Fixes AbstractMethodError in field's class's equals

CHANGELOG.md

@@ -13,6 +13,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- AbstractMethodError when a the `equals` method in a field's class calls an abstract method. ([Issue 938](https://github.com/jqno/equalsverifier/issues/938))
+
 ## [3.16] - 2024-03-22
 
 ### Added

equalsverifier-core/src/main/java/nl/jqno/equalsverifier/internal/prefabvalues/PrefabValues.java

@@ -117,6 +117,7 @@ public <T> T giveOther(TypeTag tag, T value) {
      * @param typeStack Keeps track of recursion in the type.
      * @return A value that is different from {@code value}.
      */
+    // CHECKSTYLE OFF: CyclomaticComplexity
     public <T> T giveOther(TypeTag tag, T value, LinkedHashSet<TypeTag> typeStack) {
         Class<T> type = tag.getType();
         if (
@@ -134,12 +135,21 @@ public <T> T giveOther(TypeTag tag, T value, LinkedHashSet<TypeTag> typeStack) {
         if (type.isArray() && arraysAreDeeplyEqual(tuple.getRed(), value)) {
             return tuple.getBlue();
         }
-        if (!type.isArray() && value != null && tuple.getRed().equals(value)) {
-            return tuple.getBlue();
+        if (!type.isArray() && value != null) {
+            try {
+                // red's equals can potentially call an abstract method
+                if (tuple.getRed().equals(value)) {
+                    return tuple.getBlue();
+                }
+            } catch (AbstractMethodError e) {
+                return tuple.getRed();
+            }
         }
         return tuple.getRed();
     }
 
+    // CHECKSTYLE ON: CyclomaticComplexity
+
     private boolean wraps(Class<?> expectedClass, Class<?> actualClass) {
         return PrimitiveMappers.PRIMITIVE_OBJECT_MAPPER.get(expectedClass) == actualClass;
     }

equalsverifier-core/src/test/java/nl/jqno/equalsverifier/integration/extended_contract/AbstractDelegationTest.java

@@ -205,6 +205,36 @@ public void originalMessageIsIncludedInErrorMessage_whenEqualsVerifierSignalsAnA
             .assertMessageContains("This is AbstractMethodError's original message");
     }
 
+    @Test
+    public void failGracefully_whenAFieldsEqualsMethodDoesntDoAnIdentityCheckButCallsAnAbstractField() {
+        ExpectedException
+            .when(() ->
+                EqualsVerifier
+                    .forClass(EqualsInFieldWithoutIdentityCheckDelegatesToAbstractMethod.class)
+                    .verify()
+            )
+            .assertFailure()
+            .assertCause(AbstractMethodError.class)
+            .assertMessageContains(
+                ABSTRACT_DELEGATION,
+                EQUALS_DELEGATES,
+                PREFAB,
+                AbstractEqualsWithoutIdentityCheckDelegator.class.getSimpleName()
+            );
+    }
+
+    @Test
+    public void succeed_whenAFieldsEqualsMethodDoesntDoAnIdentityCheckButCallsAnAbstractField_givenAConcretePrefabImplementationOfSaidField() {
+        EqualsVerifier
+            .forClass(EqualsInFieldWithoutIdentityCheckDelegatesToAbstractMethod.class)
+            .withPrefabValues(
+                AbstractEqualsWithoutIdentityCheckDelegator.class,
+                new AbstractEqualsWithoutIdentityCheckDelegatorImpl(1),
+                new AbstractEqualsWithoutIdentityCheckDelegatorImpl(2)
+            )
+            .verify();
+    }
+
     private abstract static class AbstractClass {
 
         private int i;
@@ -291,6 +321,48 @@ public boolean theAnswer() {
         }
     }
 
+    abstract static class AbstractEqualsWithoutIdentityCheckDelegator {
+
+        private final int i;
+
+        public AbstractEqualsWithoutIdentityCheckDelegator(int i) {
+            this.i = i;
+        }
+
+        abstract boolean theAnswer();
+
+        @Override
+        public boolean equals(Object obj) {
+            if (!(obj instanceof AbstractEqualsWithoutIdentityCheckDelegator)) {
+                return false;
+            }
+            if (theAnswer()) {
+                return true;
+            }
+            AbstractEqualsWithoutIdentityCheckDelegator other =
+                (AbstractEqualsWithoutIdentityCheckDelegator) obj;
+            return i == other.i;
+        }
+
+        @Override
+        public int hashCode() {
+            return defaultHashCode(this);
+        }
+    }
+
+    static final class AbstractEqualsWithoutIdentityCheckDelegatorImpl
+        extends AbstractEqualsWithoutIdentityCheckDelegator {
+
+        public AbstractEqualsWithoutIdentityCheckDelegatorImpl(int i) {
+            super(i);
+        }
+
+        @Override
+        public boolean theAnswer() {
+            return false;
+        }
+    }
+
     abstract static class AbstractHashCodeDelegator {
 
         private final int i;
@@ -612,4 +684,30 @@ public int hashCode() {
             return defaultHashCode(this);
         }
     }
+
+    static class EqualsInFieldWithoutIdentityCheckDelegatesToAbstractMethod {
+
+        private final AbstractEqualsWithoutIdentityCheckDelegator id;
+
+        protected EqualsInFieldWithoutIdentityCheckDelegatesToAbstractMethod(
+            AbstractEqualsWithoutIdentityCheckDelegator id
+        ) {
+            this.id = id;
+        }
+
+        @Override
+        public final boolean equals(Object other) {
+            if (!(other instanceof EqualsInFieldWithoutIdentityCheckDelegatesToAbstractMethod)) {
+                return false;
+            }
+            EqualsInFieldWithoutIdentityCheckDelegatesToAbstractMethod that =
+                (EqualsInFieldWithoutIdentityCheckDelegatesToAbstractMethod) other;
+            return Objects.equals(id, that.id);
+        }
+
+        @Override
+        public final int hashCode() {
+            return Objects.hash(id);
+        }
+    }
 }