#150: Implement class inheritance pattern

PATTERNS.md

@@ -310,8 +310,6 @@ class Foo {
 }
 ```
 
-***
-
 *Title*: Non final attributes
 
 *Code*: **P12**
@@ -628,6 +626,21 @@ class Foo {
 }
 ```
 
+***
+
+*Title*: Class inheritance
+
+*Code*: **P34**
+
+*Description*: Once a class extends another class via `extends`, it's a pattern.
+
+*Example*:
+
+```java
+class MyList extends AbstractList { // here
+}
+```
+
 ***
 *Title*: Assign null
 

aibolit/config.py

@@ -28,6 +28,7 @@
 from aibolit.patterns.assert_in_code.assert_in_code import AssertInCode as P1
 from aibolit.patterns.assign_null_finder.assign_null_finder import NullAssignment as P28
 from aibolit.patterns.classic_setter.classic_setter import ClassicSetter as P2
+from aibolit.patterns.class_inheritance.class_inheritance import ClassInheritance as P34
 from aibolit.patterns.empty_rethrow.empty_rethrow import EmptyRethrow as P3
 from aibolit.patterns.er_class.er_class import ErClass as P4
 from aibolit.patterns.force_type_casting_finder.force_type_casting_finder import \
@@ -198,6 +199,7 @@ def get_patterns_config() -> PatternsConfig:
                                      ASTNodeType.FOR_STATEMENT,
                                      ASTNodeType.WHILE_STATEMENT)},
                 {'name': 'Incomplete For', 'code': 'P33', 'make': P33},
+                {'name': 'Class inheritance', 'code': 'P34', 'make': P34},
             ],
             'metrics': [
                 {'name': 'Entropy', 'code': 'M1', 'make': M1},  # type: ignore

aibolit/patterns/class_inheritance/__init__.py

@@ -0,0 +1,2 @@
+# SPDX-FileCopyrightText: Copyright (c) 2019-2026 Aibolit
+# SPDX-License-Identifier: MIT

aibolit/patterns/class_inheritance/class_inheritance.py

@@ -0,0 +1,19 @@
+# SPDX-FileCopyrightText: Copyright (c) 2019-2026 Aibolit
+# SPDX-License-Identifier: MIT
+
+from typing import List
+
+from aibolit.ast_framework import AST, ASTNodeType
+
+
+class ClassInheritance:
+    """
+    Find classes that use implementation inheritance via the `extends` keyword.
+    """
+
+    def value(self, ast: AST) -> List[int]:
+        lines: List[int] = []
+        for class_declaration in ast.proxy_nodes(ASTNodeType.CLASS_DECLARATION):
+            if class_declaration.extends is not None:
+                lines.append(class_declaration.line)
+        return lines

test/patterns/class_inheritance/NestedInheritance.java

@@ -0,0 +1,12 @@
+// SPDX-FileCopyrightText: Copyright (c) 2019-2026 Aibolit
+// SPDX-License-Identifier: MIT
+
+package patterns.class_inheritance;
+
+class Outer {
+    static class Base {
+    }
+
+    class Inner extends Base {
+    }
+}

test/patterns/class_inheritance/OnlyImplements.java

@@ -0,0 +1,8 @@
+// SPDX-FileCopyrightText: Copyright (c) 2019-2026 Aibolit
+// SPDX-License-Identifier: MIT
+
+package patterns.class_inheritance;
+
+interface Base {}
+class MyList implements Base {
+}

test/patterns/class_inheritance/SimpleInheritance.java

@@ -0,0 +1,8 @@
+// SPDX-FileCopyrightText: Copyright (c) 2019-2026 Aibolit
+// SPDX-License-Identifier: MIT
+
+package patterns.class_inheritance;
+
+class Base {}
+class MyList extends Base {
+}

test/patterns/class_inheritance/TypeParameterExtends.java

@@ -0,0 +1,7 @@
+// SPDX-FileCopyrightText: Copyright (c) 2019-2026 Aibolit
+// SPDX-License-Identifier: MIT
+
+package patterns.class_inheritance;
+
+class GenericBox<T extends Number> {
+}

test/patterns/class_inheritance/test_class_inheritance.py

@@ -0,0 +1,41 @@
+# SPDX-FileCopyrightText: Copyright (c) 2019-2026 Aibolit
+# SPDX-License-Identifier: MIT
+
+from pathlib import Path
+from unittest import TestCase
+
+from aibolit.ast_framework import AST
+from aibolit.patterns.class_inheritance.class_inheritance import ClassInheritance
+from aibolit.utils.ast_builder import build_ast
+
+
+class ClassInheritanceTestCase(TestCase):
+    current_directory = Path(__file__).absolute().parent
+
+    def test_find_class_inheritance(self):
+        filepath = self.current_directory / 'SimpleInheritance.java'
+        ast = AST.build_from_javalang(build_ast(filepath))
+        pattern = ClassInheritance()
+        lines = pattern.value(ast)
+        self.assertEqual(lines, [7])
+
+    def test_ignore_implements_only(self):
+        filepath = self.current_directory / 'OnlyImplements.java'
+        ast = AST.build_from_javalang(build_ast(filepath))
+        pattern = ClassInheritance()
+        lines = pattern.value(ast)
+        self.assertEqual(lines, [])
+
+    def test_ignore_type_parameter_extends(self):
+        filepath = self.current_directory / 'TypeParameterExtends.java'
+        ast = AST.build_from_javalang(build_ast(filepath))
+        pattern = ClassInheritance()
+        lines = pattern.value(ast)
+        self.assertEqual(lines, [])
+
+    def test_find_nested_class_inheritance(self):
+        filepath = self.current_directory / 'NestedInheritance.java'
+        ast = AST.build_from_javalang(build_ast(filepath))
+        pattern = ClassInheritance()
+        lines = pattern.value(ast)
+        self.assertEqual(lines, [10])