The Difference Between Java Map’s compute, computeIfAbsent

Time：2020-09-11 08:23:45
Class：Weblog
Read：16

In Java, the Map (java.util.Map) is a Generic Interface that stores a key value pair:

/*
 * @see HashMap
 * @see TreeMap
 * @see Hashtable
 * @see SortedMap
 * @see Collection
 * @see Set
 * @since 1.2
*/
public interface Map<K, V> {
  .. ..
}

/*
 * @see HashMap
 * @see TreeMap
 * @see Hashtable
 * @see SortedMap
 * @see Collection
 * @see Set
 * @since 1.2
*/
public interface Map<K, V> {
  .. ..
}

Many data structures based on the hashing (e.g. HashMap, TreeMap, Hashtable etc) are implementing the Map interface – which has supported three useful methods: compute, computeIfPresent and computeIfAbsent.

computeIfAbsent on Java’s Map

The computeIfAbsent, as compute-if-absent, will put the value (and return it) in the map only if the current key is not existent in the map. If it is already existent in the map, the function will do nothing and simply return the original value.

For example:

Map<Integer, Integer> data = new HashMap<>();
data.put(1, 10);
System.out.println(data.computeIfAbsent(1, 20)); // print 10, MAP = (1: 10)
System.out.println(data.computeIfAbsent(2, 30)); // print 30, MAP = (1: 10, 2: 30);

Map<Integer, Integer> data = new HashMap<>();
data.put(1, 10);
System.out.println(data.computeIfAbsent(1, 20)); // print 10, MAP = (1: 10)
System.out.println(data.computeIfAbsent(2, 30)); // print 30, MAP = (1: 10, 2: 30);

The key-value pair (1, 10) is put first in the hash map, thus the first computeIfAbsent will not alter the hash map (the data.get(1) still equals to 10). The second computeIfAbsent(2, 30) will put key-value pair (2, 30) into the hash map.

The second parameter could be a lambda function:

1	data.computeIfAbsent(2, (key) -> key + 1);

data.computeIfAbsent(2, (key) -> key + 1);

The full Java SDK of the computeIfAbsent can be found in the java.util.Map:

   /**
     * If the specified key is not already associated with a value (or is mapped
     * to {@code null}), attempts to compute its value using the given mapping
     * function and enters it into this map unless {@code null}.
     *
     * <p>If the function returns {@code null} no mapping is recorded. If
     * the function itself throws an (unchecked) exception, the
     * exception is rethrown, and no mapping is recorded.  The most
     * common usage is to construct a new object serving as an initial
     * mapped value or memoized result, as in:
     *
     * <pre> {@code
     * map.computeIfAbsent(key, k -> new Value(f(k)));
     * }</pre>
     *
     * <p>Or to implement a multi-value map, {@code Map<K,Collection<V>>},
     * supporting multiple values per key:
     *
     * <pre> {@code
     * map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);
     * }</pre>
     *
     *
     * @implSpec
     * The default implementation is equivalent to the following steps for this
     * {@code map}, then returning the current value or {@code null} if now
     * absent:
     *
     * <pre> {@code
     * if (map.get(key) == null) {
     *     V newValue = mappingFunction.apply(key);
     *     if (newValue != null)
     *         map.put(key, newValue);
     * }
     * }</pre>
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties. In particular, all implementations of
     * subinterface {@link java.util.concurrent.ConcurrentMap} must document
     * whether the function is applied once atomically only if the value is not
     * present.
     *
     * @param key key with which the specified value is to be associated
     * @param mappingFunction the function to compute a value
     * @return the current (existing or computed) value associated with
     *         the specified key, or null if the computed value is null
     * @throws NullPointerException if the specified key is null and
     *         this map does not support null keys, or the mappingFunction
     *         is null
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @since 1.8
     */
    default V computeIfAbsent(K key,
            Function<? super K, ? extends V> mappingFunction) {
        Objects.requireNonNull(mappingFunction);
        V v;
        if ((v = get(key)) == null) {
            V newValue;
            if ((newValue = mappingFunction.apply(key)) != null) {
                put(key, newValue);
                return newValue;
            }
        }
        return v;
    }

