8372170: Implement Enhanced Local Variable Declarations (Preview)

src/java.base/share/classes/jdk/internal/javac/PreviewFeature.java

@@ -72,6 +72,8 @@ public enum Feature {
             status="Second Preview")
         PEM_API,
         LANGUAGE_MODEL,
+        @JEP(number=999, title="Enhanced Local Variable Declarations", status="First Preview")
+        ENHANCED_LOCAL_VARIABLE_DECLARATIONS(),
         /**
          * A key for testing.
          */

src/jdk.compiler/share/classes/com/sun/source/tree/EnhancedForLoopTree.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2005, 2014, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2005, 2026, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -25,6 +25,8 @@
 
 package com.sun.source.tree;
 
+import jdk.internal.javac.PreviewFeature;
+
 /**
  * A tree node for an "enhanced" {@code for} loop statement.
  *
@@ -41,12 +43,37 @@
  * @since 1.6
  */
 public interface EnhancedForLoopTree extends StatementTree {
+    /**
+     * "Enhanced" {@code for} declarations come in two forms:
+     * <ul>
+     * <li> local variable declarations and
+     * <li> record patterns
+     * </ul>
+     *
+     * @since 27
+     */
+    @PreviewFeature(feature=PreviewFeature.Feature.ENHANCED_LOCAL_VARIABLE_DECLARATIONS, reflective=true)
+    public enum DeclarationKind {
+        /** enum constant for local variable declarations */
+        VARIABLE,
+        /** enum constant for record pattern declarations */
+        PATTERN
+    }
+
     /**
      * Returns the control variable for the loop.
-     * @return the control variable
+     * @return the control variable, or {@code null} if this "enhanced" {@code for} uses a pattern
      */
     VariableTree getVariable();
 
+    /**
+     * Returns the record pattern for the loop.
+     * @return the record pattern, or {@code null} if this "enhanced" {@code for} uses a local variable declaration
+     * @since 27
+     */
+    @PreviewFeature(feature=PreviewFeature.Feature.ENHANCED_LOCAL_VARIABLE_DECLARATIONS, reflective=true)
+    PatternTree getRecordPattern();
+
     /**
      * Returns the expression yielding the values for the control variable.
      * @return the expression
@@ -58,4 +85,12 @@ public interface EnhancedForLoopTree extends StatementTree {
      * @return the body of the loop
      */
     StatementTree getStatement();
+
+    /**
+     * Returns the kind of the declaration of the "enhanced" {@code for}.
+     * @return the kind of the declaration
+     * @since 27
+     */
+    @PreviewFeature(feature=PreviewFeature.Feature.ENHANCED_LOCAL_VARIABLE_DECLARATIONS, reflective=true)
+    DeclarationKind getDeclarationKind();
 }

src/jdk.compiler/share/classes/com/sun/source/tree/EnhancedVariableDeclTree.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright (c) 2026, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package com.sun.source.tree;
+
+import jdk.internal.javac.PreviewFeature;
+
+/**
+ * A tree node for an Enhanced Local Variable Declaration Statement.
+ *
+ * For example:
+ * <pre>
+ *   <em>record pattern</em> = <em>expression</em>;
+ * </pre>
+ *
+ * @jls 14.23.1 Enhanced Local Variable Declaration Statements
+ *
+ * @author Angelos Bimpoudis
+ * @since 27
+ */
+@PreviewFeature(feature=PreviewFeature.Feature.ENHANCED_LOCAL_VARIABLE_DECLARATIONS, reflective=true)
+public interface EnhancedVariableDeclTree extends StatementTree {
+    /**
+     * Returns the pattern for the enhanced local variable declaration statement.
+     * @return pattern
+     */
+    Tree getPattern();
+
+    /**
+     * Returns the expression to be matched.
+     * @return the expression
+     */
+    ExpressionTree getExpression();
+}

src/jdk.compiler/share/classes/com/sun/source/tree/Tree.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2005, 2024, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2005, 2026, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -25,6 +25,8 @@
 
 package com.sun.source.tree;
 
