Guava学习一(字符串、函数、缓存)

Guava

一个非常有意思的，来自google的工具，maven依赖如下：

		<dependency>
            <groupId>com.google.guava</groupId>
            <artifactId>guava</artifactId>
            <version>28.1-jre</version>
        </dependency>

字符串与对象

1. Joiner(连接，list、map连接)
2. Splitter/MapSplitter（分割器，字符串按照格式分割成list或map）
3. Strings类(判空，重复，填充，公共前缀)
4. CharMatcher（字符串格式化，字符串抽取）
5. Preconditions(checkNotNull,检查下标，参数，状态)
6. MoreObjects（tostring，hashcode重写,ComparisonChain.start()比较器）

        Map map= Maps.newLinkedHashMap();
		map.putIfAbsent("testA","1234");
		map.putIfAbsent("testB","4567");
		//1.joiner
		String result=Joiner.on("#").withKeyValueSeparator("=").join(map);
		//testA=1234#testB=4567

		List<String> lt = Arrays.asList("123", "456");
		String result2=Joiner.on("+").join(lt);
		//123+456

		Splitter.on("|").split("foo|bar|baz").forEach(System.out::println);

		String startString = "Washington D.C=Redskins#New York City=Giants#Philadelphia=Eagles#Dallas=Cowboys";
		Splitter.MapSplitter mapSplitter =Splitter.on("#").withKeyValueSeparator("=");
		Map<String,String> splitMap = mapSplitter.split(startString);
		//{Washington D.C=Redskins, New York City=Giants, Philadelphia=Eagles, Dallas=Cowboys}

		String testString = "helloLk";
		boolean isNullOrEmpty=Strings.isNullOrEmpty(testString);

		String lettersAndNumbers = "foo989yxbar234";
		String expected = "989234";
		String retained = CharMatcher.digit().retainFrom(lettersAndNumbers);
		assertEquals(expected,retained);
		//true

		Preconditions.checkNotNull(testString, "XXX is null");
		MoreObjects.toStringHelper(splitMap).add("test","1234").toString();

函数式编程

1. Function<F,T>接口，使用函数达到目的而不是改变状态，转换对象，隐藏实现
2. 匿名内部类也能实现，但是较为笨重
3. Functions.forMap处理map和Functions.compose流式处理
4. Predicate接口，断言，Predicates.and/or/not/compose
5. Supplier接口，抽象了复杂性和对象如何建立的细节，Suppliers.memoize返回缓存的代理实例
   Suppliers.memoizeWithExpiration，带有消亡时间的缓存代理实例

    //函数接口，指定in和out类型，apply实现函数
  public interface Function<F,T> {
        T apply(F input);
        boolean equals(Object object);
    }

public class DateFormatFunction implements Function<Date,String> {
    @Override
    public String apply(Date input) {
        SimpleDateFormat dateFormat = new SimpleDateFormat("dd/mm/yyyy");
        return dateFormat.format(input);
    }
}

public interface Predicate<T> {
    boolean apply(T input)
    boolean equals(Object object)
}

public interface Supplier<T> {
    T get();
}

缓存

1. MapMaker类，使用fluent接口API构造CHM

   ConcurrentMap<String,Book> books = new MapMaker()
    .concurrencyLevel(2)
    .softValues()
    .makeMap();

2. Cache和LoadingCache接口,常用方式如下：

LoadingCache<String,TradeAccount> tradeAccountCache =
    CacheBuilder.newBuilder()
    .expireAfterWrite(5L, TimeUnit.Minutes)
    .maximumSize(5000L)
	.softValues()//软饮用，回收
    .removalListener(new TradeAccountRemovalListener())
    .ticker(Ticker.systemTicker())
    .build(new CacheLoader<String, TradeAccount>() {
        @Override
            public TradeAccount load(String key) throws Exception {
                return tradeAccountService.getTradeAccountById(key);
            }
    });