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SciJava Common is a shared library for SciJava software. It provides a plugin framework, with an extensible mechanism for service discovery, backed by its own annotation processor, so that plugins can be loaded dynamically. It is used by downstream projects in the SciJava ecosystem, such as ImageJ and SCIFIO.

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 * Institute of Molecular Cell Biology and Genetics.
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 * 1. Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
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package org.scijava.util;

import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.Collection;

/**
 * An {@link ArrayList} whose size can be adjusted more efficiently.
 * 
 * When sizing down, elements at the end of the list are removed in one
 * operation. When sizing up, null elements are appended to the list.
 * 
 * 
 * @author Curtis Rueden
 * @param  The type of data stored in the list.
 */
public class SizableArrayList extends ArrayList implements Sizable {

	private static final long serialVersionUID = 1L;

	// -- Constructors --

	public SizableArrayList(final int initialCapacity) {
		super(initialCapacity);
	}

	public SizableArrayList() {
		super();
	}

	public SizableArrayList(final Collection c) {
		super(c);
	}

	// -- ArrayList methods --

	@Override
	public void ensureCapacity(int capacity) {
		// HACK: With Java 1.7.0_45 (but not 1.7.0_21), when calling ensureCapacity
		// with a value <= 10, while the size is still 0, does not actually affect
		// the capacity. This seems like a bug, but we will work around it!
		super.ensureCapacity(capacity < 11 ? 11 : capacity);
	}

	// -- Sizable methods --

	@Override
	public void setSize(final int size) {
		final int oldSize = size();
		if (oldSize == size) return; // no size change
		if (size < oldSize) {
			// need to remove extra elements
			removeRange(size, oldSize);
		}
		else {
			// need to add some elements
			ensureCapacity(size);
			final boolean hackSuccessful = hackSize(size);
			if (!hackSuccessful) {
				// explicitly increase the size by adding nulls
				while (size() < size) add(null);
			}
		}
	}

	// -- Helper methods --

	private boolean hackSize(final int size) {
		// HACK: Override the size field directly.
		final int oldSize;
		try {
			final Field sizeField = ArrayList.class.getDeclaredField("size");
			sizeField.setAccessible(true);
			oldSize = (Integer) sizeField.get(this);
			sizeField.set(this, size);

			// NB: Check that it worked. In the case of Java 1.7.0_45, it is possible
			// for the capacity to not *actually* be ensured, in which case
			// subsequently attempting to get the (size - 1)th value results in
			// ArrayIndexOutOfBoundsException. So let's be safe and verify it here.
			try {
				get(size - 1);
			}
			catch (final Exception exc) {
				// NB: Restore the previous size, then fail.
				sizeField.set(this, oldSize);
				return false;
			}
		}
		catch (final Exception exc) {
			return false;
		}
		return true;
	}

}