+import jdk.internal.javac.PreviewFeature;
+
 /**
  * Common interface for all nodes in an abstract syntax tree.
  *
@@ -78,6 +80,13 @@ public enum Kind {
          */
         ASSERT(AssertTree.class),
 
+        /**
+         * Used for instances of {@link EnhancedVariableDeclTree}.
+         * @since 27
+         */
+        @PreviewFeature(feature=PreviewFeature.Feature.ENHANCED_LOCAL_VARIABLE_DECLARATIONS, reflective=true)
+        ENHANCED_VARIABLE_DECL(EnhancedVariableDeclTree.class),
+
         /**
          * Used for instances of {@link AssignmentTree}.
          */

src/jdk.compiler/share/classes/com/sun/source/tree/TreeVisitor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2005, 2024, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2005, 2026, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -25,6 +25,8 @@
 
 package com.sun.source.tree;
 
+import jdk.internal.javac.PreviewFeature;
+
 /**
  * A visitor of trees, in the style of the visitor design pattern.
  * Classes implementing this interface are used to operate
@@ -89,6 +91,17 @@ public interface TreeVisitor<R,P> {
      */
     R visitAssert(AssertTree node, P p);
 
+    /**
+     * Visits an {@code EnhancedVariableDeclTree} node.
+     * @param node the node being visited
+     * @param p a parameter value
+     * @return a result value
+     *
+     * @since 27
+     */
+    @PreviewFeature(feature=PreviewFeature.Feature.ENHANCED_LOCAL_VARIABLE_DECLARATIONS, reflective=true)
+    R visitEnhancedVariableDecl(EnhancedVariableDeclTree node, P p);
+
     /**
      * Visits an {@code AssignmentTree} node.
      * @param node the node being visited

src/jdk.compiler/share/classes/com/sun/source/util/SimpleTreeVisitor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2005, 2024, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2005, 2026, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -26,6 +26,7 @@
 package com.sun.source.util;
 
 import com.sun.source.tree.*;
+import jdk.internal.javac.PreviewFeature;
 
 /**
  * A simple visitor for tree nodes.
@@ -475,6 +476,21 @@ public R visitAssert(AssertTree node, P p) {
         return defaultAction(node, p);
     }
 
+    /**
+     * {@inheritDoc}
+     *
+     * @implSpec This implementation calls {@code defaultAction}.
+     *
+     * @param node {@inheritDoc}
+     * @param p {@inheritDoc}
+     * @return  the result of {@code defaultAction}
+     */
+    @Override
+    @PreviewFeature(feature=PreviewFeature.Feature.ENHANCED_LOCAL_VARIABLE_DECLARATIONS, reflective=true)
+    public R visitEnhancedVariableDecl(EnhancedVariableDeclTree node, P p) {
+        return defaultAction(node, p);
+    }
+
     /**
      * {@inheritDoc}
      *

src/jdk.compiler/share/classes/com/sun/source/util/TreeScanner.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2005, 2024, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2005, 2026, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -26,6 +26,7 @@
 package com.sun.source.util;
 
 import com.sun.source.tree.*;
+import jdk.internal.javac.PreviewFeature;
 
 /**
  * A TreeVisitor that visits all the child tree nodes.
@@ -333,11 +334,29 @@ public R visitForLoop(ForLoopTree node, P p) {
     @Override
     public R visitEnhancedForLoop(EnhancedForLoopTree node, P p) {
         R r = scan(node.getVariable(), p);
+        r = scanAndReduce(node.getRecordPattern(), p, r);
         r = scanAndReduce(node.getExpression(), p, r);
         r = scanAndReduce(node.getStatement(), p, r);
         return r;
     }
 
