Resizing of index.

java/src/main/java/com/spotify/voyager/jni/StringIndex.java

@@ -358,6 +358,27 @@ public void close() throws IOException {
     index.close();
   }
 
+  /**
+   * Change the maximum number of elements currently storable by this {@link Index}. This operation
+   * reallocates the memory used by the index and can be quite slow, so it may be useful to set the
+   * maximum number of elements in advance if that number is known.
+   *
+   * @param newSize The new number of maximum elements to resize this {@link Index} to.
+   */
+  public void resizeIndex(long newSize) {
+    index.resizeIndex(newSize);
+  }
+
+  /**
+   * Get the maximum number of elements currently storable by this {@link Index}. If more elements
+   * are added than {@code getMaxElements()}, the index will be automatically (but slowly) resized.
+   *
+   * @return The number of elements (vectors) that are currently storable in this {@link Index}.
+   */
+  public long getMaxElements() {
+    return index.getMaxElements();
+  }
+
   /** A wrapper class for nearest neighbor query results. */
   public static class QueryResults {
     private final String[] names;

java/src/test/java/com/spotify/voyager/jni/IndexTest.java

@@ -281,6 +281,11 @@ private void runTestWith(
 
       // And the resulting call should pass:
       assertNotNull(index.getVector(0));
+
+      // Test resizing of index
+      long currentSize = index.getMaxElements();
+      index.resizeIndex(currentSize + 1);
+      assertEquals(currentSize + 1, index.getMaxElements());
     }
   }
 }

java/src/test/java/com/spotify/voyager/jni/StringIndexTest.java

@@ -21,6 +21,7 @@
 package com.spotify.voyager.jni;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.Assert.assertEquals;
 
 import com.google.common.io.Resources;
 import com.spotify.voyager.jni.Index.SpaceType;
@@ -239,6 +240,27 @@ public void itLoadsFromInputStreamNoParams() throws Exception {
     }
   }
 
+  @Test
+  public void itResizesIndex() throws Exception {
+    List<Vector> testVectors = TestUtils.getTestVectors();
+    try (final StringIndex index =
+        new StringIndex(
+            SpaceType.Cosine,
+            testVectors.get(0).vector.length,
+            20,
+            testVectors.size(),
+            0,
+            testVectors.size(),
+            StorageDataType.E4M3)) {
+      for (Vector v : testVectors) {
+        index.addItem(v.name, v.vector);
+      }
+      long currentSize = index.getMaxElements();
+      index.resizeIndex(currentSize + 1);
+      assertEquals(currentSize + 1, index.getMaxElements());
+    }
+  }
+
   public static class CustomResult {
     private final String name;
     private final float distance;