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写该系列文章的目的是记录Guava源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,此前本人一直用比较挫的方式:不管三七二一,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
从整体上讲,在使用带有提示消息的相关check方法时需要考虑到性能问题,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(...)); //意思好像是说这种写法比较影响性能 * } * * 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"); } // 但作者在下面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述的不太OK方式,原因我没太明白。 /** * 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); } }
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当然,还有一个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(); }
// Optional及其两个实现类Absent,Present,这三个类用简洁的方式解决了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>
[Guava源码阅读笔记]-Basic Utilities篇-1
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原文地址:http://blog.csdn.net/cza55007/article/details/46391167