1. Introduction
The Collectors class is in the java.util.stream package. The Collectors class contains static methods which return a Collector to allow for various reduction operations. This article will discuss the toMap and the toConcurrentMap methods. These methods will return a Map or a ConcurrentMap. In addition, we will also be discussing the SortedMap interface. Finally, we conclude this article with a code sample and the corresponding output.
2. Some definitions for Collectors to Map
- A method is a term for function when discussed in Object Oriented Programming. As such it may have input parameters, return a type, and may be invoked or called.
- A static method is a class-level method that does not require object instantiation for the method to be invoked.
- An instance method is a method that does require object instantiation for the method to be invoked.
- An interface allows for an implementing class to fill in the code for the method signatures declared.
- Overloaded methods are methods defined in a class which share the same name and return type but have different number or types of parameters.
- A subclass can be created from a superclass or base class by extending the superclass and overriding methods found in the superclass or adding additional methods not found in the superclass.
- The base class for all classes in Java is the Object class found in the java.lang package.
- Abstract classes cannot be instantiated. A subclass must implement the abstract methods declared in the abstract class.
- Polymorphism as discussed in the context of Object Oriented Programming involves the use of all of the above.
3. Map, ConcurrentMap, and SortedMap interfaces
ConcurrentMap and SortedMap interfaces extend the Map interface. The ConcurrentMap interface allows for thread safety while the SortedMap interface allows for the ordering of Map keys.
3.1 The following classes implement the Map Interface:
- AbstractMap (abstract Class)
- Attributes
- AuthProvider (abstract class)
- ConcurrentHashMap
- ConcurrentSkipListMap
- EnumMap
- HashMap
- Hashtable
- IdentityHashMap
- LinkedHashMap
- PrinterStateReasons (final class)
- Properties
- Provider (abstract class)
- RenderingHints
- SimpleBindings
- TabularDataSupport
- TreeMap
- UIDefaults
- WeakHashMap
3.2 The following classes implement the ConcurrentMap interface:
- ConcurrentHashMap
- ConcurrentSkipListMap
3.3 The following classes implement the SortedMap interface
- ConcurrentSkipListMap
- TreeMap
4. The collectors to Map and toConcurrentMap methods
For both toMap and toConcurrentMap methods, there are 3 overloaded methods.
The default Map returned from toMap methods would be HashMap while the default Map returned from toConcurrentMap methods would be ConcurrentHashMap.
| keyMapper | a function that provides the keys for the resulting Map |
| valueMapper | a function that provides the values for the resulting Map |
| mergeFunction | a function that resolves conflicts arising from duplicate keys |
| mapSupplier | determines which would be the returned Map object |
| Method |
| ||||||||
| toMap |
| ||||||||
| toMap |
| ||||||||
| toMap |
| ||||||||
| toConcurrentMap |
| ||||||||
| toConcurrentMap |
| ||||||||
| toConcurrentMap |
| ||||||||
5. Example
Disclaimer: The data used in the following example is computer-generated and does not represent real data of real people.
The following example streams a list of transactions that contains the account number, name, phone numbers, and dollars spent and creates a map from it.
A particular account number is associated with a person. That person may have multiple phone numbers which were used for purchases. The purpose of the application is to output a list of the individuals with only one primary phone number with which to contact the person. The choice for the phone number is the number with which the greatest total purchases were made. In the rare event in which the total purchases for different phone numbers for a person are the same, there is a priority for the different area codes of the phone used.
