Maps

Java HashMap is a hash table based implementation of Java’s Map interface. A Map, as you might know, is a collection of key-value pairs. It maps keys to values.

Following are few key points to note about HashMaps in Java -

• A HashMap cannot contain duplicate keys.

• Java HashMap allows null values and the null key.

• HashMap is an unordered collection. It does not guarantee any specific order of the elements.

• Java HashMap is not thread-safe. You must explicitly synchronize concurrent modifications to the HashMap.

Creating a HashMap and Adding key-value pairs to it

The following example shows how to create a HashMap, and add new key-value pairs to it.

import java.util.HashMap;
import java.util.Map;

public class CreateHashMapExample {
    public static void main(String[] args) {
        // Creating a HashMap
        Map<String, Integer> numberMapping = new HashMap<>();

        // Adding key-value pairs to a HashMap
        numberMapping.put("One", 1);
        numberMapping.put("Two", 2);
        numberMapping.put("Three", 3);

        // Add a new key-value pair only if the key does not exist in the HashMap, or is mapped to `null`
        numberMapping.putIfAbsent("Four", 4);

        System.out.println(numberMapping);
    }
}

# Output
{One=1, Four=4, Two=2, Three=3}

Accessing keys and modifying their associated value in a HashMap

The example below shows:

• How to check if a HashMap is empty | isEmpty()

• How to find the size of a HashMap | size()

• How to check if a given key exists in a HashMap | containsKey()

• How to check if a given value exists in a HashMap | containsValue()

• How to get the value associated with a given key in the HashMap | get()

• How to modify the value associated with a given key in the HashMap | put()

import java.util.HashMap;
import java.util.Map;

public class AccessKeysFromHashMapExample {
    public static void main(String[] args) {
        Map<String, String> userCityMapping = new HashMap<>();

        // Check if a HashMap is empty
        System.out.println("is userCityMapping empty? : " + userCityMapping.isEmpty());

        userCityMapping.put("John", "New York");
        userCityMapping.put("Rajeev", "Bengaluru");
        userCityMapping.put("Steve", "London");

        System.out.println("userCityMapping HashMap : " + userCityMapping);

        // Find the size of a HashMap
        System.out.println("We have the city information of " + userCityMapping.size() + " users");

        String userName = "Steve";
        // Check if a key exists in the HashMap
        if(userCityMapping.containsKey(userName)) {
            // Get the value assigned to a given key in the HashMap
            String city = userCityMapping.get(userName);
            System.out.println(userName + " lives in " + city);
        } else {
            System.out.println("City details not found for user " + userName);
        }

        // Check if a value exists in a HashMap
        if(userCityMapping.containsValue("New York")) {
            System.out.println("There is a user in the userCityMapping who lives in New York");
        } else {
            System.out.println("There is no user in the userCityMapping who lives in New York");
        }


        // Modify the value assigned to an existing key
        userCityMapping.put(userName, "California");
        System.out.println(userName + " moved to a new city " + userCityMapping.get(userName) + ", New userCityMapping : " + userCityMapping);

        // The get() method returns `null` if the specified key was not found in the HashMap
        System.out.println("Lisa's city : " + userCityMapping.get("Lisa"));
    }
}

# Output
is userCityMapping empty? : true
userCityMapping HashMap : {Steve=London, John=New York, Rajeev=Bengaluru}
We have the city information of 3 users
Steve lives in London
There is a user in the userCityMapping who lives in New York
Steve moved to a new city California, New userCityMapping : {Steve=California, John=New York, Rajeev=Bengaluru}
Lisa's city : null

Removing keys from a HashMap

The following example shows how to :

• Remove a key from a HashMap | remove(Object key)

• Remove a key from a HashMap only if it is associated with a given value | remove(Object key, Object value)

import java.util.HashMap;
import java.util.Map;

public class RemoveKeysFromHashMapExample {
    public static void main(String[] args) {
        Map<String, String> husbandWifeMapping = new HashMap<>();
        husbandWifeMapping.put("Jack", "Marie");
        husbandWifeMapping.put("Chris", "Lisa");
        husbandWifeMapping.put("Steve", "Jennifer");

