A Java Map implementation that provides a default value.

Maps.java

import java.util.Collection;
import java.util.Map;
import java.util.Set;

public class Maps {
    
    public static <K,V> Map<K,V> withDefaultValue(Map<K,V> map, V defaultValue){
        return new DefaultMap<K, V>(map, defaultValue);
    }
    
    private static class DefaultMap<K, V> implements Map<K, V> {

        private final Map<K, V> map;
        private final V defaultValue;

        public DefaultMap(Map<K, V> map, V defaultValue) {
            this.map = map;
            this.defaultValue = defaultValue;
        }

        @Override
        public V get(Object key) {
            V ret = map.get(key);
            if (ret == null) {
                ret = defaultValue;
            }
            return ret;
        }

        @Override
        public int size() {
            return map.size();
        }

        @Override
        public boolean isEmpty() {
            return map.isEmpty();
        }

        @Override
        public boolean containsKey(Object o) {
            return map.containsKey(o);
        }

        @Override
        public boolean containsValue(Object o) {
            return map.containsValue(o);
        }

        @Override
        public V put(K k, V v) {
            return this.map.put(k, v);
        }

        @Override
        public V remove(Object o) {
            return map.remove(o);
        }

        @Override
        public void putAll(Map<? extends K, ? extends V> m) {
            this.map.putAll(m);
        }

        @Override
        public void clear() {
            this.map.clear();
        }

        @Override
        public Set<K> keySet() {
            return this.map.keySet();
        }

        @Override
        public Collection<V> values() {
            return this.map.values();
        }

        @Override
        public Set<Map.Entry<K, V>> entrySet() {
            return this.map.entrySet();
        }
    }
}