Partition.java

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.apachecon.memories.util;

import java.util.AbstractList;
import java.util.List;

public class Partition {

    public static <T> List<List<T>> partition(List<T> list, int size) {

        if (list == null)
            throw new NullPointerException("'list' must not be null");
        if (!(size > 0))
            throw new IllegalArgumentException("'size' must be greater than 0");

        return new PartitionList<T>(list, size);
    }

    private static class PartitionList<T> extends AbstractList<List<T>> {

        final List<T> list;
        final int size;

        PartitionList(List<T> list, int size) {
            this.list = list;
            this.size = size;
        }

        @Override
        public List<T> get(int index) {
            int listSize = size();
            if (listSize < 0)
                throw new IllegalArgumentException("negative size: " + listSize);
            if (index < 0)
                throw new IndexOutOfBoundsException("index " + index + " must not be negative");
            if (index >= listSize)
                throw new IndexOutOfBoundsException("index " + index + " must be less than size " + listSize);
            int start = index * size;
            int end = Math.min(start + size, list.size());
            return list.subList(start, end);
        }

        @Override
        public int size() {
            return (list.size() + size - 1) / size;
        }

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