        System.out.println("Husband-Wife Mapping : " + husbandWifeMapping);

        // Remove a key from the HashMap
        // Ex - Unfortunately, Chris got divorced. Let's remove him from the mapping
        String husband = "Chris";
        String wife = husbandWifeMapping.remove(husband);

        System.out.println("Couple (" + husband + " => " + wife + ") got divorced");
        System.out.println("New Mapping : " + husbandWifeMapping);

        // Remove a key from the HashMap only if it is mapped to the given value
        // Ex - Divorce "Jack" only if He is married to "Linda"
        boolean isRemoved = husbandWifeMapping.remove("Jack", "Linda");
        System.out.println("Did Jack get removed from the mapping? : " + isRemoved);

        // remove() returns null if the mapping was not found for the supplied key
        wife = husbandWifeMapping.remove("David");
        if(wife == null) {
            System.out.println("Looks like David is not married to anyone");
        } else {
            System.out.println("Removed David and his wife from the mapping");
        }
    }
}

# Output
Husband-Wife Mapping : {Steve=Jennifer, Chris=Lisa, Jack=Marie}
Couple (Chris => Lisa) got divorced
New Mapping : {Steve=Jennifer, Jack=Marie}
Did Jack get removed from the mapping? : false
Looks like David is not married to anyone

Obtaining the entrySet, keySet, and values from a HashMap

The Map interface provides methods to retrieve the set of entries (key-value pairs), the set of keys, and the collection of values.

The following example shows how to retrieve them from a HashMap -

import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class HashMapEntryKeySetValuesExample {
    public static void main(String[] args) {
        Map<String, String> countryISOCodeMapping = new HashMap<>();

        countryISOCodeMapping.put("India", "IN");
        countryISOCodeMapping.put("United States of America", "US");
        countryISOCodeMapping.put("Russia", "RU");
        countryISOCodeMapping.put("Japan", "JP");
        countryISOCodeMapping.put("China", "CN");

        // HashMap's entry set
        Set<Map.Entry<String, String>> countryISOCodeEntries = countryISOCodeMapping.entrySet();
        System.out.println("countryISOCode entries : " + countryISOCodeEntries);

        // HashMap's key set
        Set<String> countries = countryISOCodeMapping.keySet();
        System.out.println("countries : " + countries);

        // HashMap's values
        Collection<String> isoCodes = countryISOCodeMapping.values();
        System.out.println("isoCodes : " + isoCodes);
    }
}

# Output
countryISOCode entries : [United States of America=US, Japan=JP, China=CN, India=IN, Russia=RU]
countries : [United States of America, Japan, China, India, Russia]
isoCodes : [US, JP, CN, IN, RU]

Iterating over a HashMap

The following example shows different ways of iterating over a HashMap -

1. Iterating over a HashMap using Java 8 forEach and lambda expression.

2. Iterating over the HashMap’s entrySet using iterator().

3. Iterating over the HashMap’s entrySet using Java 8 forEach and lambda expression.

4. Iterating over the HashMap’s entrySet using simple for-each loop.

5. Iterating over the HashMap’s keySet.

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

public class IterateOverHashMap {
    public static void main(String[] args) {
        Map<String, Double> employeeSalary = new HashMap<>();
        employeeSalary.put("David", 76000.00);
        employeeSalary.put("John", 120000.00);
        employeeSalary.put("Mark", 95000.00);
        employeeSalary.put("Steven", 134000.00);

        System.out.println("=== Iterating over a HashMap using Java 8 forEach and lambda ===");
        employeeSalary.forEach((employee, salary) -> {
            System.out.println(employee + " => " + salary);
        });

        System.out.println("\n=== Iterating over the HashMap's entrySet using iterator() ===");
        Set<Map.Entry<String, Double>> employeeSalaryEntries = employeeSalary.entrySet();
        Iterator<Map.Entry<String, Double>> employeeSalaryIterator = employeeSalaryEntries.iterator();
        while (employeeSalaryIterator.hasNext()) {
            Map.Entry<String, Double> entry = employeeSalaryIterator.next();
            System.out.println(entry.getKey() + " => " + entry.getValue());
        }