   /**
     * If the specified key is not already associated with a value (or is mapped
     * to {@code null}), attempts to compute its value using the given mapping
     * function and enters it into this map unless {@code null}.
     *
     * <p>If the function returns {@code null} no mapping is recorded. If
     * the function itself throws an (unchecked) exception, the
     * exception is rethrown, and no mapping is recorded.  The most
     * common usage is to construct a new object serving as an initial
     * mapped value or memoized result, as in:
     *
     * <pre> {@code
     * map.computeIfAbsent(key, k -> new Value(f(k)));
     * }</pre>
     *
     * <p>Or to implement a multi-value map, {@code Map<K,Collection<V>>},
     * supporting multiple values per key:
     *
     * <pre> {@code
     * map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);
     * }</pre>
     *
     *
     * @implSpec
     * The default implementation is equivalent to the following steps for this
     * {@code map}, then returning the current value or {@code null} if now
     * absent:
     *
     * <pre> {@code
     * if (map.get(key) == null) {
     *     V newValue = mappingFunction.apply(key);
     *     if (newValue != null)
     *         map.put(key, newValue);
     * }
     * }</pre>
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties. In particular, all implementations of
     * subinterface {@link java.util.concurrent.ConcurrentMap} must document
     * whether the function is applied once atomically only if the value is not
     * present.
     *
     * @param key key with which the specified value is to be associated
     * @param mappingFunction the function to compute a value
     * @return the current (existing or computed) value associated with
     *         the specified key, or null if the computed value is null
     * @throws NullPointerException if the specified key is null and
     *         this map does not support null keys, or the mappingFunction
     *         is null
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (<a href="https://helloacm.com/the-difference-between-java-maps-compute-computeifabsent-and-computeifpresent/{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     *         (<a href="https://helloacm.com/the-difference-between-java-maps-compute-computeifabsent-and-computeifpresent/{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @since 1.8
     */
    default V computeIfAbsent(K key,
            Function<? super K, ? extends V> mappingFunction) {
        Objects.requireNonNull(mappingFunction);
        V v;
        if ((v = get(key)) == null) {
            V newValue;
            if ((newValue = mappingFunction.apply(key)) != null) {
                put(key, newValue);
                return newValue;
            }
        }
        return v;
    }

the computeIfPresent on Java’s Map Interface

On the other hand, the computeIfPresent, read as compute-if-present, will update the key-value pair only if the key is existent in the Map.

For example:

Map<Integer, Integer> data = new HashMap<>();
data.put(1, 10); // MAP = (1: 10)
System.out.println(data.computeIfPresent(1, (key, val) -> val + 1)); // print 11, MAP = (1: 11)
System.out.println(data.computeIfPresent(2, (key, val) -> val + 1)); // null

Map<Integer, Integer> data = new HashMap<>();
data.put(1, 10); // MAP = (1: 10)
System.out.println(data.computeIfPresent(1, (key, val) -> val + 1)); // print 11, MAP = (1: 11)
System.out.println(data.computeIfPresent(2, (key, val) -> val + 1)); // null

The Key-Value pair (1, 10) exists in the Map, thus it will compute the new value for the key using the second parameter which is a lambda function (with parameters key and value). If the key is not existent, the null value will be returned.

If the lambda function returns NULL for the new value, it will be equivantly to remove the key from the hash map.

Map<Integer, Integer> data = new HashMap<>();
data.put(1, 10);
System.out.println(data.computeIfPresent(1, (key, val) -> null)); // remove key 1
System.out.println(data.containsKey(1));  // false

Map<Integer, Integer> data = new HashMap<>();
data.put(1, 10);
System.out.println(data.computeIfPresent(1, (key, val) -> null)); // remove key 1
System.out.println(data.containsKey(1));  // false

The implementation of the computeIfPresent can be found in Java SDK: java.util.Map.

