diff --git a/pdb-api/src/main/java/org/lucares/pdb/api/UniqueStringIntegerPairs.java b/pdb-api/src/main/java/org/lucares/pdb/api/UniqueStringIntegerPairs.java
index ff987a9..da483f3 100644
--- a/pdb-api/src/main/java/org/lucares/pdb/api/UniqueStringIntegerPairs.java
+++ b/pdb-api/src/main/java/org/lucares/pdb/api/UniqueStringIntegerPairs.java
@@ -10,16 +10,16 @@ import java.io.Writer;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.util.ArrayList;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 import java.util.Set;
-import java.util.SortedMap;
-import java.util.TreeMap;
 import java.util.function.Function;
 import java.util.regex.Pattern;
 
 /**
- * A very simple {@link Set}-like or {@link Map}-like datastructure that stores
+ * A very simple {@link Set}-like or {@link Map}-like data structure that stores
  * unique¹ pairs of Strings and integers persistently.
  * <p>
  * (1) Unique means, that neither the string, nor the integer may occur twice.
@@ -42,7 +42,7 @@ public class UniqueStringIntegerPairs {
 	/**
 	 * Maps an integer to a string. E.g. 123 -> "myLongValue"
 	 */
-	private final SortedMap<Integer, String> intToString = new TreeMap<>();
+	private final List<String> intToString = new ArrayList<>();
 
 	private final Path file;
 
@@ -75,7 +75,7 @@ public class UniqueStringIntegerPairs {
 					if (tokens.length == 2) {
 						final String string = tokens[0];
 						final int value = Integer.parseInt(tokens[1]);
-						intToString.put(value, string);
+						intToStringPut(value, string);
 						stringToInt.put(string, value);
 					}
 				}
@@ -85,9 +85,19 @@ public class UniqueStringIntegerPairs {
 		}
 	}
 
-	public void put(final String first, final int second) {
+	private void intToStringPut(final int value, final String string) {
+		if (intToString.size() <= value) {
+			// list is not long enough -> grow list
+			while (intToString.size() <= value) {
+				intToString.add(null);
+			}
+		}
+		intToString.set(value, string);
+	}
 
-		if (stringToInt.containsKey(first) || intToString.containsKey(second)) {
+	void put(final String first, final int second) {
+
+		if (stringToInt.containsKey(first) || (intToString.size() > second && intToString.get(second) != null)) {
 			throw new IllegalArgumentException("Unique key constraint violation for (" + first + ", " + second + ")");
 		}
 		if (file != null) {
@@ -101,7 +111,7 @@ public class UniqueStringIntegerPairs {
 			}
 		}
 
-		intToString.put(second, first);
+		intToStringPut(second, first);
 		stringToInt.put(first, second);
 	}
 
@@ -110,16 +120,15 @@ public class UniqueStringIntegerPairs {
 		return stringToInt.get(first);
 	}
 
-	public String getKey(final Integer second) {
+	public String getKey(final int second) {
 		return intToString.get(second);
 	}
 
 	public Integer getHighestInteger() {
-		return intToString.size() == 0 ? -1 : intToString.lastKey();
+		return intToString.size() == 0 ? -1 : intToString.size() - 1;
 	}
 
 	public Integer computeIfAbsent(final String first, final Function<String, Integer> mappingFunction) {
-
 		if (!stringToInt.containsKey(first)) {
 			synchronized (stringToInt) {
 				if (!stringToInt.containsKey(first)) {