Idiomatic AssertJ assertions for empty Strings (#525)

* Idiomatic AssertJ assertions for empty Strings

Fixes #513

* Add recipe to AssertJ best practices

* Resolve automated findings

* Reuse the method matcher for precondition too

src/main/java/org/openrewrite/java/testing/assertj/IsEqualToEmptyString.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2024 the original author or authors.
+ * <p>
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * <p>
+ * https://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.openrewrite.java.testing.assertj;
+
+import org.openrewrite.ExecutionContext;
+import org.openrewrite.Preconditions;
+import org.openrewrite.Recipe;
+import org.openrewrite.TreeVisitor;
+import org.openrewrite.java.JavaIsoVisitor;
+import org.openrewrite.java.MethodMatcher;
+import org.openrewrite.java.search.UsesMethod;
+import org.openrewrite.java.tree.J;
+import org.openrewrite.java.tree.JavaType;
+
+import java.util.Collections;
+
+public class IsEqualToEmptyString extends Recipe {
+
+    private static final MethodMatcher IS_EQUAL_TO = new MethodMatcher("org.assertj.core.api.AbstractStringAssert isEqualTo(java.lang.String)");
+
+    @Override
+    public String getDisplayName() {
+        return "Convert `assertThat(String).isEqualTo(\"\")` to `isEmpty()`";
+    }
+
+    @Override
+    public String getDescription() {
+        return "Adopt idiomatic AssertJ assertion for empty Strings.";
+    }
+
+    @Override
+    public TreeVisitor<?, ExecutionContext> getVisitor() {
+        return Preconditions.check(
+                new UsesMethod<>(IS_EQUAL_TO),
+                new JavaIsoVisitor<ExecutionContext>() {
+                    @Override
+                    public J.MethodInvocation visitMethodInvocation(J.MethodInvocation method, ExecutionContext ctx) {
+                        J.MethodInvocation mi = super.visitMethodInvocation(method, ctx);
+                        if (IS_EQUAL_TO.matches(mi) && J.Literal.isLiteralValue(mi.getArguments().get(0), "")) {
+                            JavaType.Method isEmptyMethodType = mi.getMethodType().withName("isEmpty");
+                            return mi
+                                    .withName(mi.getName().withSimpleName("isEmpty").withType(isEmptyMethodType))
+                                    .withMethodType(isEmptyMethodType)
+                                    .withArguments(Collections.emptyList());
+                        }
+                        return mi;
+                    }
+                }
+        );
+    }
+}

src/main/resources/META-INF/rewrite/assertj.yml

@@ -26,6 +26,7 @@ recipeList:
   - org.openrewrite.java.testing.assertj.JUnitToAssertj
   - org.openrewrite.java.testing.assertj.StaticImports
   - org.openrewrite.java.testing.assertj.SimplifyChainedAssertJAssertions
+  - org.openrewrite.java.testing.assertj.IsEqualToEmptyString
 
 ---
 type: specs.openrewrite.org/v1beta/recipe

src/test/java/org/openrewrite/java/testing/assertj/IsEqualToEmptyStringTest.java

@@ -0,0 +1,61 @@
+/*
+ * Copyright 2024 the original author or authors.
+ * <p>
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * <p>
+ * https://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.openrewrite.java.testing.assertj;
+
+import org.junit.jupiter.api.Test;
+import org.openrewrite.DocumentExample;
+import org.openrewrite.InMemoryExecutionContext;
+import org.openrewrite.java.JavaParser;
+import org.openrewrite.test.RecipeSpec;
+import org.openrewrite.test.RewriteTest;
+
+import static org.openrewrite.java.Assertions.java;
+
+class IsEqualToEmptyStringTest implements RewriteTest {
+
+    @Override
+    public void defaults(RecipeSpec spec) {
+        spec
+          .parser(JavaParser.fromJavaVersion().classpathFromResources(new InMemoryExecutionContext(), "assertj-core-3.24"))
+          .recipe(new IsEqualToEmptyString());
+    }
+
+    @DocumentExample
+    @Test
+    void convertsIsEqualToEmptyString() {
+        rewriteRun(
+          //language=java
+          java(
+            """
+              import static org.assertj.core.api.Assertions.assertThat;
+              class Test {
+                  void test() {
+                      assertThat("test").isEqualTo("");
+                  }
+              }
+              """,
+            """
+              import static org.assertj.core.api.Assertions.assertThat;
+              class Test {
+                  void test() {
+                      assertThat("test").isEmpty();
+                  }
+              }
+              """
+          )
+        );
+    }
+}