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

public class CreateHashMap {
    public static void main(String[] args) { // creating a HashMap
        Map<String, Integer>numMap = new HashMap<>();

        // adding key-value pairs to a HashMap

        numMap.put("one", 1);
        numMap.put("two", 2);
        numMap.put("three", 3);

        // adding new key-value pairs to a HashMap
        numMap.putIfAbsent("four", 4);
    }
}

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

public class KeyHashMap {
    public static void main(String[] args) {
        // creating a HashMap
        Map<String, Integer>checkMap = new HashMap<>();

        // check if a hashmap is empty
        System.out.println("is empty？： " + checkMap.isEmpty());

        // find the  size of a hashmap
        checkMap.put("xxx", "yyy");
        checkMap.put("xxx1", "yyy1");
        checkMap.put("xxx2", "yyy2");

        System.out.println("is empty？： " + checkMap.size());

        //check if the key exists in the HashMap
        String username = "xxx";

        if(checkMap.containsKey(username)) {
            String useradd = checkMap.get(username);
            System.out.println(username + useradd);
        } else {
            System.out.println("not found： " + username);
        }


        //check if the value exists in the HashMap

        if(checkMap.containsValue("yyy")) {

            System.out.println("ff");
        } else {
            System.out.println("xx");
        }



        numMap.put("one", 1);
        numMap.put("two", 2);
        numMap.put("three", 3);

        // adding new key-value pairs to a HashMap
        numMap.putIfAbsent("four", 4);

        // modify the value assigned to an existing key
        checkMap.put(username, "yyy");

    }
}

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

public class removeHashMap {
    public static void main(String[] args) {
        // creating a HashMap
        Map<String, Integer>remMap = new HashMap<>();
        remMap.put("one", 1);
        remMap.put("two", 2);
        remMap.put("three", 3);

        String rev= "one";
        String rem = remMap.remove(rev);

        System.out.println("new " + remMap);
        //output:
        //{two=2, three=3}


        boolean isRemoved = remMap.remove("one",1)；
         System.out.println(isRemoved);
         //one, 1 not in a pair anymore, so the output is false;

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


public class EntryKeyMap {
    public static void main(String[] args) {
        // creating a HashMap
        Map<String, String> enkMap = new HashMap<>();
        enkMap.put("one", "1");
        enkMap.put("two", "2");
        enkMap.put("three", "3");


        // HashMap's entry set

        Set<Map.Entry<String, String>> enkEntries = enkMap.entrySet();
        System.out.println(enkEntries);
        // print hashmap in { xxx=yyy, xxx1=yyy1};
        // print entrySet is[xxx=yyy, xxx1=yyy1];

        // hashmap's key set
        Set<String> xname = enkMap.keySet();
        System.out.println(xname);
        //output:
        //[one, two]

        // hashmap's value
        Collection<String> yname = enkMap.values();
        System.out.println(yname);
        //output:
        //[yyy, yyy1]
        ```
`Iterating over a hashmap`

 ```java
 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);

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


        // 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());
        }

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

        //  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());
        }

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

`user defined objects`

```java
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);
    }

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