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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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/*
 * Copyright (c) 1999, 2017, 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
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 */

package com.sun.tools.javac.comp;

import com.sun.tools.javac.tree.*;
import java.util.Iterator;
import java.util.NoSuchElementException;

/** A class for environments, instances of which are passed as
 *  arguments to tree visitors.  Environments refer to important ancestors
 *  of the subtree that's currently visited, such as the enclosing method,
 *  the enclosing class, or the enclosing toplevel node. They also contain
 *  a generic component, represented as a type parameter, to carry further
 *  information specific to individual passes.
 *
 *  This is NOT part of any supported API.
 *  If you write code that depends on this, you do so at your own risk.
 *  This code and its internal interfaces are subject to change or
 *  deletion without notice.
 */
public class Env implements Iterable> {

    /** The next enclosing environment.
     */
    public Env next;

    /** The environment enclosing the current class.
     */
    public Env outer;

    /** The tree with which this environment is associated.
     */
    public JCTree tree;

    /** The enclosing toplevel tree.
     */
    public JCTree.JCCompilationUnit toplevel;

    /** The next enclosing class definition.
     */
    public JCTree.JCClassDecl enclClass;

    /** The next enclosing method definition.
     */
    public JCTree.JCMethodDecl enclMethod;

    /** A generic field for further information.
     */
    public A info;

    /** Is this an environment for evaluating a base clause?
     */
    public boolean baseClause = false;

    /** Create an outermost environment for a given (toplevel)tree,
     *  with a given info field.
     */
    public Env(JCTree tree, A info) {
        this.next = null;
        this.outer = null;
        this.tree = tree;
        this.toplevel = null;
        this.enclClass = null;
        this.enclMethod = null;
        this.info = info;
    }

    /** Duplicate this environment, updating with given tree and info,
     *  and copying all other fields.
     */
    public Env dup(JCTree tree, A info) {
        return dupto(new Env<>(tree, info));
    }

    /** Duplicate this environment into a given Environment,
     *  using its tree and info, and copying all other fields.
     */
    public Env dupto(Env that) {
        that.next = this;
        that.outer = this.outer;
        that.toplevel = this.toplevel;
        that.enclClass = this.enclClass;
        that.enclMethod = this.enclMethod;
        return that;
    }

    /** Duplicate this environment, updating with given tree,
     *  and copying all other fields.
     */
    public Env dup(JCTree tree) {
        return dup(tree, this.info);
    }

    /** Return closest enclosing environment which points to a tree with given tag.
     */
    public Env enclosing(JCTree.Tag tag) {
        Env env1 = this;
        while (env1 != null && !env1.tree.hasTag(tag)) env1 = env1.next;
        return env1;
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append("Env[").append(info);
        if (outer != null)
            sb.append(",outer=").append(outer);
        sb.append("]");
        return sb.toString();
    }

    public Iterator> iterator() {
        return new Iterator>() {
            Env next = Env.this;
            public boolean hasNext() {
                return next.outer != null;
            }
            public Env next() {
                if (hasNext()) {
                    Env current = next;
                    next = current.outer;
                    return current;
                }
                throw new NoSuchElementException();
            }
            public void remove() {
                throw new UnsupportedOperationException();
            }
        };
    }
}