ExplicitOrdering from Guava that will not throw when element is outside of the initial collection (unknown to the explicit ordering)

ExplicitOrdering.java

/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.spartez.nbf.utils;

import com.google.common.annotations.GwtCompatible;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.Ordering;

import javax.annotation.Nullable;
import java.io.Serializable;
import java.util.List;

/** An ordering that compares objects according to a given order. */
@GwtCompatible(serializable = true)
public final class ExplicitOrdering<T> extends Ordering<T> implements Serializable {
    final ImmutableMap<T, Integer> rankMap;

    public ExplicitOrdering(List<T> valuesInOrder) {
        this(buildRankMap(valuesInOrder));
    }

    ExplicitOrdering(ImmutableMap<T, Integer> rankMap) {
        this.rankMap = rankMap;
    }

    @Override public int compare(T left, T right) {
        return rank(left) - rank(right); // safe because both are nonnegative
    }

    private int rank(T value) {
        final Integer rank = rankMap.get(value);
        if (rank == null) {
            return rankMap.size();
        }
        return rank;
    }

    private static <T> ImmutableMap<T, Integer> buildRankMap(
            List<T> valuesInOrder) {
        final ImmutableMap.Builder<T, Integer> builder = ImmutableMap.builder();
        int rank = 0;
        for (final T value : valuesInOrder) {
            builder.put(value, rank++);
        }
        return builder.build();
    }

    @Override public boolean equals(@Nullable Object object) {
        if (object instanceof ExplicitOrdering) {
            final ExplicitOrdering<?> that = (ExplicitOrdering<?>) object;
            return this.rankMap.equals(that.rankMap);
        }
        return false;
    }

    @Override public int hashCode() {
        return rankMap.hashCode();
    }

    @Override public String toString() {
        return "Ordering.explicit(" + rankMap.keySet() + ")";
    }

    private static final long serialVersionUID = 0;
}