diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackDirectory.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackDirectory.java
index 5f30970..73745d8 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackDirectory.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackDirectory.java
@@ -22,6 +22,7 @@
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.EnumMap;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -512,13 +513,15 @@
 		for (String name : nameList) {
 			try {
 				PackFile pack = new PackFile(directory, name);
-				Map<PackExt, PackFile> packByExt = packFilesByExtById
-						.get(pack.getId());
-				if (packByExt == null) {
-					packByExt = new HashMap<>(PackExt.values().length);
-					packFilesByExtById.put(pack.getId(), packByExt);
+				if (pack.getPackExt() != null) {
+					Map<PackExt, PackFile> packByExt = packFilesByExtById
+							.get(pack.getId());
+					if (packByExt == null) {
+						packByExt = new EnumMap<>(PackExt.class);
+						packFilesByExtById.put(pack.getId(), packByExt);
+					}
+					packByExt.put(pack.getPackExt(), pack);
 				}
-				packByExt.put(pack.getPackExt(), pack);
 			} catch (IllegalArgumentException e) {
 				continue;
 			}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackExt.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackExt.java
index bedc693..6fb775d 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackExt.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackExt.java
@@ -13,66 +13,26 @@
 /**
  * A pack file extension.
  */
-public class PackExt {
-	private static volatile PackExt[] VALUES = new PackExt[] {};
-
+public enum PackExt {
 	/** A pack file extension. */
-	public static final PackExt PACK = newPackExt("pack"); //$NON-NLS-1$
+	PACK("pack"), //$NON-NLS-1$
 
 	/** A pack index file extension. */
-	public static final PackExt INDEX = newPackExt("idx"); //$NON-NLS-1$
+	INDEX("idx"), //$NON-NLS-1$
 
 	/** A keep pack file extension. */
-	public static final PackExt KEEP = newPackExt("keep"); //$NON-NLS-1$
+	KEEP("keep"), //$NON-NLS-1$
 
 	/** A pack bitmap index file extension. */
-	public static final PackExt BITMAP_INDEX = newPackExt("bitmap"); //$NON-NLS-1$
+	BITMAP_INDEX("bitmap"), //$NON-NLS-1$
 
 	/** A reftable file. */
-	public static final PackExt REFTABLE = newPackExt("ref"); //$NON-NLS-1$
-
-	/**
-	 * Get all of the PackExt values.
-	 *
-	 * @return all of the PackExt values.
-	 */
-	public static PackExt[] values() {
-		return VALUES;
-	}
-
-	/**
-	 * Returns a PackExt for the file extension and registers it in the values
-	 * array.
-	 *
-	 * @param ext
-	 *            the file extension.
-	 * @return the PackExt for the ext
-	 */
-	public static synchronized PackExt newPackExt(String ext) {
-		PackExt[] dst = new PackExt[VALUES.length + 1];
-		for (int i = 0; i < VALUES.length; i++) {
-			PackExt packExt = VALUES[i];
-			if (packExt.getExtension().equals(ext))
-				return packExt;
-			dst[i] = packExt;
-		}
-		if (VALUES.length >= 32)
-			throw new IllegalStateException(
-					"maximum number of pack extensions exceeded"); //$NON-NLS-1$
-
-		PackExt value = new PackExt(ext, VALUES.length);
-		dst[VALUES.length] = value;
-		VALUES = dst;
-		return value;
-	}
+	REFTABLE("ref"); //$NON-NLS-1$
 
 	private final String ext;
 
-	private final int pos;
-
-	private PackExt(String ext, int pos) {
+	private PackExt(String ext) {
 		this.ext = ext;
-		this.pos = pos;
 	}
 
 	/**
@@ -85,12 +45,12 @@
 	}
 
 	/**
-	 * Get the position of the extension in the values array.
+	 * Get the position of the extension in the enum declaration.
 	 *
-	 * @return the position of the extension in the values array.
+	 * @return the position of the extension in the enum declaration.
 	 */
 	public int getPosition() {
-		return pos;
+		return ordinal();
 	}
 
 	/**