Handle empty lambda expressions in UnitsCompiler (fix #248)

core/src/org/sbml/jsbml/math/compiler/UnitsCompiler.java

@@ -727,12 +727,23 @@ private <T> ASTNode2Value<?> invalid() {
   @Override
   // TODO: Specify generic type T i.e. ASTNode2Value<?>
   public <T> ASTNode2Value<?> lambda(List<ASTNode2> values) throws SBMLException {
+    // When the lambda node is first constructed, its child list may be empty;
+    // in that case, there is no body and no arguments yet, so we cannot derive
+    // any meaningful units. Returning an invalid unit definition here avoids
+    // out-of-bounds exceptions while signalling "units unknown".
+    if (values == null || values.isEmpty()) {
+      return invalid();
+    }
+
+    // For a fully-formed lambda expression, all but the last child are
+    // arguments; the last child is the body whose units we want to derive.
     for (int i = 0; i < values.size() - 1; i++) {
-      namesToUnits.put(values.get(i).toString(),
-        values.get(i).compile(this));
+      namesToUnits.put(values.get(i).toString(), values.get(i).compile(this));
     }
-    return new ASTNode2Value(values.get(values.size() - 1).compile(this)
-      .getUnits(), this);
+
+    return new ASTNode2Value(
+        values.get(values.size() - 1).compile(this).getUnits(),
+        this);
   }
 
   /* (non-Javadoc)

core/test/org/sbml/jsbml/math/compiler/UnitsCompilerLambdaTest.java

@@ -0,0 +1,37 @@
+package org.sbml.jsbml.math.compiler;
+
+import static org.junit.Assert.*;
+
+import java.util.Collections;
+
+import org.junit.Test;
+import org.sbml.jsbml.math.ASTNode2;
+// no import needed for ASTNode2Value since we are in the same package
+// or, if you prefer, you could import:
+// import org.sbml.jsbml.math.compiler.ASTNode2Value;
+
+/**
+ * Regression tests for lambda handling in {@link UnitsCompiler}.
+ */
+public class UnitsCompilerLambdaTest {
+
+  /**
+   * Issue #248: Deriving units for a lambda expression while its child list
+   * is still empty must not cause an IndexOutOfBoundsException.
+   */
+  @Test
+  public void testLambdaWithNoChildrenDoesNotThrow() throws Exception {
+    UnitsCompiler compiler = new UnitsCompiler(3, 1); // arbitrary level/version
+
+    ASTNode2Value<?> result = null;
+    try {
+      result = compiler.lambda(Collections.<ASTNode2>emptyList());
+    } catch (IndexOutOfBoundsException ex) {
+      fail("lambda() must not throw IndexOutOfBoundsException when values list is empty: " + ex);
+    }
+
+    assertNotNull("lambda() should return a non-null ASTNode2Value", result);
+    assertTrue("lambda() with no children should return invalid units",
+               result.getUnits().isInvalid());
+  }
+}