Java : ConcurrentMap with Examples

void test(Map<String, Integer> map) throws InterruptedException {
    map.put("a", 10);
    map.put("b", 20);
    map.put("c", 30);
    map.put("d", 40);
    map.put("e", 50);

    try (final var executor = Executors.newSingleThreadScheduledExecutor()) {

        executor.schedule(() -> {
            System.out.println("-- put entry! --");
            map.put("X", 999);
        }, 5, TimeUnit.SECONDS);

        try {
            for (final var entry : map.entrySet()) {
                System.out.println("entry = " + entry);
                TimeUnit.SECONDS.sleep(2);
            }
        } catch (ConcurrentModificationException e) {
            System.out.println("ConcurrentModificationException!");
        }
    }

    System.out.println("-- end --");
    System.out.println("map = " + map);
}

test(new HashMap<>());

// Result
// ↓
//entry = a=10
//entry = b=20
//entry = c=30
//-- put entry! --
//ConcurrentModificationException!
//-- end --
//map = {a=10, b=20, c=30, d=40, e=50, X=999}

final ConcurrentMap<String, Integer> concurrentMap = new ConcurrentHashMap<>();
test(concurrentMap);

// Result
// ↓
//entry = a=10
//entry = b=20
//entry = c=30
//-- put entry! --
//entry = d=40
//entry = e=50
//entry = X=999
//-- end --
//map = {a=10, b=20, c=30, d=40, e=50, X=999}

default V compute (K key, BiFunction<? super K,? super V,? extends V> remappingFunction)

Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping).

final ConcurrentMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
System.out.println(map); // {a=1, b=2, c=3}

final var func = new BiFunction<String, Integer, Integer>() {
    @Override
    public Integer apply(String key, Integer value) {
        if (value == null) {
            return -100;
        } else {
            return value * 10;
        }
    }
};

final var ret1 = map.compute("a", func);
System.out.println(ret1); // 10
System.out.println(map); // {a=10, b=2, c=3}

final var ret2 = map.compute("b", func);
System.out.println(ret2); // 20
System.out.println(map); // {a=10, b=20, c=3}

final var ret3 = map.compute("c", func);
System.out.println(ret3); // 30
System.out.println(map); // {a=10, b=20, c=30}

final var ret4 = map.compute("X", func);
System.out.println(ret4); // -100
System.out.println(map); // {a=10, b=20, c=30, X=-100}

final ConcurrentMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
System.out.println(map); // {a=1, b=2, c=3}

final var ret = map.compute("b", (key, value) -> null);
System.out.println(ret); // null
System.out.println(map); // {a=1, c=3}

default V computeIfAbsent (K key, Function<? super K,? extends V> mappingFunction)

If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless null.

final ConcurrentMap<String, String> map = new ConcurrentHashMap<>();
map.put("a", "aaa");
map.put("b", "bbb");
System.out.println(map); // {a=aaa, b=bbb}

final var func = new Function<String, String>() {
    @Override
    public String apply(String key) {
        if ("c".equals(key)) {
            return "ccc";
        } else {
            return null;
        }
    }
};

final var ret1 = map.computeIfAbsent("a", func);
System.out.println(ret1); // aaa
System.out.println(map); // {a=aaa, b=bbb}

final var ret2 = map.computeIfAbsent("b", func);
System.out.println(ret2); // bbb
System.out.println(map); // {a=aaa, b=bbb}

final var ret3 = map.computeIfAbsent("c", func);
System.out.println(ret3); // ccc
System.out.println(map); // {a=aaa, b=bbb, c=ccc}

final var ret4 = map.computeIfAbsent("d", func);
System.out.println(ret4); // null
System.out.println(map); // {a=aaa, b=bbb, c=ccc}

final ConcurrentMap<String, List<String>> map = new ConcurrentHashMap<>();
System.out.println(map); // {}

map.computeIfAbsent("a", (key) -> new ArrayList<>()).add("a-1");
System.out.println(map); // {a=[a-1]}

map.computeIfAbsent("a", (key) -> new ArrayList<>()).add("a-2");
System.out.println(map); // {a=[a-1, a-2]}

map.computeIfAbsent("b", (key) -> new ArrayList<>()).add("b-1");
System.out.println(map); // {a=[a-1, a-2], b=[b-1]}

default V computeIfPresent (K key, BiFunction<? super K,? super V,? extends V> remappingFunction)

If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.

final ConcurrentMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
System.out.println(map); // {a=1, b=2, c=3}

final var func = new BiFunction<String, Integer, Integer>() {
    @Override
    public Integer apply(String key, Integer value) {
        return value * 10;
    }
};

final var ret1 = map.computeIfPresent("a", func);
System.out.println(ret1); // 10
System.out.println(map); // {a=10, b=2, c=3}

final var ret2 = map.computeIfPresent("b", func);
System.out.println(ret2); // 20
System.out.println(map); // {a=10, b=20, c=3}

final var ret3 = map.computeIfPresent("c", func);
System.out.println(ret3); // 30
System.out.println(map); // {a=10, b=20, c=30}

final var ret4 = map.computeIfPresent("X", func);
System.out.println(ret4); // null
System.out.println(map); // {a=10, b=20, c=30}

final ConcurrentMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
System.out.println(map); // {a=1, b=2, c=3}

final var ret = map.computeIfPresent("b", (k, v) -> null);
System.out.println(ret); // null
System.out.println(map); // {a=1, c=3}

default void forEach (BiConsumer<? super K,? super V> action)

Performs the given action for each entry in this map until all entries have been processed or the action throws an exception.

final ConcurrentMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
System.out.println(map); // {a=1, b=2, c=3}

