Iterate over a Map in sorted order in Java

This post will discuss how to iterate over a Map in sorted order in Java.

1. Use `TreeMap`

The HashMap in Java provides good performance but doesn’t maintain any order of its elements. If you want insertion-order iteration with near-HashMap performance, you can use LinkedHashMap. If you want sorted-order iteration, you can use the TreeMap implementation of the Map interface.

A TreeMap is implemented as a Red-black tree and has keys sorted according to their natural ordering, or by a custom comparator. Here’s a working example using Stream API:

import java.util.Map;

import java.util.Random;

import java.util.TreeMap;

import java.util.stream.Collectors;

import java.util.stream.IntStream;

public class Main

{

private static final String chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

public static String generateRandomWord(int len) {

return IntStream.range(0, len)

.map(i -> new Random().nextInt(chars.length()))

.mapToObj(randomIndex -> String.valueOf(chars.charAt(randomIndex)))

.collect(Collectors.joining());

}

public static void main(String[] args)

{

Map<String, Integer> map = IntStream.rangeClosed(1, 10).boxed()

.collect(Collectors.toMap(Main::generateRandomWord, i -> i,

(oldValue, newValue) -> newValue, TreeMap::new));

map.entrySet().stream().forEach(System.out::println);

}

Download Run Code

Output (will vary):

ACGU=4
AMUNXIUAF=9
EO=2
H=1
OCZJWMMAQD=10
RSTFJKPM=8
RXE=3
VICPVI=6
WVNHBUV=7
XTWGP=5

Before Java 8, you can do like,

import java.util.Map;

import java.util.Random;

import java.util.TreeMap;

public class Main

{

private static final String chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

public static String generateRandomWord(int len) {

StringBuilder sb = new StringBuilder();

for (int i = 0; i < len; i++) {

int randomIndex = new Random().nextInt(chars.length());

sb.append(chars.charAt(randomIndex));

}

return sb.toString();

}

public static void main(String[] args)

{

Map<String, Integer> map = new TreeMap<>();

for (int i = 1; i <= 10; i++) {

map.put(generateRandomWord(i), i);

}

for (Map.Entry<String, Integer> stringIntegerEntry : map.entrySet()) {

System.out.println(stringIntegerEntry);

}

Download Run Code

Output (will vary):

ACGU=4
AMUNXIUAF=9
EO=2
H=1
OCZJWMMAQD=10
RSTFJKPM=8
RXE=3
VICPVI=6
WVNHBUV=7
XTWGP=5

2. Sorting keys

If you prefer not to use TreeMap and stick to HashMap, you can sort its keys. This can be easily achieved using Java 8:

import java.util.*;

import java.util.stream.Collectors;

import java.util.stream.IntStream;

public class Main

{

private static final String chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

public static String generateRandomWord(int len) {

return IntStream.range(0, len)

.map(i -> new Random().nextInt(chars.length()))

.mapToObj(randomIndex -> String.valueOf(chars.charAt(randomIndex)))

.collect(Collectors.joining());

}

public static void main(String[] args)

{

Map<String, Integer> map = IntStream.rangeClosed(1, 10).boxed()

.collect(Collectors.toMap(Main::generateRandomWord, i -> i,

(oldValue, newValue) -> newValue, HashMap::new));

map.entrySet().stream()

.sorted(Comparator.comparing(Map.Entry::getKey))

.forEach(System.out::println);

}

Download Run Code

Output (will vary):

BKXVAGACLF=10
CDDJWE=6
CLQVZWO=7
CNRUW=5
EEWT=4
MASGNOIQ=8
N=1
OOJ=3
PBMLONYQZ=9
SI=2

That’s all about iterating over a Map in sorted order in Java.

Iterate over a Map in sorted order in Java

1. Use TreeMap

2. Sorting keys

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1. Use `TreeMap`