Batch renamer (preserves extensions)

Renamer.java

import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.Arrays;
import java.util.Set;
import java.util.TreeSet;

public class Renamer {

	public static void main(final String[] args) throws FileNotFoundException, IOException {
		if (args.length != 2) {
			throw new IllegalArgumentException();
		}

		final String srcDirName = args[0].trim();
		final File srcDir = new File(srcDirName);
		if (!srcDir.isDirectory()) {
			throw new FileNotFoundException(srcDir.getCanonicalPath());
		}

		final String dstDirName = args[1].trim();
		final File dstDir = new File(dstDirName);
		if (!dstDir.isDirectory()) {
			throw new FileNotFoundException(dstDir.getCanonicalPath());
		}

		final Set<String> srcFileNames = new TreeSet<>();
		Arrays.asList(srcDir.list()).stream().forEach(srcFileNames::add);

		final Set<String> dstFileNames = new TreeSet<>();
		Arrays.asList(dstDir.list()).stream().forEach(dstFileNames::add);

		if (srcFileNames.size() != dstFileNames.size()) {
			throw new IllegalStateException();
		}

		final String[] srcFileNamesArray = srcFileNames.toArray(new String[] {});
		final String[] dstFileNamesArray = dstFileNames.toArray(new String[] {});

		for (int i = 0; i < srcFileNames.size(); i++) {
			final String dstFileName = dstFileNamesArray[i];
			final String srcFileName = srcFileNamesArray[i];

			final File oldNamedFile = new File(dstDir + File.separator + dstFileName);
			final File newNamedFile = new File(dstDir + File.separator + srcFileName.replace(srcFileName.substring(srcFileName.lastIndexOf('.')), dstFileName.substring(dstFileName.lastIndexOf('.'))));

			if (!oldNamedFile.toString().equals(newNamedFile.toString())) { // case-sensitive
				System.out.printf("%s -> %s%s", oldNamedFile.toString(), newNamedFile.getName(), System.lineSeparator());
				if (!oldNamedFile.renameTo(newNamedFile)) {
					throw new RuntimeException();
				}
			}
		}
	}

}