Hashmaps

Hashmaps are helpful tools that allow us to store a key and a value. Unlike Arrays and Arraylists, Hashmaps allow us to easily look up a value if we know its key. With that said, let's say you are creating a piece of software that stores student grades for a class. We want to be able to look up a student's grade by their name, so we will need to use a Hashmap.

What a Hashmap Looks Like

With Hashmaps we assign a key and a value associated with that key. If we are creating a piece of software to store student grades, our keys are the students' names and the value is the grade.

This would look like:

Key	Value
Wezley Sherman	85
Wade Adams	95
Julio Rodriguez	84
Trevor Forrey	100
Ada Lovelace	96
Sally Pants	98
Sarah Abe	86

The Key is the element we use to look up a specific Value in the map. The Value is the result we get when we look up a specific item with a Key. The Key-Value Pair is the combination of the Key and the Value associated.

It is important to remember that each key can only have one value, and key-value pairs are not ordered.

Utilizing Hashmaps

Creating Hashmaps:

In order to create a Hashmap we use: Hashmap<key_type, value_type> name = new Hashmap<key_type, value_type>();. If we were to create our grade book Hashmap we would use:

Hashmap<String, Integer> gradeBook = new Hashmap<String, Integer>();

Adding Items:

To add an item to a Hashmap we use: map.put(key, value); If we were to add grades to our grade book Hashmap we would use:

gradeBook.put("Wezley Sherman", 85);
gradeBook.put("Wade Adams", 95);
gradeBook.put("Julio Rodriguez", 84);
...

Retrieving Items:

To retrieve an item from a Hashmap we use: map.get(key); Let's say we wanted to access Trevor Forry's and Ada Lovelace's grades. To do so we would use:

// `adaGrade` would contain `96`
int adaGrade = gradeBook.get("Ada Lovelace");
// `trevGrade` would contain `100`
int trevGrade = gradeBook.get("Trevor Forrey");

Iterating Over a Hashmap:

Now, let's say we want to print out every grade in our grade book. In order to do this, we would have to iterate over our Hashmap. Here is an example of what that would look like:

System.out.println("Class Grades: ");

// For each `key` in gradeBook.keySet()
for(String key: in gradeBook.keySet) 
{
  int grade = gradeBook.get(key);
  System.out.println(key + ": " + grade);
}