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Basic utilities to compensate shortcomings of Java SE. Part of chicory.
There is a newer version: 0.36
/*
 * Copyright (c) 2014, Victor Nazarov 
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *     * Redistributions of source code must retain the above copyright
 *       notice, this list of conditions and the following disclaimer.
 *     * 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.
 *     * Neither the name of the Victor Nazarov nor the
 *       names of its contributors may be used to endorse or promote products
 *       derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "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  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 com.github.sviperll;

/**
 * Objects used as pure functions.
 *
 * @see Applicable
 *
 * @author Victor Nazarov 
 * @param  argument type
 * @param  result type
 */
public class Function implements Applicable {
    @SuppressWarnings({"rawtypes", "unchecked"})
    private static final Function ID = new Function(new IdApplicable());

    public static  Function identity() {
        return typedIdentity();
    }

    @SuppressWarnings({"unchecked"})
    public static  Function typedIdentity() {
        return (Function)ID;
    }

    @SuppressWarnings({"unchecked"})
    public static  Function of(Applicable function) {
        if (function instanceof Function) {
            // Applicable is covariant in it's first argument
            // so this cast is safe
            return (Function)function;
        } else
            return new Function(function);
    }

    private final Applicable function;
    private Function(Applicable function) {
        this.function = function;
    }

    @Override
    public R apply(T t) {
        return function.apply(t);
    }

    public  Function composeWith(Applicable thatFunction) {
        return new Function(new ComposedApplicable(this.function, thatFunction));
    }

    public  Function andThen(Applicable thatFunction) {
        return new Function(new ComposedApplicable(thatFunction, this.function));
    }

    public Function passNullThrough() {
        return new Function(new PassingNullThroughApplicable(function));
    }

    public Function throwOnNull() {
        return new Function(new ThrowingOnNullApplicable(function));
    }

    private static class IdApplicable implements Applicable {
        @Override
        public R apply(T t) {
            return t;
        }
    }

    private static class ComposedApplicable implements Applicable {
        private final Applicable f;
        private final Applicable g;
        public ComposedApplicable(Applicable f, Applicable g) {
            this.f = f;
            this.g = g;
        }

        @Override
        public R apply(T t) {
            return f.apply(g.apply(t));
        }
    }

    private static class PassingNullThroughApplicable implements Applicable {
        private final Applicable function;
        public PassingNullThroughApplicable(Applicable function) {
            this.function = function;
        }

        @Override
        public R apply(T t) {
            if (t == null)
                return null;
            else
                return function.apply(t);
        }
    }

    private static class ThrowingOnNullApplicable implements Applicable {
        private final Applicable function;
        public ThrowingOnNullApplicable(Applicable function) {
            this.function = function;
        }

        @Override
        public R apply(T t) {
            if (t == null)
                throw new NullPointerException();
            else
                return function.apply(t);
        }
    }
}