+    /**
+     * {@inheritDoc}
+     *
+     * @implSpec This implementation scans the children in left to right order.
+     *
+     * @param node  {@inheritDoc}
+     * @param p  {@inheritDoc}
+     * @return the result of scanning
+     */
+    @Override
+    @PreviewFeature(feature=PreviewFeature.Feature.ENHANCED_LOCAL_VARIABLE_DECLARATIONS, reflective=true)
+    public R visitEnhancedVariableDecl(EnhancedVariableDeclTree node, P p) {
+        R r = scan(node.getPattern(), p);
+        r = scanAndReduce(node.getExpression(), p, r);
+        return r;
+    }
+
     /**
      * {@inheritDoc}
      *

src/jdk.compiler/share/classes/com/sun/tools/javac/code/Preview.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018, 2025, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2018, 2026, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -31,14 +31,11 @@
 import com.sun.tools.javac.jvm.Target;
 import com.sun.tools.javac.resources.CompilerProperties.Errors;
 import com.sun.tools.javac.resources.CompilerProperties.LintWarnings;
-import com.sun.tools.javac.resources.CompilerProperties.Warnings;
 import com.sun.tools.javac.util.Assert;
 import com.sun.tools.javac.util.Context;
 import com.sun.tools.javac.util.JCDiagnostic.DiagnosticPosition;
 import com.sun.tools.javac.util.JCDiagnostic.Error;
-import com.sun.tools.javac.util.JCDiagnostic.LintWarning;
 import com.sun.tools.javac.util.JCDiagnostic.SimpleDiagnosticPosition;
-import com.sun.tools.javac.util.JCDiagnostic.Warning;
 import com.sun.tools.javac.util.Log;
 import com.sun.tools.javac.util.Names;
 import com.sun.tools.javac.util.Options;
@@ -50,7 +47,6 @@
 import java.util.Set;
 
 import static com.sun.tools.javac.main.Option.PREVIEW;
-import com.sun.tools.javac.util.JCDiagnostic;
 
 /**
  * Helper class to handle preview language features. This class maps certain language features
@@ -212,6 +208,7 @@ public boolean isEnabled() {
     public boolean isPreview(Feature feature) {
         return switch (feature) {
             case PRIMITIVE_PATTERNS -> true;
+            case ENHANCED_VARIABLE_DECLS -> true;
             //Note: this is a backdoor which allows to optionally treat all features as 'preview' (for testing).
             //When real preview features will be added, this method can be implemented to return 'true'
             //for those selected features, and 'false' for all the others.

src/jdk.compiler/share/classes/com/sun/tools/javac/code/Source.java

@@ -285,6 +285,7 @@ public enum Feature {
         PRIVATE_MEMBERS_IN_PERMITS_CLAUSE(JDK19),
         ERASE_POLY_SIG_RETURN_TYPE(JDK24),
         CAPTURE_MREF_RETURN_TYPE(JDK26),
+        ENHANCED_VARIABLE_DECLS(JDK26, Fragments.FeatureEnhancedVariableDecls, DiagKind.PLURAL),
         ;
 
         enum DiagKind {

src/jdk.compiler/share/classes/com/sun/tools/javac/code/Symtab.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1999, 2025, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1999, 2026, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -191,6 +191,7 @@ public static Symtab instance(Context context) {
     public final Type incompatibleClassChangeErrorType;
     public final Type cloneNotSupportedExceptionType;
     public final Type matchExceptionType;
+    public final Type nullPointerExceptionType;
     public final Type annotationType;
     public final TypeSymbol enumSym;
     public final Type listType;
@@ -570,6 +571,7 @@ public <R, P> R accept(ElementVisitor<R, P> v, P p) {
         incompatibleClassChangeErrorType = enterClass("java.lang.IncompatibleClassChangeError");
         cloneNotSupportedExceptionType = enterClass("java.lang.CloneNotSupportedException");
         matchExceptionType = enterClass("java.lang.MatchException");
+        nullPointerExceptionType = enterClass("java.lang.NullPointerException");
         annotationType = enterClass("java.lang.annotation.Annotation");
         classLoaderType = enterClass("java.lang.ClassLoader");
         enumSym = enterClass(java_base, names.java_lang_Enum);