5.1 Sample Code
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;
import java.util.WeakHashMap;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentSkipListMap;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.stream.Collectors;
public class ProcessTransactions {
public static String methodName() {
return new Exception().getStackTrace()[1].getMethodName();
}
public static List<Transaction> getTransactionListWithNoDups(List<Transaction>transactions) {
ArrayList<Transaction>newList = new ArrayList<Transaction>();
for (Transaction transaction: transactions) {
boolean gotMatch = false;
for (Transaction listTransaction: newList) {
if (listTransaction.equals(transaction)) {
gotMatch = true;
break;
}
}
if (!gotMatch) {
newList.add(transaction);
}
}
return newList;
}
public static List<Transaction> createTransactions() {
String [][] data = {
{ "Julie", "Barrister", "21144", "(917) 555-3287", "8205.09"},
{ "Julie", "Barrister", "21144", "(917) 555-1675", "1744.65"},
{ "Julie", "Barrister", "21144", "(646) 555-7644", "274.67"},
{ "Julie", "Barrister", "21144", "(631) 555-6938", "3309.86"},
{ "Julie", "Barrister", "21144", "(718) 555-2965", "783.60"},
{ "Michelle", "Albertson", "45581", "(917) 555-1363", "721.76"},
{ "Michelle", "Albertson", "45581", "(917) 555-6071", "1272.34"},
{ "Michelle", "Albertson", "45581", "(518) 555-4079", "1272.34"},
{ "Michelle", "Albertson", "45581", "(212) 555-5955", "1272.34"},
{ "Michelle", "Albertson", "45581", "(917) 555-4045", "1056.65"},
{ "Michelle", "Albertson", "45581", "(917) 555-0107", "185.05"},
{ "John", "Tillson", "22839", "(646) 555-4314", "5752.88"},
{ "John", "Tillson", "22839", "(917) 555-4602", "321.18"},
{ "John", "Tillson", "22839", "(212) 555-8018", "669.74"},
{ "John", "Tillson", "22839", "(718) 555-8923", "6116.53"},
{ "Warren", "Lee", "94328", "(718) 555-4652", "1141.72"},
{ "Warren", "Lee", "94328", "(212) 555-5515", "1289.81"},
{ "Warren", "Lee", "94328", "(718) 555-7599", "66.12"},
{ "Warren", "Lee", "94328", "(631) 555-8512", "2846.22"},
{ "Warren", "Lee", "94328", "(212) 555-9497", "3847.88"},
{ "Warren", "Lee", "94328", "(212) 555-9357", "341.63"},
{ "Zenia", "Lasher", "63937", "(516) 555-8570", "115.11"},
{ "Zenia", "Lasher", "63937", "(646) 555-9940", "811.40"},
{ "Zenia", "Lasher", "63937", "(212) 555-5194", "482.39"},
{ "Zenia", "Lasher", "63937", "(917) 555-1854", "1551.28"},
{ "Kevin", "Mickens", "77205", "(718) 555-4650", "1991.75"},
{ "Kevin", "Mickens", "77205", "(212) 555-9212", "2128.45"},
{ "Kevin", "Mickens", "77205", "(518) 555-1061", "2943.39"},
{ "Yvonne", "Quantico", "83341", "(516) 555-6968", "2856.09"},
{ "Yvonne", "Quantico", "83341", "(631) 555-7763", "713.36"},
{ "Yvonne", "Quantico", "83341", "(631) 555-5896", "880.25"},
{ "Yvonne", "Quantico", "83341", "(631) 555-1493", "750.15"},
{ "Yvonne", "Quantico", "83341", "(518) 555-3394", "3563.78"},
{ "Oliver", "Roberts", "66822", "(516) 555-6366", "5792.25"},
{ "Oliver", "Roberts", "66822", "(631) 555-4212", "559.85"},
{ "Oliver", "Roberts", "66822", "(631) 555-9948", "307.82"},
{ "Oliver", "Roberts", "66822", "(631) 555-6474", "102.38"},
{ "Oliver", "Roberts", "66822", "(646) 555-3666", "2253.37"},
{ "Victoria", "Tillson", "85361", "(516) 555-7517", "947.92"},
{ "Victoria", "Tillson", "85361", "(631) 555-8346", "2857.27"},
{ "Victoria", "Tillson", "85361", "(631) 555-2377", "632.28"},
{ "Paula", "Harrison", "26262", "(518) 555-4495", "4280.57"},
{ "Paula", "Harrison", "26262", "(212) 555-5639", "7794.50"},
{ "Paula", "Harrison", "26262", "(516) 555-8609", "5178.87"},
{ "Paula", "Harrison", "26262", "(631) 555-8516", "2086.11"},
{ "Freda", "Stapleton", "72652", "(212) 555-0800", "4665.87"},
{ "Freda", "Stapleton", "72652", "(631) 555-6978", "574.62"},
{ "Freda", "Stapleton", "72652", "(718) 555-2429", "1322.35"},
{ "Queen", "Tillson", "67169", "(646) 555-0590", "665.89"},
{ "Queen", "Tillson", "67169", "(516) 555-7907", "315.66"},
{ "Queen", "Tillson", "67169", "(516) 555-6804", "7217.85"},
{ "Queen", "Tillson", "67169", "(718) 555-8152", "778.31"},
{ "Queen", "Tillson", "67169", "(212) 555-5951", "1181.02"},
{ "Queen", "Tillson", "67169", "(917) 555-0052", "1012.82"},
{ "Laura", "Gavin", "77790", "(516) 555-0615", "2155.50"},
{ "Laura", "Gavin", "77790", "(646) 555-4320", "87.39"},
