Hashmaps

HashMap stores sorted key - value pairs. The keys are unique, can't be modified but can be deleted.

Introduction

In order to use it, the following line needs to be added:

import java.util.HashMap;

Syntax

HashMap<keyType, valueType> mapName = new HashMap<keyType, valueType>();

Inserting Data

The inserted data must correspond to the key's and values type.

Example:

HashMap<String, Integer> scores = new HashMap<String, Integer>(); scores.put("Smith", 89); scores.put("Alex", 85); // scores.put(87, "John");

In this example, 2 records have been inserted with String keys and Integer values.

If we uncomment the last line, an exception will be thrown.

Accessing Values

Values can be accessed via their keys.

Let's look at the following example:

import java.util.HashMap; public class Main { public static void main(String[] args) { HashMap<String, Double> drinks = new HashMap<String, Double>(); drinks.put("Coke", 1.55); drinks.put("Water", 0.80); System.out.print(drinks.get("Coke")); } }

Output:

1.55

The key-value pair for Coke is set at line 7 and accessed at line 10.

Not Set Key

Accessing key that's not set in the map results in a default value - null.

Let's look at the following example:

import java.util.HashMap; public class Main { public static void main(String[] args) { HashMap<String, Double> drinks = new HashMap<String, Double>(); drinks.put("Orange Juice", 1.55); drinks.put("Water", 0.80); System.out.print(drinks.get("Coke")); } }

Output:

null

The map doesn't contain any Coke key, so null is printed.

Iterating Elements

Let's have a program in which students scores are stored in map and printed afterwards.

import java.util.HashMap; public class Main { public static void main(String[] args) { HashMap<String, Integer> scores = new HashMap<String, Integer>(); scores.put("Smith", 89); scores.put("Alex", 75); scores.put("John", 87); // keySet gets HashMap keys. for (String name : scores.keySet()) { System.out.println(name + " " + scores.get(name)); } } }

Output:

Alex 75 Smith 89 John 87

Notice that maps store data sorted by their keys. For that reason the names are printed in alphabetical order.

Removing Elements

That's done via the remove function which accepts key as argument. Like in this example:

HashMap<String, Double> drinks = new HashMap<String, Double>(); drinks.put("Coke", 1.50); drinks.put("Orange Juice", 2.30); drinks.remove("Coke");

Size

Size is the count of the key-value pairs.

It can be accessed via the size function.

Let's look at this example:

import java.util.HashMap; public class Main { public static void main(String[] args) { HashMap<String, Double> drinks = new HashMap<String, Double>(); drinks.put("Coke", 1.50); drinks.put("Orange Juice", 2.30); drinks.put("Water", 0.80); drinks.put("Apple Juice", 2.35); drinks.remove("Coke"); System.out.print(drinks.size()); } }

Output:

3

4 records were inserted and 1 is deleted afterwards. This makes for 3 key-value pairs.