Java 10+ Local-Variable Type Inference

src/main/java/org/openrewrite/java/migrate/lang/VarKeyword.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2021 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.migrate.lang;
+
+import lombok.EqualsAndHashCode;
+import lombok.Value;
+import org.openrewrite.ExecutionContext;
+import org.openrewrite.Recipe;
+import org.openrewrite.TreeVisitor;
+import org.openrewrite.internal.lang.Nullable;
+import org.openrewrite.java.JavaVisitor;
+import org.openrewrite.java.search.HasJavaVersion;
+
+import java.time.Duration;
+
+@Value
+@EqualsAndHashCode(callSuper = false)
+public class VarKeyword extends Recipe {
+    public String getDisplayName() {
+        return "Use local variable type-inference (var) where possible";
+    }
+
+    @Override
+    public String getDescription() {
+        return "Local variable type-inference reduce the noise produces by repeating the type definitions in Java 10 or higher.";
+    }
+
+    @Override
+    public @Nullable Duration getEstimatedEffortPerOccurrence() {
+        return Duration.ofMinutes(1);
+    }
+
+    @Override
+    protected TreeVisitor<?, ExecutionContext> getSingleSourceApplicableTest() {
+        return new HasJavaVersion("11", true).getVisitor();
+    }
+
+    @Override
+    protected TreeVisitor<?, ExecutionContext> getVisitor() {
+        return new JavaVisitor<ExecutionContext>() {
+        };
+    }
+}

src/test/java/org/openrewrite/java/migrate/lang/VarUsageTest.java

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+/*
+ * Copyright 2021 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.migrate.lang;
+
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+import org.openrewrite.test.RecipeSpec;
+import org.openrewrite.test.RewriteTest;
+import org.openrewrite.test.TypeValidation;
+
+import static org.openrewrite.java.Assertions.java;
+import static org.openrewrite.java.Assertions.version;
+
+class VarUsageTest implements RewriteTest {
+    public void defaults(RecipeSpec spec) {
+        spec.recipe(new VarKeyword())
+          .typeValidationOptions(new TypeValidation().methodInvocations(false));
+    }
+
+    @Test
+    void donotTransformNull() {
+        //language=java
+        rewriteRun(
+          version(
+            java("""
+              package com.example.app;
+
+              class A {
+                void m() {
+                  String str = null;
+                }
+              }
+              """),
+            10
+          )
+        );
+    }
+
+    @Nested
+    class TransformObject {
+        @Test
+        void transformObjectInMethodBody() {
+            //language=java
+            rewriteRun(
+              version(
+                java("""
+                  package com.example.app;
+
+                  class A {
+                    void m() {
+                        Object o = new Object();
+                    }
+                  }
+                  """, """
+                  package com.example.app;
+
+                  class A {
+                    void m() {
+                      var o = new Object();
+                    }
+                  }
+                  """),
+                10
+              )
+            );
+        }
+
+        @Test
+        void donotTransformObjectInParameter() {
+            //language=java
+            rewriteRun(
+              version(
+                java("""
+                  package com.example.app;
+
+                  class A {
+                    Object m(Object o) {
+                        return o;
+                    }
+                  }
+                  """),
+                10
+              )
+            );
+        }
+
+        @Test
+        void donotTransformObjectInClass() {
+            //language=java
+            rewriteRun(
+              version(
+                java("""
+                  package com.example.app;
+
+                  class A {
+                    Object o;
+                    Object m() {
+                        return o;
+                    }
+                  }
+                  """),
+                10
+              )
+            );
+        }
+    }
+
+    @Nested
+    class Generics {
+        @Test
+        void transformFullGenerics() {
+            //language=java
+            rewriteRun(
+              version(
+                java("""
+                  package com.example.app;
+
+                  class A {
+                    void m() {
+                        List<String> strs = new ArrayList<String>();
+                    }
+                  }
+                  """, """
+                  package com.example.app;
+
+                  class A {
+                    void m() {
+                      var str = new ArrayList<String>();
+                    }
+                  }
+                  """),
+                10
+              )
+            );
+        }
+
+        @Test
+        void transformGenericsWithDiamond() {
+            //language=java
+            rewriteRun(
+              version(
+                java("""
+                  package com.example.app;
+
+                  class A {
+                    void m() {
+                        List<String> strs = new ArrayList<>();
+                    }
+                  }
+                  """, """
+                  package com.example.app;
+
+                  class A {
+                    void m() {
+                      var str = new ArrayList<String>();
+                    }
+                  }
+                  """),
+                10
+              )
+            );
+        }
+
+        @Test
+        void transformGenericsOnFactory() {
+            //language=java
+            rewriteRun(
+              version(
+                java(
+                  """
+                    package com.example.app;
+
+                    import java.util.ArrayList;class A {
+                      void m() {
+                          List<String> strs = List.of("one","two");
+                      }
+                    }
+                    """, """
+                    package com.example.app;
+
+                    class A {
+                      void m() {
+                        var str = List.of("one","two");
+                      }
+                    }
+                    """),
+                10
+              )
+            );
+        }
+
+        @Test
+        void donotTransformGenericsOnEmptyFactory() {
+            //language=java
+            rewriteRun(
+              version(
+                java(
+                  """
+                      package com.example.app;
+
+                      class A {
+                        void m() {
+                          List<String> strs = List.of();
+                        }
+                      }
+                    """),
+                10
+              )
+            );
+        }
+    }
+}