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With inspiration from other libraries
There is a newer version: 2.12.15
/*
 * Copyright (C) 2015 Square, Inc.
 *
 * 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
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * 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 com.squareup.javapoet;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import javax.lang.model.element.Modifier;

import static java.lang.Character.isISOControl;

/**
 * Like Guava, but worse and standalone. This makes it easier to mix JavaPoet with libraries that
 * bring their own version of Guava.
 */
final class Util {
  private Util() {
  }

  /** Modifier.DEFAULT doesn't exist until Java 8, but we want to run on earlier releases. */
  static final Modifier DEFAULT;
  static {
    Modifier def = null;
    try {
      def = Modifier.valueOf("DEFAULT");
    } catch (IllegalArgumentException ignored) {
    }
    DEFAULT = def;
  }

  static  Map> immutableMultimap(Map> multimap) {
    LinkedHashMap> result = new LinkedHashMap<>();
    for (Map.Entry> entry : multimap.entrySet()) {
      if (entry.getValue().isEmpty()) continue;
      result.put(entry.getKey(), immutableList(entry.getValue()));
    }
    return Collections.unmodifiableMap(result);
  }

  static  Map immutableMap(Map map) {
    return Collections.unmodifiableMap(new LinkedHashMap<>(map));
  }

  static void checkArgument(boolean condition, String format, Object... args) {
    if (!condition) throw new IllegalArgumentException(String.format(format, args));
  }

  static  T checkNotNull(T reference, String format, Object... args) {
    if (reference == null) throw new NullPointerException(String.format(format, args));
    return reference;
  }

  static void checkState(boolean condition, String format, Object... args) {
    if (!condition) throw new IllegalStateException(String.format(format, args));
  }

  static  List immutableList(Collection collection) {
    return Collections.unmodifiableList(new ArrayList<>(collection));
  }

  static  Set immutableSet(Collection set) {
    return Collections.unmodifiableSet(new LinkedHashSet<>(set));
  }

  static String join(String separator, List parts) {
    if (parts.isEmpty()) return "";
    StringBuilder result = new StringBuilder();
    result.append(parts.get(0));
    for (int i = 1; i < parts.size(); i++) {
      result.append(separator).append(parts.get(i));
    }
    return result.toString();
  }

  static  Set union(Set a, Set b) {
    Set result = new LinkedHashSet<>();
    result.addAll(a);
    result.addAll(b);
    return result;
  }

  static void requireExactlyOneOf(Set modifiers, Modifier... mutuallyExclusive) {
    int count = 0;
    for (Modifier modifier : mutuallyExclusive) {
      if (modifier == null && Util.DEFAULT == null) continue; // Skip 'DEFAULT' if it doesn't exist!
      if (modifiers.contains(modifier)) count++;
    }
    checkArgument(count == 1, "modifiers %s must contain one of %s",
        modifiers, Arrays.toString(mutuallyExclusive));
  }

  static boolean hasDefaultModifier(Collection modifiers) {
    return DEFAULT != null && modifiers.contains(DEFAULT);
  }

  static String characterLiteralWithoutSingleQuotes(char c) {
    // see https://docs.oracle.com/javase/specs/jls/se7/html/jls-3.html#jls-3.10.6
    switch (c) {
      case '\b': return "\\b"; /* \u0008: backspace (BS) */
      case '\t': return "\\t"; /* \u0009: horizontal tab (HT) */
      case '\n': return "\\n"; /* \u000a: linefeed (LF) */
      case '\f': return "\\f"; /* \u000c: form feed (FF) */
      case '\r': return "\\r"; /* \u000d: carriage return (CR) */
      case '\"': return "\"";  /* \u0022: double quote (") */
      case '\'': return "\\'"; /* \u0027: single quote (') */
      case '\\': return "\\\\";  /* \u005c: backslash (\) */
      default:
        return isISOControl(c) ? String.format("\\u%04x", (int) c) : Character.toString(c);
    }
  }

  /** Returns the string literal representing {@code value}, including wrapping double quotes. */
  static String stringLiteralWithDoubleQuotes(String value, String indent) {
    StringBuilder result = new StringBuilder(value.length() + 2);
    result.append('"');
    for (int i = 0; i < value.length(); i++) {
      char c = value.charAt(i);
      // trivial case: single quote must not be escaped
      if (c == '\'') {
        result.append("'");
        continue;
      }
      // trivial case: double quotes must be escaped
      if (c == '\"') {
        result.append("\\\"");
        continue;
      }
      // default case: just let character literal do its work
      result.append(characterLiteralWithoutSingleQuotes(c));
      // need to append indent after linefeed?
      if (c == '\n' && i + 1 < value.length()) {
        result.append("\"\n").append(indent).append(indent).append("+ \"");
      }
    }
    result.append('"');
    return result.toString();
  }
}