- Java Introduction
- Java Getting Started
- Java Syntax
- Java Comments
- Java Variables
- Java Data Types
- Java Type Casting
- Java Operators
- Java Strings
- Java Math
- Java Booleans
- Java If ... Else
- Java Switch
- Java While Loop
- Java For Loop
- Java Break And Continue
- Java Arrays
- Java Methods
- Java Method Parameters
- Java Method Overloading
- Java Scope
- Java Recursion
- Java OOP
- Java Classes And Objects
- Java Class Attributes
- Java Class Methods
- Java Constructors
- Java Modifiers
- Java Encapsulation
- Java Packages
- Java Inheritance
- Java Polymorphism
- Java Inner Classes
- Java Abstraction
- Java Interface
- Java Enums
- Java User Input (Scanner)
- Java Date And Time
- Java ArrayList
- Java LinkedList
- Java HashMap
- Java HashSet
- Java Iterator
- Java Wrapper Classes
- Java Exceptions - Try...Catch
- Java Regular Expressions
- Java Threads
- Java Lambda Expressions
- Java Files
- Java Create And Write To Files
- Java Read Files
- Java Delete Files
Java HashMap
Java HashMap
In the ArrayList
chapter, you learned that Arrays store items as an ordered collection, and you have to access them with an index number (int
type). A HashMap
however, store items in "key/value" pairs, and you can access them by an index of another type (e.g. a String
).
One object is used as a key (index) to another object (value). It can store different types: String
keys and Integer
values, or the same type, like: String
keys and String
values:
Example
Create a HashMap
object called capitalCities that will store String
keys and String
values:
import java.util.HashMap; // import the HashMap class
HashMap<String, String> capitalCities = new HashMap<String, String>();
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Add Items
The HashMap
class has many useful methods. For example, to add items to it, use the put()
method:
Example
// Import the HashMap class
import java.util.HashMap;
public class Main {
public static void main(String[] args) {
// Create a HashMap object called capitalCities
HashMap<String, String> capitalCities = new HashMap<String, String>();
// Add keys and values (Country, City)
capitalCities.put("England", "London");
capitalCities.put("Germany", "Berlin");
capitalCities.put("Norway", "Oslo");
capitalCities.put("USA", "Washington DC");
System.out.println(capitalCities);
}
}
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Access An Item
To access a value in the HashMap
, use the get()
method and refer to its key:
Example
capitalCities.get("England");
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Remove An Item
To remove an item, use the remove()
method and refer to the key:
Example
capitalCities.remove("England");
To remove all items, use the clear()
method:
Example
capitalCities.clear();
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HashMap Size
To find out how many items there are, use the size()
method:
Example
capitalCities.size();
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Loop Through A HashMap
Loop through the items of a HashMap
with a for-each loop.
Note: Use the keySet()
method if you only want the keys, and use the values()
method if you only want the values:
Example
// Print keys
for (String i : capitalCities.keySet()) {
System.out.println(i);
}
Example
// Print values
for (String i : capitalCities.values()) {
System.out.println(i);
}
Example
// Print keys and values
for (String i : capitalCities.keySet()) {
System.out.println("key: " + i + " value: " + capitalCities.get(i));
}
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Other Types
Keys and values in a HashMap are actually objects. In the examples above, we used objects of type "String". Remember that a String in Java is an object (not a primitive type). To use other types, such as int, you must specify an equivalent wrapper class: Integer
. For other primitive types, use: Boolean
for boolean, Character
for char, Double
for double, etc:
Example
Create a HashMap
object called people that will store String
keys and Integer
values:
// Import the HashMap class
import java.util.HashMap;
public class Main {
public static void main(String[] args) {
// Create a HashMap object called people
HashMap<String, Integer> people = new HashMap<String, Integer>();
// Add keys and values (Name, Age)
people.put("John", 32);
people.put("Steve", 30);
people.put("Angie", 33);
for (String i : people.keySet()) {
System.out.println("key: " + i + " value: " + people.get(i));
}
}
}
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