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A versatile Java(TM) library that implements many useful container and utility classes.
There is a newer version: 1.1.12

/*
 * de.unkrig.commons - A general-purpose Java class library
 *
 * Copyright (c) 2014, Arno Unkrig
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modification, are permitted provided that the
 * following conditions are met:
 *
 *    1. Redistributions of source code must retain the above copyright notice, this list of conditions and the
 *       following disclaimer.
 *    2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the
 *       following disclaimer in the documentation and/or other materials provided with the distribution.
 *    3. The name of the author may not be used to endorse or promote products derived from this software without
 *       specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
 * THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

package de.unkrig.commons.lang.protocol;

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

import de.unkrig.commons.nullanalysis.NotNullByDefault;
import de.unkrig.commons.nullanalysis.Nullable;

/**
 * A map that computes a value only when {@link #get(Object)} is invoked.
 * Consequently, in contrast with {@link Map}, it has no specific {@link Map#size() size()}, and thus no {@link
 * Map#entrySet() entrySet()}, {@link Map#keySet() keySet()} and {@link Map#values() values()} set.
 * Also the modifying operations ({@link Map#put(Object, Object) put}, {@link Map#putAll(Map) putAll}, {@link
 * Map#remove(Object) remove} and {@link Map#clear() clear}) are all missing, because a {@link Mapping} is not changed
 * by "putting" key-value-pairs into it.
 * 
 *   Actually {@link Map} should extend {@link Mapping}, but it doesn't - thus there are the {@link #asMap()} and
 *   {@link Mappings#fromMap(Map)} helper methods.
 * 
 * 
 *   The relationship between {@link Mapping} and {@link Map} is very much like that between {@link Predicate} and
 *   {@link Collection}.
 * 
 *
 * IMPORTANT NOTICE:
 * 
 *   When using this type in a variable, parameter or field declaration, never write:
 * 
 * Mapping<key-type, value-type>
 * 
 *   , but always:
 * 
 * Mapping<? super key-type, ? extends value-type>
 * 
 *   That way many type conversions are possible  which otherwise aren't.
 * 
 *
 * @param  The 'key' type
 * @param  The 'value' type
 */
public abstract
class Mapping {

    /** @see Map#containsKey(Object) */
    public abstract boolean
    containsKey(@Nullable Object key);

    /** @see Map#get(Object) */
    @Nullable public abstract V
    get(@Nullable Object key);

    /**
     * Returns a proxy {@link Map} for a {@link Mapping} where all methods declared by {@link Map} but not by
     * {@link Mapping} throw an {@link UnsupportedOperationException}.
     */
    @NotNullByDefault(false) public final Map
    asMap() {

        return new Map() {

            @Override public boolean containsKey(Object key) { return Mapping.this.containsKey(key); }
            @Override public V       get(Object key)         { return Mapping.this.get(key); }

            // SUPPRESS CHECKSTYLE LineLength:16
            @Override public int              size()                                  { throw new UnsupportedOperationException("size");          }
            @Override public boolean          isEmpty()                               { throw new UnsupportedOperationException("isEmpty");       }
            @Override public boolean          containsValue(Object value)             { throw new UnsupportedOperationException("containsValue"); }
            @Override public V                put(K key, V value)                     { throw new UnsupportedOperationException("put");           }
            @Override public V                remove(Object key)                      { throw new UnsupportedOperationException("remove");        }
            @Override public void             putAll(Map t) { throw new UnsupportedOperationException("putAll");        }
            @Override public void             clear()                                 { throw new UnsupportedOperationException("clear");         }
            @Override public Set           keySet()                                { throw new UnsupportedOperationException("keySet");        }
            @Override public Collection    values()                                { throw new UnsupportedOperationException("values");        }
            @Override public Set> entrySet()                              { throw new UnsupportedOperationException("entrySet");      }

            @Override public boolean equals(Object o) { return o == this;                     }
            @Override public int     hashCode()       { return System.identityHashCode(this); }
            @Override public String  toString()       { return "Mapping";                     }

            @Override protected Object clone() throws CloneNotSupportedException { throw new CloneNotSupportedException("Mapping"); }
        };
    }
}