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Create java-hashmap-examples #2

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225 changes: 225 additions & 0 deletions java-hashmap-examples
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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 not 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"));
}
}

***************************************************************************************************************************************************************************

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);
}
}
***************************************************************************************************************************************************************************

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);
}
}
***************************************************************************************************************************************************************************

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));
});
}
}



************************************************************************************************************************************************************************

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));
});
}
}

************************************************************************************************************************************************************************
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");
}
}
}