System.out.println("-- forEach --");
map.forEach((key, value) -> {
    System.out.println("key = %s : value = %d".formatted(key, value));
});

// Result
// ↓
//-- forEach --
//key = a : value = 1
//key = b : value = 2
//key = c : value = 3

default V getOrDefault (Object key, V defaultValue)

Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key.

final ConcurrentMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
System.out.println(map); // {a=1, b=2, c=3}

System.out.println(map.getOrDefault("a", 10)); // 1
System.out.println(map.getOrDefault("b", 20)); // 2
System.out.println(map.getOrDefault("c", 30)); // 3
System.out.println(map.getOrDefault("X", 40)); // 40
System.out.println(map.getOrDefault("", 50)); // 50

default V merge (K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction)

If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.

final ConcurrentMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
System.out.println(map); // {a=1, b=2, c=3}

final var func = new BiFunction<Integer, Integer, Integer>() {
    @Override
    public Integer apply(Integer oldValue, Integer newValue) {
        return oldValue + newValue;
    }
};

final var ret1 = map.merge("a", 1000, func);
System.out.println(ret1); // 1001
System.out.println(map); // {a=1001, b=2, c=3}

final var ret2 = map.merge("b", 1000, func);
System.out.println(ret2); // 1002
System.out.println(map); // {a=1001, b=1002, c=3}

final var ret3 = map.merge("c", 1000, func);
System.out.println(ret3); // 1003
System.out.println(map); // {a=1001, b=1002, c=1003}

final var ret4 = map.merge("d", 1000, func);
System.out.println(ret4); // 1000
System.out.println(map); // {a=1001, b=1002, c=1003, d=1000}

final var ret5 = map.merge("b", 1000, (k, v) -> null);
System.out.println(ret5); // null
System.out.println(map); // {a=1001, c=1003, d=1000}

V putIfAbsent (K key, V value)

If the specified key is not already associated with a value, associates it with the given value.

final ConcurrentMap<String, Integer> map = new ConcurrentHashMap<>();
System.out.println(map); // {}

System.out.println(map.putIfAbsent("a", 1)); // null
System.out.println(map); // {a=1}

System.out.println(map.putIfAbsent("b", 2)); // null
System.out.println(map); // {a=1, b=2}

System.out.println(map.putIfAbsent("c", 3)); // null
System.out.println(map); // {a=1, b=2, c=3}

System.out.println(map.putIfAbsent("a", 999)); // 1
System.out.println(map); // {a=1, b=2, c=3}

boolean remove (Object key, Object value)

Removes the entry for a key only if currently mapped to a given value.

final ConcurrentMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
System.out.println(map); // {a=1, b=2, c=3}

System.out.println(map.remove("a", 1)); // true
System.out.println(map); // {b=2, c=3}

System.out.println(map.remove("b", 1)); // false
System.out.println(map); // {b=2, c=3}

System.out.println(map.remove("b", 2)); // true
System.out.println(map); // {c=3}

System.out.println(map.remove("C", 3)); // false
System.out.println(map); // {c=3}

System.out.println(map.remove("d", 4)); // false
System.out.println(map); // {c=3}

V replace (K key, V value)

Replaces the entry for a key only if currently mapped to some value.

final ConcurrentMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
System.out.println(map); // {a=1, b=2, c=3}

System.out.println(map.replace("a", 10)); // 1
System.out.println(map); // {a=10, b=2, c=3}

System.out.println(map.replace("b", 20)); // 2
System.out.println(map); // {a=10, b=20, c=3}

System.out.println(map.replace("z", 99)); // null
System.out.println(map); // {a=10, b=20, c=3}

boolean replace (K key, V oldValue, V newValue)

Replaces the entry for a key only if currently mapped to a given value.

final ConcurrentMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
System.out.println(map); // {a=1, b=2, c=3}

System.out.println(map.replace("a", 1, 10)); // true
System.out.println(map); // {a=10, b=2, c=3}

System.out.println(map.replace("b", 999, 20)); // false
System.out.println(map); // {a=10, b=2, c=3}

System.out.println(map.replace("b", 2, 20)); // true
System.out.println(map); // {a=10, b=20, c=3}

System.out.println(map.replace("z", 3, 999)); // false
System.out.println(map); // {a=10, b=20, c=3}

default void replaceAll (BiFunction<? super K,? super V,? extends V> function)

Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.

final var src = Map.of("a", 1, "b", 2, "c", 3);

{
    final ConcurrentMap<String, Integer> map = new ConcurrentHashMap<>(src);
    System.out.println(map); // {a=1, b=2, c=3}

    map.replaceAll((k, v) -> v * 10);
    System.out.println(map); // {a=10, b=20, c=30}
}
{
    final ConcurrentMap<String, Integer> map = new ConcurrentHashMap<>(src);
    System.out.println(map); // {a=1, b=2, c=3}

    map.replaceAll((k, v) -> {
        if (k.equals("a")) {
            return v * 1000;
        } else {
            return v * 10;
        }
    });
    System.out.println(map); // {a=1000, b=20, c=30}
}
{
    final ConcurrentMap<String, Integer> map = new ConcurrentHashMap<>(src);
    System.out.println(map); // {a=1, b=2, c=3}

    map.replaceAll((k, v) -> {
        if (v == 2) {
            return v * 1000;
        } else {
            return v * 10;
        }
    });
    System.out.println(map); // {a=10, b=2000, c=30}
}

clear, containsKey, containsValue, entrySet, equals, get, hashCode, isEmpty, keySet, put, putAll, remove, size, values

Please see the link below.

• Java API Examples : Map