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Helpers classes for collections.

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/*
 * Copyright (c) 2010-2017, KolibriFX AS. Licensed under the Apache License, version 2.0.
 */

package com.kolibrifx.common.collections;

import java.util.HashMap;
import java.util.function.Supplier;

/**
 * Specialized hash map that for missing keys calls a supplier function to set a default value,
 * instead of returning null. The supplier can be a standard {@link Supplier} (which
 * takes no arguments) or a {@link ValueSupplier} which takes the key as an argument.
 * 
 * Inspired by python's "collections.defaultdict".
 */
public class DefaultHashMap extends HashMap {
    @FunctionalInterface
    public interface ValueSupplier {
        V get(K key);
    }

    private static final long serialVersionUID = -6024567039608325689L;
    private final ValueSupplier defaultValueSupplier;

    public DefaultHashMap(final ValueSupplier defaultValueSupplier) {
        this.defaultValueSupplier = defaultValueSupplier;
    }

    public DefaultHashMap(final Supplier defaultValueSupplier) {
        this(k -> defaultValueSupplier.get());
    }

    @Override
    public V get(final Object key) {
        if (!containsKey(key)) {
            @SuppressWarnings("unchecked")
            final K typedKey = (K) key; // bah
            final V value = defaultValueSupplier.get(typedKey);
            put(typedKey, value);
            return value;
        }
        return super.get(key);
    }
}