Fix for multithreaded execution

core/src/main/java/com/github/gumtreediff/tree/TypeSet.java

@@ -35,7 +35,7 @@ private TypeSet() {}
      * Build a type with the provided name. If the provided name is null or
      * the empty string, the empty type will be returned.
      */
-    public static Type type(String value) {
+    public static synchronized Type type(String value) {
         return implementation.makeOrGetType(value);
     }
 

core/src/test/java/com/github/gumtreediff/test/TestTree.java

@@ -27,6 +27,9 @@
 import com.github.gumtreediff.tree.*;
 import org.junit.jupiter.api.Test;
 
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
 import static org.junit.jupiter.api.Assertions.*;
 
 public class TestTree {
@@ -285,4 +288,25 @@ public void testToString() {
         t3.setLength(2);
         assertEquals("foo: hello [1,3]", t3.toString());
     }
+
+    @Test
+    public void testTypeThreading() throws InterruptedException {
+        int n = 20;
+        ExecutorService exec = Executors.newFixedThreadPool(n);
+        List<Type> types = new ArrayList<>();
+
+        for (int i = 0; i < n; i++) {
+            exec.submit(() -> {
+                types.add(TypeSet.type("foo"));
+            });
+        }
+
+        exec.awaitTermination(1, java.util.concurrent.TimeUnit.SECONDS);
+
+        for (Type t1 : types) {
+            for (Type t2: types) {
+                assertSame(t1, t2);
+            }
+        }
+    }
 }