{ "Laura", "Gavin", "77790", "(518) 555-4944", "628.97"},
{ "Laura", "Gavin", "77790", "(646) 555-8332", "826.65"},
{ "Julie", "Harris", "32221", "(631) 555-4728", "3387.69"},
{ "Julie", "Harris", "32221", "(646) 555-3643", "38776.50"},
{ "Julie", "Harris", "32221", "(917) 555-3847", "419.75"},
{ "Julie", "Harris", "32221", "(631) 555-4924", "788.83"},
{ "Julie", "Harris", "32221", "(212) 555-6936", "15.25"},
{ "Michelle", "Harrison", "44460", "(212) 555-5522", "497.93"},
{ "Michelle", "Harrison", "44460", "(212) 555-3738", "352.73"},
{ "Michelle", "Harrison", "44460", "(212) 555-1351", "983.13"},
{ "Michelle", "Harrison", "44460", "(917) 555-8179", "108.81"},
{ "Garry", "Lee", "31471", "(516) 555-7130", "1573.11"},
{ "Garry", "Lee", "31471", "(212) 555-5324", "619.44"},
{ "Garry", "Lee", "31471", "(631) 555-2783", "2089.00"},
{ "Garry", "Lee", "31471", "(212) 555-4763", "622.21"},
{ "Garry", "Lee", "31471", "(516) 555-1300", "1071.09"},
{ "Ida", "Olden", "86092", "(518) 555-2803", "2690.21"},
{ "Ida", "Olden", "86092", "(917) 555-2207", "512.03"},
{ "Ida", "Olden", "86092", "(631) 555-0085", "631.42"},
{ "Ida", "Olden", "86092", "(518) 555-9402", "688.26"},
{ "Donna", "Dallas", "57010", "(718) 555-0330", "43928.50"},
{ "Donna", "Dallas", "57010", "(631) 555-0852", "1623.95"},
{ "Donna", "Dallas", "57010", "(718) 555-9695", "1244.96"},
{ "Donna", "Dallas", "57010", "(646) 555-3638", "729.54"},
{ "Victor", "Barrister", "93245", "(631) 555-0304", "949.33"},
{ "Victor", "Barrister", "93245", "(516) 555-7210", "3575.09"},
{ "Victor", "Barrister", "93245", "(212) 555-7660", "5264.27"},
{ "Victor", "Barrister", "93245", "(212) 555-0380", "2388.30"},
{ "Calvin", "Ellsworth", "48508", "(516) 555-6529", "288.45"},
{ "Calvin", "Ellsworth", "48508", "(646) 555-1321", "787.00"},
{ "Calvin", "Ellsworth", "48508", "(917) 555-8595", "948.15"},
{ "Calvin", "Ellsworth", "48508", "(917) 555-4224", "3212.30"},
{ "Calvin", "Ellsworth", "48508", "(518) 555-3741", "1036.52"},
{ "Calvin", "Ellsworth", "48508", "(516) 555-1910", "1848.41"},
{ "Kevin", "Stapleton", "45565", "(516) 555-0930", "3224.65"},
{ "Kevin", "Stapleton", "45565", "(516) 555-2136", "885.96"},
{ "Kevin", "Stapleton", "45565", "(646) 555-2048", "10510.67"},
{ "Kevin", "Stapleton", "45565", "(718) 555-9657", "580.86"},
{ "Joseph", "Kendricks", "62068", "(518) 555-0249", "1026.78"},
{ "Joseph", "Kendricks", "62068", "(212) 555-7486", "150.52"},
{ "Joseph", "Kendricks", "62068", "(516) 555-7813", "444.96"},
{ "Joseph", "Kendricks", "62068", "(631) 555-4661", "1098.24"},
{ "Joseph", "Kendricks", "62068", "(518) 555-0167", "95.97"},
{ "Joseph", "Kendricks", "62068", "(212) 555-8809", "677.55"},
{ "Garry", "Paulson", "86036", "(917) 555-4386", "6.94"},
{ "Garry", "Paulson", "86036", "(917) 555-7134", "3835.47"},
{ "Garry", "Paulson", "86036", "(718) 555-0187", "350.00"},
{ "Garry", "Paulson", "86036", "(212) 555-3707", "2019.02"},
};
List<Transaction>transactions = new ArrayList<Transaction>();
for (String [] row: data) {
transactions.add(new Transaction(new Customer(row[0], row[1], row[2]), row[3], Double.parseDouble(row[4])));
}
return transactions;
}
static public Map<String, Object> listWithUniqueKeysToMap(List<Transaction> transactions) {
Map<String, Object> result = new HashMap<String, Object>();
result.put("methodDescription", String.format("Executing %s with toMap(keyMapper, valueMapper)\n", methodName()));
result.put("result", transactions.stream().collect(Collectors.toMap(Transaction::getAccountNumber, Function.identity())));
return result;
}
static public Map<String, Object> listToMap(List<Transaction> transactions) {
Map<String, Object> result = new HashMap<String, Object>();
result.put("methodDescription", String.format("Executing %s with toMap(keyMapper, valueMapper, mergeFunction)\n", methodName()));
result.put("result", transactions.stream().collect(Collectors.toMap(Transaction::getAccountNumber, Function.identity(), (existing, replacement) -> existing)));
return result;
}
static public Map<String, Object> listToWeakHashMap(List<Transaction> transactions) {
Map<String, Object> result = new HashMap<String, Object>();
result.put("methodDescription", String.format("Executing %s with toMap(keyMapper, valueMapper, mergeFunction, mapSupplier)\n", methodName()));
