Java HashMap

HashMap is a hash table that stores key-value pairs.

HashMap implements the Map interface, stores data based on the hash code of the key, provides fast access, allows at most one null key, and does not support thread synchronization.

HashMap is unordered, meaning it does not record the insertion order.

HashMap extends AbstractMap and implements the Map, Cloneable, and java.io.Serializable interfaces.

The key and value types in HashMap can be the same or different, and can be of type String or Integer.

The elements in HashMap are actually objects. Common primitive types can use their wrapper classes.

The table for primitive types and their corresponding wrapper classes is as follows:

Primitive Type	Reference Type
boolean	Boolean
byte	Byte
short	Short
int	Integer
long	Long
float	Float
double	Double
char	Character

The HashMap class is located in the java.util package and needs to be imported before use, with the following syntax:

import java.util.HashMap; // Import the HashMap class

The following example creates a HashMap object named Sites with Integer keys and String values:

HashMap&lt;Integer, String> Sites = new HashMap&lt;Integer, String>();

Adding Elements

HashMap provides many useful methods. The put() method can be used to add key-value pairs:

Example

// Import the HashMap class      
import java.util.HashMap;

public class tutorialproTest {
    public static void main(String[] args) {
        // Create a HashMap object named Sites
        HashMap&lt;Integer, String> Sites = new HashMap&lt;Integer, String>();
        // Add key-value pairs
        Sites.put(1, "Google");
        Sites.put(2, "tutorialpro");
        Sites.put(3, "Taobao");
        Sites.put(4, "Zhihu");
        System.out.println(Sites);
    }
}

Executing the above code will produce the following output:

{1=Google, 2=tutorialpro, 3=Taobao, 4=Zhihu}

The following example creates a HashMap with String keys and String values:

Example

// Import the HashMap class      
import java.util.HashMap;

public class tutorialproTest {
    public static void main(String[] args) {
        // Create a HashMap object named Sites
        HashMap&lt;String, String> Sites = new HashMap&lt;String, String>();
        // Add key-value pairs
        Sites.put("one", "Google");
        Sites.put("two", "tutorialpro");
        Sites.put("three", "Taobao");
        Sites.put("four", "Zhihu");
        System.out.println(Sites);
    }
}

Executing the above code will produce the following output:

{four=Zhihu, one=Google, two=tutorialpro, three=Taobao}

Accessing Elements

We can use the get(key) method to retrieve the value corresponding to a key:

Example

// Import the HashMap class      
import java.util.HashMap;

public class tutorialproTest {
    public static void main(String[] args) {
        // Create a HashMap object named Sites
        HashMap&lt;String, String> Sites = new HashMap&lt;String, String>();
        // Add key-value pairs
        Sites.put("one", "Google");
        Sites.put("two", "tutorialpro");
        Sites.put("three", "Taobao");
        Sites.put("four", "Zhihu");
        System.out.println(Sites.get("one")); // Output the value for key "one"
    }
}

Executing the above code will produce the following output:

Google

public class tutorialproTest {
    public static void main(String[] args) {
        // Create HashMap object Sites
        HashMap&lt;Integer, String> Sites = new HashMap&lt;Integer, String>();
        // Add key-value pairs
        Sites.put(1, "Google");
        Sites.put(2, "tutorialpro");
        Sites.put(3, "Taobao");
        Sites.put(4, "Zhihu");
        System.out.println(Sites.get(3));
    }
}

Executing the above code will output the following:

Taobao

Deleting Elements

We can use the remove(key) method to delete the key-value pair corresponding to the key:

Example

// Import HashMap class
import java.util.HashMap;

public class tutorialproTest {
    public static void main(String[] args) {
        // Create HashMap object Sites
        HashMap&lt;Integer, String> Sites = new HashMap&lt;Integer, String>();
        // Add key-value pairs
        Sites.put(1, "Google");
        Sites.put(2, "tutorialpro");
        Sites.put(3, "Taobao");
        Sites.put(4, "Zhihu");
        Sites.remove(4);
        System.out.println(Sites);
    }
}

Executing the above code will output the following:

