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With inspiration from other libraries
There is a newer version: 2.12.15
/*
 * Copyright (c) 2012-2017 The ANTLR Project. All rights reserved.
 * Use of this file is governed by the BSD 3-clause license that
 * can be found in the LICENSE.txt file in the project root.
 */

package org.antlr.v4.codegen.target;

import org.antlr.v4.Tool;
import org.antlr.v4.codegen.CodeGenerator;
import org.antlr.v4.codegen.Target;
import org.antlr.v4.codegen.UnicodeEscapes;
import org.antlr.v4.tool.ast.GrammarAST;
import org.stringtemplate.v4.STGroup;
import org.stringtemplate.v4.StringRenderer;

import java.util.Arrays;
import java.util.HashSet;
import java.util.Locale;
import java.util.Set;

public class JavaTarget extends Target {

	/**
	 * The Java target can cache the code generation templates.
	 */
	private static final ThreadLocal targetTemplates = new ThreadLocal();

	protected static final String[] javaKeywords = {
		"abstract", "assert", "boolean", "break", "byte", "case", "catch",
		"char", "class", "const", "continue", "default", "do", "double", "else",
		"enum", "extends", "false", "final", "finally", "float", "for", "goto",
		"if", "implements", "import", "instanceof", "int", "interface",
		"long", "native", "new", "null", "package", "private", "protected",
		"public", "return", "short", "static", "strictfp", "super", "switch",
		"synchronized", "this", "throw", "throws", "transient", "true", "try",
		"void", "volatile", "while"
	};

	/** Avoid grammar symbols in this set to prevent conflicts in gen'd code. */
	protected final Set badWords = new HashSet();

	public JavaTarget(CodeGenerator gen) {
		super(gen, "Java");
	}

    @Override
    public String getVersion() {
        return Tool.VERSION; // Java and tool versions move in lock step
    }

    public Set getBadWords() {
		if (badWords.isEmpty()) {
			addBadWords();
		}

		return badWords;
	}

	protected void addBadWords() {
		badWords.addAll(Arrays.asList(javaKeywords));
		badWords.add("rule");
		badWords.add("parserRule");
	}

	@Override
	public int getSerializedATNSegmentLimit() {
		// 65535 is the class file format byte limit for a UTF-8 encoded string literal
		// 3 is the maximum number of bytes it takes to encode a value in the range 0-0xFFFF
		return 65535 / 3;
	}

	@Override
	protected boolean visibleGrammarSymbolCausesIssueInGeneratedCode(GrammarAST idNode) {
		return getBadWords().contains(idNode.getText());
	}

	@Override
	protected STGroup loadTemplates() {
		STGroup result = targetTemplates.get();
		if (result == null) {
			result = super.loadTemplates();
			result.registerRenderer(String.class, new JavaStringRenderer(), true);
			targetTemplates.set(result);
		}

		return result;
	}

	protected static class JavaStringRenderer extends StringRenderer {

		@Override
		public String toString(Object o, String formatString, Locale locale) {
			if ("java-escape".equals(formatString)) {
				// 5C is the hex code for the \ itself
				return ((String)o).replace("\\u", "\\u005Cu");
			}

			return super.toString(o, formatString, locale);
		}

	}

	@Override
	protected void appendUnicodeEscapedCodePoint(int codePoint, StringBuilder sb) {
		UnicodeEscapes.appendJavaStyleEscapedCodePoint(codePoint, sb);
	}
}