result.put("result", transactions.stream().collect(Collectors.toMap(Transaction::getAccountNumber, Function.identity(), (existing, replacement) -> existing, WeakHashMap::new)));
return result;
}
static public Map<String, Object> listToSortedMap(List<Transaction> transactions) {
Map<String, Object> result = new HashMap<String, Object>();
BinaryOperator<Transaction> breakSame = (v1, v2) -> v1.getTotalPurchases() > v2.getTotalPurchases() ? v1: v2;
result.put("methodDescription", String.format("Executing %s with toMap(keyMapper, valueMapper, mergeFunction, mapSupplier)\n", methodName()));
result.put("result", transactions.stream().collect(Collectors.toMap(Transaction::getAccountNumber, Function.identity(),
breakSame, TreeMap::new)));
return result;
}
static public Map<String, Object> listToSortedConcurrentMap(List<Transaction> transactions) {
Map<String, Object> result = new HashMap<String, Object>();
BinaryOperator<Transaction> breakSame = (v1, v2) -> v1.getTotalPurchases() > v2.getTotalPurchases() ? v1: v2;
result.put("methodDescription", String.format("Executing %s with toMap(keyMapper, valueMapper, mergeFunction, mapSupplier)\n", methodName()));
result.put("result", transactions.stream().collect(Collectors.toConcurrentMap(Transaction::getAccountNumber, Function.identity(),
breakSame, ConcurrentSkipListMap::new)));
return result;
}
static public Map<String, Object> listWithUniqueKeysToConcurrentMap(List<Transaction> transactions) {
Map<String, Object> result = new HashMap<String, Object>();
result.put("methodDescription", String.format("Executing %s with toConcurrentMap(keyMapper, valueMapper)\n", methodName()));
result.put("result", transactions.stream().collect(Collectors.toConcurrentMap(Transaction::getAccountNumber, Function.identity())));
return result;
}
static public Map<String, Object> listToConcurrentMap(List<Transaction> transactions) {
Map<String, Object> result = new HashMap<String, Object>();
BinaryOperator<Transaction> breakSame = (v1, v2) -> v1.getTotalPurchases() > v2.getTotalPurchases() ? v1: v2;
result.put("methodDescription", String.format("Executing %s with toConcurrentMap(keyMapper, valueMapper, mergeFunction, mapSupplier)\n", methodName()));
result.put("result", transactions.stream().collect(Collectors.toConcurrentMap(Transaction::getAccountNumber, Function.identity(),
breakSame, ConcurrentSkipListMap::new)));
return result;
}
static public Map<String, Object> listToConcurrentMap(List<Transaction> transactions, BinaryOperator<Transaction> breakSame) {
Map<String, Object> result = new HashMap<String, Object>();
result.put("methodDescription", String.format("Executing %s with toConcurrentMap(keyMapper, valueMapper, mergeFunction)\n", methodName()));
result.put("result", transactions.stream().collect(Collectors.toConcurrentMap(Transaction::getAccountNumber, Function.identity(),
breakSame)));
return result;
}
public static void printBanner(String type, String methodDescription) {
int bannerLength = 110;
StringBuffer stars = new StringBuffer();
StringBuffer dashes = new StringBuffer();
String typeformat = "\n%" + String.format("%d", (bannerLength-type.length())/2 + type.length()) + "s\n\n";
String descformat = "%" + String.format("%d", (bannerLength-methodDescription.length())/2 + methodDescription.length()) + "s\n";
for (int i = 0; i < bannerLength; i++) stars.append('*');
for (int i = 0; i < bannerLength; i++) dashes.append('-');
System.out.println(stars);
System.out.printf(typeformat, type);
System.out.printf(descformat, methodDescription);
System.out.println(dashes);
System.out.printf("Account Number Phone Number Name Total Purchases\n");
System.out.println(stars + "\n");
}
@SuppressWarnings("unchecked")
public static void printTransactions(Map<String, Object>transactionData) {
Map<String, Transaction>transactions = (Map<String, Transaction>)transactionData.get("result");
String methodDescription = (String)transactionData.get("methodDescription");
BiConsumer<? super String, ? super Transaction>printLine = (key, transaction) -> {
System.out.printf("%-17s %-18s %-20s %22.2f\n", key, transaction.getPhoneNumber(), transaction.getFirstName() + " " + transaction.getLastName(), transaction.getTotalPurchases());
};
printBanner(transactions.getClass().getName(), methodDescription);
transactions.forEach(printLine);
System.out.println("\n\n");
}
