[Guava源代码阅读笔记]-Basic Utilities篇-1

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写该系列文章的目的是记录Guava源代码中个人感觉不错且值得借鉴的内容。

一、MoreObjects类

//MoreObjects.ToStringHelper类的toString()方法：对于字符串拼接的写法蛮不错的，此前本人一直用比較挫的方式：无论三七二一，先拼接然后再subString()
@Override public String toString() {
      // create a copy to keep it consistent in case value changes
      boolean omitNullValuesSnapshot = omitNullValues;
      String nextSeparator = "";
      StringBuilder builder = new StringBuilder(32).append(className)
          .append('{');
      for (ValueHolder valueHolder = holderHead.next; valueHolder != null;
          valueHolder = valueHolder.next) {
        if (!omitNullValuesSnapshot || valueHolder.value != null) {
          builder.append(nextSeparator);
          nextSeparator = ", ";

          if (valueHolder.name != null) {
            builder.append(valueHolder.name).append('=');
          }
          builder.append(valueHolder.value);
        }
      }
      return builder.append('}').toString();
    }

    private ValueHolder addHolder() {
      ValueHolder valueHolder = new ValueHolder();
      holderTail = holderTail.next = valueHolder;
      return valueHolder;
    }

    private ToStringHelper addHolder(@Nullable Object value) {
      ValueHolder valueHolder = addHolder();
      valueHolder.value = value;
      return this;
    }

    private ToStringHelper addHolder(String name, @Nullable Object value) {
      ValueHolder valueHolder = addHolder();
      valueHolder.value = value;
      valueHolder.name = checkNotNull(name);
      return this;
    }

    private static final class ValueHolder {
      String name;
      Object value;
      ValueHolder next;
    }

二、Preconditions类

从总体上讲，在使用带有提示消息的相关check方法时须要考虑到性能问题，在一些性能敏感产品中可能。

/*
   * All recent hotspots (as of 2009) *really* like to have the natural code
   *
   * if (guardExpression) {
   *    throw new BadException(messageExpression);
   * }
   *
   * refactored so that messageExpression is moved to a separate String-returning method.
   *
   * if (guardExpression) {
   *    throw new BadException(badMsg(...)); //意思好像是说这样的写法比較影响性能
   * }
   *
   * The alternative natural refactorings into void or Exception-returning methods are much slower.
   * This is a big deal - we're talking factors of 2-8 in microbenchmarks, not just 10-20%.  (This
   * is a hotspot optimizer bug, which should be fixed, but that's a separate, big project).
   *
   * The coding pattern above is heavily used in java.util, e.g. in ArrayList.  There is a
   * RangeCheckMicroBenchmark in the JDK that was used to test this.
   *
   * But the methods in this class want to throw different exceptions, depending on the args, so it
   * appears that this pattern is not directly applicable.  But we can use the ridiculous, devious
   * trick of throwing an exception in the middle of the construction of another exception.  Hotspot
   * is fine with that.
   */

  /**
   * Ensures that {@code index} specifies a valid <i>element</i> in an array, list or string of size
   * {@code size}. An element index may range from zero, inclusive, to {@code size}, exclusive.
   *
   * @param index a user-supplied index identifying an element of an array, list or string
   * @param size the size of that array, list or string
   * @return the value of {@code index}
   * @throws IndexOutOfBoundsException if {@code index} is negative or is not less than {@code size}
   * @throws IllegalArgumentException if {@code size} is negative
   */
  public static int checkElementIndex(int index, int size) {
    return checkElementIndex(index, size, "index");
  }
// 但作者在以下的方法中为了实现抛出不同类型的异常时，还是使用了上述所描写叙述的不太OK方式，原因我没太明确。
  /**
   * Ensures that {@code index} specifies a valid <i>element</i> in an array, list or string of size
   * {@code size}. An element index may range from zero, inclusive, to {@code size}, exclusive.
   *
   * @param index a user-supplied index identifying an element of an array, list or string
   * @param size the size of that array, list or string
   * @param desc the text to use to describe this index in an error message
   * @return the value of {@code index}
   * @throws IndexOutOfBoundsException if {@code index} is negative or is not less than {@code size}
   * @throws IllegalArgumentException if {@code size} is negative
   */
  public static int checkElementIndex(
      int index, int size, @Nullable String desc) {
    // Carefully optimized for execution by hotspot (explanatory comment above)
    if (index < 0 || index >= size) {
      throw new IndexOutOfBoundsException(badElementIndex(index, size, desc));
    }
    return index;
  }

  private static String badElementIndex(int index, int size, String desc) {
    if (index < 0) {
      return format("%s (%s) must not be negative", desc, index);
    } else if (size < 0) {
      throw new IllegalArgumentException("negative size: " + size);
    } else { // index >= size
      return format("%s (%s) must be less than size (%s)", desc, index, size);
    }
  }

当然，另一个format方法（仍然是字符串处理）

/**
   * Substitutes each {@code %s} in {@code template} with an argument. These are matched by
   * position: the first {@code %s} gets {@code args[0]}, etc.  If there are more arguments than
   * placeholders, the unmatched arguments will be appended to the end of the formatted message in
   * square braces.
   *
   * @param template a non-null string containing 0 or more {@code %s} placeholders.
   * @param args the arguments to be substituted into the message template. Arguments are converted
   *     to strings using {@link String#valueOf(Object)}. Arguments can be null.
   */
  // Note that this is somewhat-improperly used from Verify.java as well.
  static String format(String template, @Nullable Object... args) {
    template = String.valueOf(template); // null -> "null"

    // start substituting the arguments into the '%s' placeholders
    StringBuilder builder = new StringBuilder(template.length() + 16 * args.length);
    int templateStart = 0;
    int i = 0;
    while (i < args.length) {
      int placeholderStart = template.indexOf("%s", templateStart);
      if (placeholderStart == -1) {
        break;
      }
      builder.append(template.substring(templateStart, placeholderStart));
      builder.append(args[i++]);
      templateStart = placeholderStart + 2;
    }
    builder.append(template.substring(templateStart));

    // if we run out of placeholders, append the extra args in square braces
    if (i < args.length) {
      builder.append(" [");
      builder.append(args[i++]);
      while (i < args.length) {
        builder.append(", ");
        builder.append(args[i++]);
      }
      builder.append(']');
    }

    return builder.toString();
  }

三、Optional及事实上现类

// Optional及其两个实现类Absent,Present。这三个类用简洁的方式攻克了Java中null值的不确定性问题（其设计哲学值得学习）
// public abstract class Optional<T> implements Serializable
// final class Absent<T> extends Optional<T>
// final class Present<T> extends Optional<T>