        System.out.println("\n=== Iterating over the HashMap's entrySet using Java 8 forEach and lambda ===");
        employeeSalary.entrySet().forEach(entry -> {
            System.out.println(entry.getKey() + " => " + entry.getValue());
        });

        System.out.println("\n=== Iterating over the HashMap's entrySet using simple for-each loop ===");
        for(Map.Entry<String, Double> entry: employeeSalary.entrySet()) {
            System.out.println(entry.getKey() + " => " + entry.getValue());
        }

        System.out.println("\n=== Iterating over the HashMap's keySet ===");
        employeeSalary.keySet().forEach(employee -> {
            System.out.println(employee + " => " + employeeSalary.get(employee));
        });
    }
}

# Output
=== Iterating over a HashMap using Java 8 forEach and lambda ===
David => 76000.0
John => 120000.0
Mark => 95000.0
Steven => 134000.0

=== Iterating over the HashMap's entrySet using iterator() ===
David => 76000.0
John => 120000.0
Mark => 95000.0
Steven => 134000.0

=== Iterating over the HashMap's entrySet using Java 8 forEach and lambda ===
David => 76000.0
John => 120000.0
Mark => 95000.0
Steven => 134000.0

=== Iterating over the HashMap's entrySet using simple for-each loop ===
David => 76000.0
John => 120000.0
Mark => 95000.0
Steven => 134000.0

=== Iterating over the HashMap's keySet ===
David => 76000.0
John => 120000.0
Mark => 95000.0
Steven => 134000.0

Java HashMap with User defined objects

Check out the following example to learn how to create and work with a HashMap of user defined objects.

import java.util.HashMap;
import java.util.Map;

class Employee {
    private Integer id;
    private String name;
    private String city;

    public Employee(Integer id, String name, String city) {
        this.id = id;
        this.name = name;
        this.city = city;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getCity() {
        return city;
    }

    public void setCity(String city) {
        this.city = city;
    }

    @Override
    public String toString() {
        return "Employee{" +
                "name='" + name + '\'' +
                ", city='" + city + '\'' +
                '}';
    }
}

public class HashMapUserDefinedObjectExample {
    public static void main(String[] args) {
        Map<Integer, Employee> employeesMap = new HashMap<>();

        employeesMap.put(1001, new Employee(1001, "Rajeev", "Bengaluru"));
        employeesMap.put(1002, new Employee(1002, "David", "New York"));
        employeesMap.put(1003, new Employee(1003, "Jack", "Paris"));

        System.out.println(employeesMap);
    }
}

# Output
{1001=Employee{name='Rajeev', city='Bengaluru'}, 1002=Employee{name='David', city='New York'}, 1003=Employee{name='Jack', city='Paris'}}

Problems:

1. Write a Java program to associate the specified value with the specified key in a HashMap. Go to the editor Click me to see the solution

2. Write a Java program to count the number of key-value (size) mappings in a map. Go to the editor Click me to see the solution

3. Write a Java program to copy all of the mappings from the specified map to another map. Go to the editor Click me to see the solution

4. Write a Java program to remove all of the mappings from a map. Go to the editor Click me to see the solution

5. Write a Java program to check whether a map contains key-value mappings (empty) or not. Go to the editor Click me to see the solution

6. Write a Java program to get a shallow copy of a HashMap instance. Go to the editor Click me to see the solution

7. Write a Java program to test if a map contains a mapping for the specified key. Go to the editor Click me to see the solution

8. Write a Java program to test if a map contains a mapping for the specified value. Go to the editor Click me to see the solution

9. Write a Java program to create a set view of the mappings contained in a map. Go to the editor Click me to see the solution

10. Write a Java program to get the value of a specified key in a map. Go to the editor Click me to see the solution

11. Write a Java program to get a set view of the keys contained in this map. Go to the editor Click me to see the solution

12. Write a Java program to get a collection view of the values contained in this map. Go to the editor Click me to see the solution

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