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"nb-javac" is a patched version of OpenJDK "javac", i.e., the Java compiler. This has long been part of NetBeans, providing a highly tuned Java compiler specifically for the Java editor i.e., parsing and lexing for features such as syntax coloring, code completion.
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 * 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).
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package javax.lang.model.util;

import java.util.List;
import java.util.ArrayList;
import javax.lang.model.element.*;
import javax.annotation.processing.SupportedSourceVersion;
import javax.lang.model.SourceVersion;
import static javax.lang.model.SourceVersion.*;

/**
 * A scanning visitor of program elements with default behavior
 * appropriate for the {@link SourceVersion#RELEASE_14 RELEASE_14}
 * source version.
 *
 * The visitXyz methods in this
 * class scan their component elements by calling {@code scan} on
 * their {@linkplain Element#getEnclosedElements enclosed elements},
 * {@linkplain ExecutableElement#getParameters parameters}, etc., as
 * indicated in the individual method specifications.  A subclass can
 * control the order elements are visited by overriding the
 * visitXyz methods.  Note that clients of a scanner
 * may get the desired behavior be invoking {@code v.scan(e, p)} rather
 * than {@code v.visit(e, p)} on the root objects of interest.
 *
 * When a subclass overrides a visitXyz method, the
 * new method can cause the enclosed elements to be scanned in the
 * default way by calling super.visitXyz.  In this
 * fashion, the concrete visitor can control the ordering of traversal
 * over the component elements with respect to the additional
 * processing; for example, consistently calling
 * super.visitXyz at the start of the overridden
 * methods will yield a preorder traversal, etc.  If the component
 * elements should be traversed in some other order, instead of
 * calling super.visitXyz, an overriding visit method
 * should call {@code scan} with the elements in the desired order.
 *
 * @apiNote
 * Methods in this class may be overridden subject to their general
 * contract.
 *
 * @param  the return type of this visitor's methods.  Use {@link
 *            Void} for visitors that do not need to return results.
 * @param  the type of the additional parameter to this visitor's
 *            methods.  Use {@code Void} for visitors that do not need an
 *            additional parameter.
 *
 * @see Compatibility note for subclasses
 * @see ElementScanner6
 * @see ElementScanner7
 * @see ElementScanner8
 * @see ElementScanner9
 * @since 16
 */
@SupportedSourceVersion(RELEASE_17)
public class ElementScanner14 extends ElementScanner9 {
    /**
     * Constructor for concrete subclasses; uses {@code null} for the
     * default value.
     */
    protected ElementScanner14(){
        super(null);
    }

    /**
     * Constructor for concrete subclasses; uses the argument for the
     * default value.
     *
     * @param defaultValue the default value
     */
    protected ElementScanner14(R defaultValue){
        super(defaultValue);
    }

    /**
     * {@inheritDoc}
     *
     * @implSpec This implementation scans the type parameters, if
     * any, and then the enclosed elements.
     *
     *
     * @param e  {@inheritDoc}
     * @param p  {@inheritDoc}
     * @return the result of scanning
     */
    @Override
    public R visitType(TypeElement e, P p) {
        return scan(createScanningList(e, e.getEnclosedElements()), p);
    }

    /**
     * {@inheritDoc}
     *
     * @implSpec This implementation first scans the type parameters, if any, and then
     * the parameters.
     *
     * @param e  {@inheritDoc}
     * @param p  {@inheritDoc}
     * @return the result of scanning
     */
    public R visitExecutable(ExecutableElement e, P p) {
        return scan(createScanningList(e, e.getParameters()), p);
    }

    private List createScanningList(Parameterizable element,
                                                       List toBeScanned) {
        List typeParameters = element.getTypeParameters();
        if (typeParameters.isEmpty()) {
            return toBeScanned;
        } else {
            List scanningList = new ArrayList<>(typeParameters);
            scanningList.addAll(toBeScanned);
            return scanningList;
        }
    }

    /**
     * {@inheritDoc}
     *
     * @implSpec This implementation scans the enclosed elements.
     *
     * @param e the element to visit
     * @param p a visitor-specified parameter
     * @return  the result of the scan
     */
    @Override
    public R visitRecordComponent(RecordComponentElement e, P p) {
        return scan(e.getEnclosedElements(), p);
    }
}