Hashmap must be unique, and the keySet() method ensures that the key set it returns does not contain duplicate values. We can also iterate over a Hashmap using a for loop with the keySet() method . Here's an example of how to do this:

ashMap<String, Integer> map = new HashMap<>();
map.put("A", 1);
map.put("B", 2);
map.put("C", 3);

for (String key : map.keySet()) {
    Integer value = map.get(key);
    System.out.println("Key: " + key + ", Value: " + value);

In this example, we again create a new Hashmap and add several key-value pairs to it. We then use a for loop with the keySet() method to iterate through the Hashmap , extracting each key and using it to get the corresponding value from the Hashmap . We then print the key and value to the console.

Conclusion

In this article, you learned three ways to iterate over a Hashmap using different types of loops. By following these guidelines, you will be able to work better with Hashmap and other data structures. Please note that changing the Hashmap during an iteration may produce unexpected results, so this should be avoided if possible.

Understanding Void and Null in Java with Examples

Source: Medium This guide will teach you the differences between Void and Null , as well as a few examples of how to use them. There are two very commonly used keywords in the Java programming language - Void and Null . Both are used to indicate absence, but in different contexts. Moreover, their usage and behavior are different from each other.

Void

In Java, the Void keyword is used to indicate the absence of a value. It is typically used as the return type for methods that do not return a value. Void methods are executed because of their side effects, such as printing something to the console or changing the state of an object. Here is an example of the Void method :

public void printMessage(String message) {
    System.out.println(message);
}

In this example, the printMessage method takes a string argument named message and prints it to the console using the System.out.println() method . The return type of the method is Void , which means the method does not return a value.

Null

Null is used to indicate that there is no reference to an object. It is commonly used in Java to initialize variables or to indicate that an object does not exist. Null is not a keyword in Java, but rather a reserved word that represents a literal value that can be assigned to any object reference variable. Here is an example of using Null to initialize a variable:

String message = null;

In this example, the message variable is initialized to Null . This means that it does not point to any object in memory.

What are the differences between Void and Null

The main difference between Void and Null is that Void represents the absence of a value, while Null represents the absence of an object reference. Void is used to declare methods that do not return a value, while Null is used to initialize variables or to indicate that an object does not exist. Another difference between Void and Null is that Void is a keyword in Java and Null is a reserved word. Void is also used in Java to declare generic types such as Void<T> , which represent a generic type that does not contain any data. Additionally, Void can only be used as a return type for methods, but Null can be assigned to any object reference variable.

Conclusion

Void and Null are important concepts in the Java programming language. Void represents the absence of a value and is commonly used as the return type for methods that do not return a value. Null , on the other hand, represents the absence of a reference to an object and is typically used to initialize variables or to indicate that an object does not exist. By clearly understanding the difference between Void and Null , you can write more efficient code.