    /**
     * 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.
     *
     * <p>If the function returns {@code null}, the mapping is removed.  If the
     * function itself throws an (unchecked) exception, the exception is
     * rethrown, and the current mapping is left unchanged.
    *
     * @implSpec
     * The default implementation is equivalent to performing the following
     * steps for this {@code map}, then returning the current value or
     * {@code null} if now absent:
     *
     * <pre> {@code
     * if (map.get(key) != null) {
     *     V oldValue = map.get(key);
     *     V newValue = remappingFunction.apply(key, oldValue);
     *     if (newValue != null)
     *         map.put(key, newValue);
     *     else
     *         map.remove(key);
     * }
     * }</pre>
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties. In particular, all implementations of
     * subinterface {@link java.util.concurrent.ConcurrentMap} must document
     * whether the function is applied once atomically only if the value is not
     * present.
     *
     * @param key key with which the specified value is to be associated
     * @param remappingFunction the function to compute a value
     * @return the new value associated with the specified key, or null if none
     * @throws NullPointerException if the specified key is null and
     *         this map does not support null keys, or the
     *         remappingFunction is null
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @since 1.8
     */
    default V computeIfPresent(K key,
            BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
        Objects.requireNonNull(remappingFunction);
        V oldValue;
        if ((oldValue = get(key)) != null) {
            V newValue = remappingFunction.apply(key, oldValue);
            if (newValue != null) {
                put(key, newValue);
                return newValue;
            } else {
                remove(key);
                return null;
            }
        } else {
            return null;
        }
    }

    /**
     * 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.
     *
     * <p>If the function returns {@code null}, the mapping is removed.  If the
     * function itself throws an (unchecked) exception, the exception is
     * rethrown, and the current mapping is left unchanged.
    *
     * @implSpec
     * The default implementation is equivalent to performing the following
     * steps for this {@code map}, then returning the current value or
     * {@code null} if now absent:
     *
     * <pre> {@code
     * if (map.get(key) != null) {
     *     V oldValue = map.get(key);
     *     V newValue = remappingFunction.apply(key, oldValue);
     *     if (newValue != null)
     *         map.put(key, newValue);
     *     else
     *         map.remove(key);
     * }
     * }</pre>
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties. In particular, all implementations of
     * subinterface {@link java.util.concurrent.ConcurrentMap} must document
     * whether the function is applied once atomically only if the value is not
     * present.
     *
     * @param key key with which the specified value is to be associated
     * @param remappingFunction the function to compute a value
     * @return the new value associated with the specified key, or null if none
     * @throws NullPointerException if the specified key is null and
     *         this map does not support null keys, or the
     *         remappingFunction is null
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (<a href="https://helloacm.com/the-difference-between-java-maps-compute-computeifabsent-and-computeifpresent/{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     *         (<a href="https://helloacm.com/the-difference-between-java-maps-compute-computeifabsent-and-computeifpresent/{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @since 1.8
     */
    default V computeIfPresent(K key,
            BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
        Objects.requireNonNull(remappingFunction);
        V oldValue;
        if ((oldValue = get(key)) != null) {
            V newValue = remappingFunction.apply(key, oldValue);
            if (newValue != null) {
                put(key, newValue);
                return newValue;
            } else {
                remove(key);
                return null;
            }
        } else {
            return null;
        }
    }

compute() on Map

The compute() is a bit similar to computeIfPresent() except that when the key isn’t existent in the map, calling the compute() will raise an exception if you reference the val parameter (the current value for the key in the Map).

Map<Integer, Integer> data = new HashMap<>();
data.put(1, 10);
System.out.println(data.compute(1, (key, val) -> val + 1)); // print 11, MAP = (1: 11)
System.out.println(data.compute(2, (key, val) -> val + 1));

Map<Integer, Integer> data = new HashMap<>();
data.put(1, 10);
System.out.println(data.compute(1, (key, val) -> val + 1)); // print 11, MAP = (1: 11)
System.out.println(data.compute(2, (key, val) -> val + 1));

The second compute() will throw: Exception in thread “main” java.lang.NullPointerException.

However, if you set the new value to null – it will be the same as removing the key-value pair:

Map<Integer, Integer> data = new HashMap<>();
data.put(1, 10);
System.out.println(data.compute(1, (key, val) -> null)); // remove key 1
System.out.println(data.containsKey(1));  // false

Map<Integer, Integer> data = new HashMap<>();
data.put(1, 10);
System.out.println(data.compute(1, (key, val) -> null)); // remove key 1
System.out.println(data.containsKey(1));  // false

The full implementation of the java.util.Map.compute() is as follows:

   /**
     * Attempts to compute a mapping for the specified key and its current
     * mapped value (or {@code null} if there is no current mapping). For
     * example, to either create or append a {@code String} msg to a value
     * mapping:
     *
     * <pre> {@code
     * map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))}</pre>
     * (Method {@link #merge merge()} is often simpler to use for such purposes.)
     *
     * <p>If the function returns {@code null}, the mapping is removed (or
     * remains absent if initially absent).  If the function itself throws an
     * (unchecked) exception, the exception is rethrown, and the current mapping
     * is left unchanged.
     *
     * @implSpec
     * The default implementation is equivalent to performing the following
     * steps for this {@code map}, then returning the current value or
     * {@code null} if absent:
     *
     * <pre> {@code
     * V oldValue = map.get(key);
     * V newValue = remappingFunction.apply(key, oldValue);
     * if (oldValue != null ) {
     *    if (newValue != null)
     *       map.put(key, newValue);
     *    else
     *       map.remove(key);
     * } else {
     *    if (newValue != null)
     *       map.put(key, newValue);
     *    else
     *       return null;
     * }
     * }</pre>
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties. In particular, all implementations of
     * subinterface {@link java.util.concurrent.ConcurrentMap} must document
     * whether the function is applied once atomically only if the value is not
     * present.
     *
     * @param key key with which the specified value is to be associated
     * @param remappingFunction the function to compute a value
     * @return the new value associated with the specified key, or null if none
     * @throws NullPointerException if the specified key is null and
     *         this map does not support null keys, or the
     *         remappingFunction is null
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @since 1.8
     */
    default V compute(K key,
            BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
        Objects.requireNonNull(remappingFunction);
        V oldValue = get(key);
 
        V newValue = remappingFunction.apply(key, oldValue);
        if (newValue == null) {
            // delete mapping
            if (oldValue != null || containsKey(key)) {
                // something to remove
                remove(key);
                return null;
            } else {
                // nothing to do. Leave things as they were.
                return null;
            }
        } else {
            // add or replace old mapping
            put(key, newValue);
            return newValue;
        }
    }

   /**
     * Attempts to compute a mapping for the specified key and its current
     * mapped value (or {@code null} if there is no current mapping). For
     * example, to either create or append a {@code String} msg to a value
     * mapping:
     *
     * <pre> {@code
     * map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))}</pre>
     * (Method {@link #merge merge()} is often simpler to use for such purposes.)
     *
     * <p>If the function returns {@code null}, the mapping is removed (or
     * remains absent if initially absent).  If the function itself throws an
     * (unchecked) exception, the exception is rethrown, and the current mapping
     * is left unchanged.
     *
     * @implSpec
     * The default implementation is equivalent to performing the following
     * steps for this {@code map}, then returning the current value or
     * {@code null} if absent:
     *
     * <pre> {@code
     * V oldValue = map.get(key);
     * V newValue = remappingFunction.apply(key, oldValue);
     * if (oldValue != null ) {
     *    if (newValue != null)
     *       map.put(key, newValue);
     *    else
     *       map.remove(key);
     * } else {
     *    if (newValue != null)
     *       map.put(key, newValue);
     *    else
     *       return null;
     * }
     * }</pre>
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties. In particular, all implementations of
     * subinterface {@link java.util.concurrent.ConcurrentMap} must document
     * whether the function is applied once atomically only if the value is not
     * present.
     *
     * @param key key with which the specified value is to be associated
     * @param remappingFunction the function to compute a value
     * @return the new value associated with the specified key, or null if none
     * @throws NullPointerException if the specified key is null and
     *         this map does not support null keys, or the
     *         remappingFunction is null
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (<a href="https://helloacm.com/the-difference-between-java-maps-compute-computeifabsent-and-computeifpresent/{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     *         (<a href="https://helloacm.com/the-difference-between-java-maps-compute-computeifabsent-and-computeifpresent/{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @since 1.8
     */
    default V compute(K key,
            BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
        Objects.requireNonNull(remappingFunction);
        V oldValue = get(key);

        V newValue = remappingFunction.apply(key, oldValue);
        if (newValue == null) {
            // delete mapping
            if (oldValue != null || containsKey(key)) {
                // something to remove
                remove(key);
                return null;
            } else {
                // nothing to do. Leave things as they were.
                return null;
            }
        } else {
            // add or replace old mapping
            put(key, newValue);
            return newValue;
        }
    }

–EOF (The Ultimate Computing & Technology Blog) —

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