public static void main(String[] args)
{
List<Transaction> transactions = createTransactions();
List<Transaction> transactionsListWithUniqueKeys = getTransactionListWithNoDups(transactions);
ArrayList<Map<String, Object>>transactionMaps = new ArrayList<Map<String, Object>>();
transactionMaps.add(listWithUniqueKeysToMap(transactionsListWithUniqueKeys));
transactionMaps.add(listToMap(transactions));
transactionMaps.add(listToWeakHashMap(transactions));
transactionMaps.add(listToSortedMap(transactions));
transactionMaps.add(listWithUniqueKeysToConcurrentMap(transactionsListWithUniqueKeys));
transactionMaps.add(listToConcurrentMap(transactions, new ChooseTransaction()));
BinaryOperator<Transaction> breakSame = (v1, v2) -> v1.getTotalPurchases() > v2.getTotalPurchases() ? v1: v2;
transactionMaps.add(listToConcurrentMap(transactions, breakSame));
transactionMaps.add(listToConcurrentMap(transactions));
transactionMaps.add(listToSortedConcurrentMap(transactions));
for (Map<String, Object> transactionMap: transactionMaps) {
printTransactions(transactionMap);
}
}
}
class Customer {
String firstName;
String lastName;
String accountNumber;
String phoneNumber;
double totalPurchases;
public Customer(String firstName, String lastName, String accountNumber) {
this.firstName = firstName;
this.lastName = lastName;
this.accountNumber = accountNumber;
}
public String getFirstName() {
return firstName;
}
public String getLastName() {
return lastName;
}
public String getAccountNumber() {
return accountNumber;
}
}
class Transaction {
private Customer customer;
private String phoneNumber;
private double totalPurchases;
public Transaction(Customer customer, String phoneNumber, double totalPurchases) {
this.customer = customer;
this.phoneNumber = phoneNumber;
this.totalPurchases = totalPurchases;
}
public String getFirstName() {
return customer.getFirstName();
}
public String getLastName() {
return customer.getLastName();
}
public String getAccountNumber() {
return customer.getAccountNumber();
}
public String getPhoneNumber() {
return phoneNumber;
}
public double getTotalPurchases() {
return totalPurchases;
}
public boolean equals(Transaction transaction) {
boolean rv = false;
if (this.getAccountNumber().equals(transaction.getAccountNumber()))
rv = true;
return rv;
}
}
class ChooseTransaction implements BinaryOperator<Transaction> {
public int preferredAreaCode(String phoneNumber) {
String [] areaCodes = {"516", "631", "212", "646", "718", "917", "518"};
int rv = -1;
for (int i = 0; i < areaCodes.length; i++) {
if (phoneNumber.substring(1, 4).equals(areaCodes[i])) {
rv = i;
}
}
return rv;
}
@Override
public Transaction apply(Transaction transaction1, Transaction transaction2) {
Transaction rv;
if (transaction1.getTotalPurchases() > transaction2.getTotalPurchases()) {
rv = transaction1;
} else if (transaction1.getTotalPurchases() < transaction2.getTotalPurchases()) {
rv = transaction2;
} else if (preferredAreaCode(transaction1.getPhoneNumber()) < preferredAreaCode(transaction2.getPhoneNumber())) {
rv = transaction1;
} else
rv = transaction2;
return rv;
}
}5.2 Sample Output
**************************************************************************************************************
java.util.HashMap
Executing listWithUniqueKeysToMap with toMap(keyMapper, valueMapper)
--------------------------------------------------------------------------------------------------------------
Account Number Phone Number Name Total Purchases
**************************************************************************************************************
67169 (646) 555-0590 Queen Tillson 665.89
93245 (631) 555-0304 Victor Barrister 949.33
86092 (518) 555-2803 Ida Olden 2690.21
94328 (718) 555-4652 Warren Lee 1141.72
21144 (917) 555-3287 Julie Barrister 8205.09
62068 (518) 555-0249 Joseph Kendricks 1026.78
45565 (516) 555-0930 Kevin Stapleton 3224.65
72652 (212) 555-0800 Freda Stapleton 4665.87
22839 (646) 555-4314 John Tillson 5752.88
57010 (718) 555-0330 Donna Dallas 43928.50
66822 (516) 555-6366 Oliver Roberts 5792.25
48508 (516) 555-6529 Calvin Ellsworth 288.45
77790 (516) 555-0615 Laura Gavin 2155.50
63937 (516) 555-8570 Zenia Lasher 115.11
26262 (518) 555-4495 Paula Harrison 4280.57
32221 (631) 555-4728 Julie Harris 3387.69
31471 (516) 555-7130 Garry Lee 1573.11
