Maps.java Source code

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Here is the source code for Maps.java

Source

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

/**
 * Utility methods for operating on memory-efficient maps. All maps of size 0 or
 * 1 are assumed to be immutable. All maps of size greater than 1 are assumed to
 * be mutable.
 */
public class Maps {

    private static final Class<?> MULTI_MAP_CLASS = HashMap.class;
    private static final Class<?> SINGLETON_MAP_CLASS = Collections.singletonMap(null, null).getClass();

    public static <K, V> Map<K, V> create() {
        return Collections.emptyMap();
    }

    public static <K, V> Map<K, V> create(K key, V value) {
        return Collections.singletonMap(key, value);
    }

    public static <K, V> Map<K, V> normalize(Map<K, V> map) {
        switch (map.size()) {
        case 0:
            return create();
        case 1: {
            if (map.getClass() == SINGLETON_MAP_CLASS) {
                return map;
            }
            K key = map.keySet().iterator().next();
            return create(key, map.get(key));
        }
        default:
            if (map.getClass() == MULTI_MAP_CLASS) {
                return map;
            }
            return new HashMap<K, V>(map);
        }
    }

    public static <K, V> Map<K, V> normalizeUnmodifiable(Map<K, V> map) {
        if (map.size() < 2) {
            return normalize(map);
        } else {
            // TODO: implement an UnmodifiableHashMap?
            return Collections.unmodifiableMap(normalize(map));
        }
    }

    public static <K, V> Map<K, V> put(Map<K, V> map, K key, V value) {
        switch (map.size()) {
        case 0:
            // Empty -> Singleton
            return Collections.singletonMap(key, value);
        case 1: {
            if (map.containsKey(key)) {
                return create(key, value);
            }
            // Singleton -> HashMap
            Map<K, V> result = new HashMap<K, V>();
            result.put(map.keySet().iterator().next(), map.values().iterator().next());
            result.put(key, value);
            return result;
        }
        default:
            // HashMap
            map.put(key, value);
            return map;
        }
    }

    public static <K, V> Map<K, V> putAll(Map<K, V> map, Map<K, V> toAdd) {
        switch (toAdd.size()) {
        case 0:
            // No-op.
            return map;
        case 1: {
            // Add one element.
            K key = toAdd.keySet().iterator().next();
            return put(map, key, toAdd.get(key));
        }
        default:
            // True list merge, result >= 2.
            switch (map.size()) {
            case 0:
                return new HashMap<K, V>(toAdd);
            case 1: {
                HashMap<K, V> result = new HashMap<K, V>();
                K key = map.keySet().iterator().next();
                result.put(key, map.get(key));
                result.putAll(toAdd);
                return normalize(result);
            }
            default:
                map.putAll(toAdd);
                return map;
            }
        }
    }

    public static <K, V> Map<K, V> remove(Map<K, V> map, K key) {
        switch (map.size()) {
        case 0:
            // Empty
            return map;
        case 1:
            // Singleton -> Empty
            if (map.containsKey(key)) {
                return create();
            }
            return map;
        case 2:
            // HashMap -> Singleton
            if (map.containsKey(key)) {
                map.remove(key);
                key = map.keySet().iterator().next();
                return create(key, map.get(key));
            }
            return map;
        default:
            // IdentityHashMap
            map.remove(key);
            return map;
        }
    }
}