{1=Google, 2=tutorialpro, 3=Taobao}

To delete all key-value pairs, you can use the clear method:

Example

// Import HashMap class
import java.util.HashMap;

public class tutorialproTest {
    public static void main(String[] args) {
        // Create HashMap object Sites
        HashMap&lt;Integer, String> Sites = new HashMap&lt;Integer, String>();
        // Add key-value pairs
        Sites.put(1, "Google");
        Sites.put(2, "tutorialpro");
        Sites.put(3, "Taobao");
        Sites.put(4, "Zhihu");
        Sites.clear();
        System.out.println(Sites);
    }
}

Executing the above code will output the following:

{}

Calculating Size

To calculate the number of elements in the HashMap, you can use the size() method:

Example

// Import HashMap class
import java.util.HashMap;

public class tutorialproTest {
    public static void main(String[] args) {
        // Create HashMap object Sites
        HashMap&lt;Integer, String> Sites = new HashMap&lt;Integer, String>();
        // Add key-value pairs
        Sites.put(1, "Google");
        Sites.put(2, "tutorialpro");
        Sites.put(3, "Taobao");
        Sites.put(4, "Zhihu");
        System.out.println(Sites.size());
    }
}

Executing the above code will output the following:

Iterating Over HashMap

You can use a for-each loop to iterate over the elements in the HashMap.

If you only want to retrieve the keys, you can use the `keySet()` method, and then you can get the corresponding value using `get(key)`. If you only want to retrieve the values, you can use the `values()` method.

## Example

```java
// Import the HashMap class    
import java.util.HashMap;

public class tutorialproTest {
    public static void main(String[] args) {
        // Create a HashMap object called Sites
        HashMap&lt;Integer, String> Sites = new HashMap&lt;Integer, String>();
        // Add key-value pairs
        Sites.put(1, "Google");
        Sites.put(2, "tutorialpro");
        Sites.put(3, "Taobao");
        Sites.put(4, "Zhihu");
        // Output keys and values
        for (Integer i : Sites.keySet()) {
            System.out.println("key: " + i + " value: " + Sites.get(i));
        }
        // Return all values
        for(String value: Sites.values()) {
            // Output each value
            System.out.print(value + ", ");
        }
    }
}

Executing the above code will produce the following output:

key: 1 value: Google
key: 2 value: tutorialpro
key: 3 value: Taobao
key: 4 value: Zhihu
Google, tutorialpro, Taobao, Zhihu,

Java HashMap Methods

Here is a list of common methods for Java HashMap:

Method	Description
clear()	Removes all key-value pairs from the hashMap
clone()	Clones a hashMap
isEmpty()	Checks if the hashMap is empty
size()	Calculates the number of key-value pairs in the hashMap
put()	Adds a key-value pair to the hashMap
putAll()	Adds all key-value pairs to the hashMap
putIfAbsent()	Inserts a key-value pair into the hashMap if the specified key is not already present
remove()	Removes the mapping for a key from the hashMap if it is present
containsKey()	Checks if the hashMap contains a mapping for the specified key
containsValue()	Checks if the hashMap contains a mapping for the specified value
replace()	Replaces the value for the specified key in the hashMap
replaceAll()	Replaces all mappings in the hashMap with the result of the given function
get()	Retrieves the value to which the specified key is mapped
getOrDefault()	Retrieves the value to which the specified key is mapped, or returns the default value if the key is not found
forEach()	Performs the specified action for each mapping in the hashMap.
entrySet()	Returns a Set view of the mappings contained in the hashMap.
keySet	Returns a Set view of the keys contained in the hashMap.
values()	Returns a Collection view of the values contained in the hashMap.
merge()	Adds a key-value pair to the hashMap.
compute()	Recalculates the value for the specified key in the hashMap.
computeIfAbsent()	Recalculates the value for the specified key in the hashMap, and adds it if the key is not present.
computeIfPresent()	Recalculates the value for the specified key in the hashMap, if the key is present.

For more API methods, see: https://www.tutorialpro.org/manual/jdk11api/java.base/java/util/HashMap.html

Java Collections Framework

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