77205 (718) 555-4650 Kevin Mickens 1991.75
83341 (516) 555-6968 Yvonne Quantico 2856.09
45581 (917) 555-1363 Michelle Albertson 721.76
86036 (917) 555-4386 Garry Paulson 6.94
44460 (212) 555-5522 Michelle Harrison 497.93
85361 (516) 555-7517 Victoria Tillson 947.92
**************************************************************************************************************
java.util.HashMap
Executing listToMap with toMap(keyMapper, valueMapper, mergeFunction)
--------------------------------------------------------------------------------------------------------------
Account Number Phone Number Name Total Purchases
**************************************************************************************************************
67169 (646) 555-0590 Queen Tillson 665.89
93245 (631) 555-0304 Victor Barrister 949.33
86092 (518) 555-2803 Ida Olden 2690.21
94328 (718) 555-4652 Warren Lee 1141.72
21144 (917) 555-3287 Julie Barrister 8205.09
62068 (518) 555-0249 Joseph Kendricks 1026.78
45565 (516) 555-0930 Kevin Stapleton 3224.65
72652 (212) 555-0800 Freda Stapleton 4665.87
22839 (646) 555-4314 John Tillson 5752.88
57010 (718) 555-0330 Donna Dallas 43928.50
66822 (516) 555-6366 Oliver Roberts 5792.25
48508 (516) 555-6529 Calvin Ellsworth 288.45
77790 (516) 555-0615 Laura Gavin 2155.50
63937 (516) 555-8570 Zenia Lasher 115.11
26262 (518) 555-4495 Paula Harrison 4280.57
32221 (631) 555-4728 Julie Harris 3387.69
31471 (516) 555-7130 Garry Lee 1573.11
77205 (718) 555-4650 Kevin Mickens 1991.75
83341 (516) 555-6968 Yvonne Quantico 2856.09
45581 (917) 555-1363 Michelle Albertson 721.76
86036 (917) 555-4386 Garry Paulson 6.94
44460 (212) 555-5522 Michelle Harrison 497.93
85361 (516) 555-7517 Victoria Tillson 947.92
**************************************************************************************************************
java.util.WeakHashMap
Executing listToWeakHashMap with toMap(keyMapper, valueMapper, mergeFunction, mapSupplier)
--------------------------------------------------------------------------------------------------------------
Account Number Phone Number Name Total Purchases
**************************************************************************************************************
45565 (516) 555-0930 Kevin Stapleton 3224.65
22839 (646) 555-4314 John Tillson 5752.88
32221 (631) 555-4728 Julie Harris 3387.69
21144 (917) 555-3287 Julie Barrister 8205.09
57010 (718) 555-0330 Donna Dallas 43928.50
72652 (212) 555-0800 Freda Stapleton 4665.87
26262 (518) 555-4495 Paula Harrison 4280.57
94328 (718) 555-4652 Warren Lee 1141.72
44460 (212) 555-5522 Michelle Harrison 497.93
31471 (516) 555-7130 Garry Lee 1573.11
62068 (518) 555-0249 Joseph Kendricks 1026.78
85361 (516) 555-7517 Victoria Tillson 947.92
86036 (917) 555-4386 Garry Paulson 6.94
93245 (631) 555-0304 Victor Barrister 949.33
66822 (516) 555-6366 Oliver Roberts 5792.25
67169 (646) 555-0590 Queen Tillson 665.89
86092 (518) 555-2803 Ida Olden 2690.21
63937 (516) 555-8570 Zenia Lasher 115.11
77790 (516) 555-0615 Laura Gavin 2155.50
45581 (917) 555-1363 Michelle Albertson 721.76
48508 (516) 555-6529 Calvin Ellsworth 288.45
77205 (718) 555-4650 Kevin Mickens 1991.75
83341 (516) 555-6968 Yvonne Quantico 2856.09
**************************************************************************************************************
java.util.TreeMap
Executing listToSortedMap with toMap(keyMapper, valueMapper, mergeFunction, mapSupplier)
--------------------------------------------------------------------------------------------------------------
Account Number Phone Number Name Total Purchases
**************************************************************************************************************
21144 (917) 555-3287 Julie Barrister 8205.09
22839 (718) 555-8923 John Tillson 6116.53
26262 (212) 555-5639 Paula Harrison 7794.50
31471 (631) 555-2783 Garry Lee 2089.00
32221 (646) 555-3643 Julie Harris 38776.50
44460 (212) 555-1351 Michelle Harrison 983.13
45565 (646) 555-2048 Kevin Stapleton 10510.67
45581 (212) 555-5955 Michelle Albertson 1272.34
48508 (917) 555-4224 Calvin Ellsworth 3212.30
57010 (718) 555-0330 Donna Dallas 43928.50
62068 (631) 555-4661 Joseph Kendricks 1098.24
63937 (917) 555-1854 Zenia Lasher 1551.28
66822 (516) 555-6366 Oliver Roberts 5792.25
67169 (516) 555-6804 Queen Tillson 7217.85
72652 (212) 555-0800 Freda Stapleton 4665.87
77205 (518) 555-1061 Kevin Mickens 2943.39
77790 (516) 555-0615 Laura Gavin 2155.50
83341 (518) 555-3394 Yvonne Quantico 3563.78
85361 (631) 555-8346 Victoria Tillson 2857.27
86036 (917) 555-7134 Garry Paulson 3835.47
86092 (518) 555-2803 Ida Olden 2690.21
93245 (212) 555-7660 Victor Barrister 5264.27
94328 (212) 555-9497 Warren Lee 3847.88
**************************************************************************************************************
java.util.concurrent.ConcurrentHashMap
Executing listWithUniqueKeysToConcurrentMap with toConcurrentMap(keyMapper, valueMapper)
--------------------------------------------------------------------------------------------------------------
Account Number Phone Number Name Total Purchases
**************************************************************************************************************
67169 (646) 555-0590 Queen Tillson 665.89
93245 (631) 555-0304 Victor Barrister 949.33
21144 (917) 555-3287 Julie Barrister 8205.09
94328 (718) 555-4652 Warren Lee 1141.72
86092 (518) 555-2803 Ida Olden 2690.21
22839 (646) 555-4314 John Tillson 5752.88
72652 (212) 555-0800 Freda Stapleton 4665.87
45565 (516) 555-0930 Kevin Stapleton 3224.65
62068 (518) 555-0249 Joseph Kendricks 1026.78
57010 (718) 555-0330 Donna Dallas 43928.50
66822 (516) 555-6366 Oliver Roberts 5792.25
77790 (516) 555-0615 Laura Gavin 2155.50
48508 (516) 555-6529 Calvin Ellsworth 288.45
63937 (516) 555-8570 Zenia Lasher 115.11
26262 (518) 555-4495 Paula Harrison 4280.57
32221 (631) 555-4728 Julie Harris 3387.69
31471 (516) 555-7130 Garry Lee 1573.11
77205 (718) 555-4650 Kevin Mickens 1991.75
45581 (917) 555-1363 Michelle Albertson 721.76
83341 (516) 555-6968 Yvonne Quantico 2856.09
44460 (212) 555-5522 Michelle Harrison 497.93
86036 (917) 555-4386 Garry Paulson 6.94
85361 (516) 555-7517 Victoria Tillson 947.92
**************************************************************************************************************
java.util.concurrent.ConcurrentHashMap
Executing listToConcurrentMap with toConcurrentMap(keyMapper, valueMapper, mergeFunction)
--------------------------------------------------------------------------------------------------------------
Account Number Phone Number Name Total Purchases
**************************************************************************************************************
67169 (516) 555-6804 Queen Tillson 7217.85
93245 (212) 555-7660 Victor Barrister 5264.27
21144 (917) 555-3287 Julie Barrister 8205.09
94328 (212) 555-9497 Warren Lee 3847.88
86092 (518) 555-2803 Ida Olden 2690.21
22839 (718) 555-8923 John Tillson 6116.53
72652 (212) 555-0800 Freda Stapleton 4665.87
45565 (646) 555-2048 Kevin Stapleton 10510.67
62068 (631) 555-4661 Joseph Kendricks 1098.24
57010 (718) 555-0330 Donna Dallas 43928.50
66822 (516) 555-6366 Oliver Roberts 5792.25
77790 (516) 555-0615 Laura Gavin 2155.50
48508 (917) 555-4224 Calvin Ellsworth 3212.30
63937 (917) 555-1854 Zenia Lasher 1551.28
26262 (212) 555-5639 Paula Harrison 7794.50
32221 (646) 555-3643 Julie Harris 38776.50
31471 (631) 555-2783 Garry Lee 2089.00
77205 (518) 555-1061 Kevin Mickens 2943.39
45581 (212) 555-5955 Michelle Albertson 1272.34
83341 (518) 555-3394 Yvonne Quantico 3563.78
44460 (212) 555-1351 Michelle Harrison 983.13
86036 (917) 555-7134 Garry Paulson 3835.47
85361 (631) 555-8346 Victoria Tillson 2857.27
**************************************************************************************************************
java.util.concurrent.ConcurrentHashMap
Executing listToConcurrentMap with toConcurrentMap(keyMapper, valueMapper, mergeFunction)
--------------------------------------------------------------------------------------------------------------
Account Number Phone Number Name Total Purchases
**************************************************************************************************************
67169 (516) 555-6804 Queen Tillson 7217.85
93245 (212) 555-7660 Victor Barrister 5264.27
21144 (917) 555-3287 Julie Barrister 8205.09
94328 (212) 555-9497 Warren Lee 3847.88
86092 (518) 555-2803 Ida Olden 2690.21
22839 (718) 555-8923 John Tillson 6116.53
72652 (212) 555-0800 Freda Stapleton 4665.87
45565 (646) 555-2048 Kevin Stapleton 10510.67
62068 (631) 555-4661 Joseph Kendricks 1098.24
57010 (718) 555-0330 Donna Dallas 43928.50
66822 (516) 555-6366 Oliver Roberts 5792.25
77790 (516) 555-0615 Laura Gavin 2155.50
48508 (917) 555-4224 Calvin Ellsworth 3212.30
63937 (917) 555-1854 Zenia Lasher 1551.28
26262 (212) 555-5639 Paula Harrison 7794.50
32221 (646) 555-3643 Julie Harris 38776.50
31471 (631) 555-2783 Garry Lee 2089.00
77205 (518) 555-1061 Kevin Mickens 2943.39
45581 (212) 555-5955 Michelle Albertson 1272.34
83341 (518) 555-3394 Yvonne Quantico 3563.78
44460 (212) 555-1351 Michelle Harrison 983.13
86036 (917) 555-7134 Garry Paulson 3835.47
85361 (631) 555-8346 Victoria Tillson 2857.27
**************************************************************************************************************
java.util.concurrent.ConcurrentSkipListMap
Executing listToConcurrentMap with toConcurrentMap(keyMapper, valueMapper, mergeFunction, mapSupplier)
--------------------------------------------------------------------------------------------------------------
Account Number Phone Number Name Total Purchases
**************************************************************************************************************
21144 (917) 555-3287 Julie Barrister 8205.09
22839 (718) 555-8923 John Tillson 6116.53
26262 (212) 555-5639 Paula Harrison 7794.50
31471 (631) 555-2783 Garry Lee 2089.00
32221 (646) 555-3643 Julie Harris 38776.50
44460 (212) 555-1351 Michelle Harrison 983.13
45565 (646) 555-2048 Kevin Stapleton 10510.67
45581 (212) 555-5955 Michelle Albertson 1272.34
48508 (917) 555-4224 Calvin Ellsworth 3212.30
57010 (718) 555-0330 Donna Dallas 43928.50
62068 (631) 555-4661 Joseph Kendricks 1098.24
63937 (917) 555-1854 Zenia Lasher 1551.28
66822 (516) 555-6366 Oliver Roberts 5792.25
67169 (516) 555-6804 Queen Tillson 7217.85
72652 (212) 555-0800 Freda Stapleton 4665.87
77205 (518) 555-1061 Kevin Mickens 2943.39
77790 (516) 555-0615 Laura Gavin 2155.50
83341 (518) 555-3394 Yvonne Quantico 3563.78
85361 (631) 555-8346 Victoria Tillson 2857.27
86036 (917) 555-7134 Garry Paulson 3835.47
86092 (518) 555-2803 Ida Olden 2690.21
93245 (212) 555-7660 Victor Barrister 5264.27
94328 (212) 555-9497 Warren Lee 3847.88
**************************************************************************************************************
java.util.concurrent.ConcurrentSkipListMap
Executing listToSortedConcurrentMap with toMap(keyMapper, valueMapper, mergeFunction, mapSupplier)
--------------------------------------------------------------------------------------------------------------
Account Number Phone Number Name Total Purchases
**************************************************************************************************************
21144 (917) 555-3287 Julie Barrister 8205.09
22839 (718) 555-8923 John Tillson 6116.53
26262 (212) 555-5639 Paula Harrison 7794.50
31471 (631) 555-2783 Garry Lee 2089.00
32221 (646) 555-3643 Julie Harris 38776.50
44460 (212) 555-1351 Michelle Harrison 983.13
45565 (646) 555-2048 Kevin Stapleton 10510.67
45581 (212) 555-5955 Michelle Albertson 1272.34
48508 (917) 555-4224 Calvin Ellsworth 3212.30
57010 (718) 555-0330 Donna Dallas 43928.50
62068 (631) 555-4661 Joseph Kendricks 1098.24
63937 (917) 555-1854 Zenia Lasher 1551.28
66822 (516) 555-6366 Oliver Roberts 5792.25
67169 (516) 555-6804 Queen Tillson 7217.85
72652 (212) 555-0800 Freda Stapleton 4665.87
77205 (518) 555-1061 Kevin Mickens 2943.39
77790 (516) 555-0615 Laura Gavin 2155.50
83341 (518) 555-3394 Yvonne Quantico 3563.78
85361 (631) 555-8346 Victoria Tillson 2857.27
86036 (917) 555-7134 Garry Paulson 3835.47
86092 (518) 555-2803 Ida Olden 2690.21
93245 (212) 555-7660 Victor Barrister 5264.27
94328 (212) 555-9497 Warren Lee 3847.886. Related Article(s)
7. Summary
The Collectors toMap and toConcurrentMap methods fulfill the Map paradigm of the Map, Reduce, Filter cornerstone paradigms of the functional programming methodology. In the case of duplicate keys used in mapping, the use of the mergeFunction was invoked to resolve these conflicts.
8. Download the Source Code
This was an example of the use of collectors to Map and toConcurrentMap to create maps from a list.
Download
You can download the full source code of this example here: Collectors toMap in Java
You can download the full source code of this example here: Collectors toMap in Java
