Java 8 Stream API features --- filter/reduce & predicate Interface test method

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现在要对下面一组 number ，过滤掉被5整除的，再*2 .
jdk1.7 之前是这么做的：

    public static void main(String... args){        List<Integer> values = Arrays.asList(11,22,33,44,55,60,75);        int result = 0;        for (int i : values) {            if(i % 5 == 0){                result += i;            }        }        System.out.println(result);       }

如果我们以这种方式去实现的话，forEach 循环里的 code 我们把它称为 embedded recording ,意味着你要 focus on what to do and how to do，但是到了 jdk 1.8，使用 Stream API后，它的理念是不需要 focus on how to do , 只需要 focus on what to do .看下面的代码，而且上面的 code 还是属于 external iteration ,data 多的时候效率相比于下面的 internal ireration 要慢许多，这也是 Stream API 的一个优点。

System.out.println(values.stream()                         .filter(i -> i%5==0)                         .map(i -> i*2));

看一下 filter method :
Returns a stream consisting of the elements of this stream that match the given predicate.

public interface Stream<T> extends BaseStream<T, Stream<T>> {    /**     * Returns a stream consisting of the elements of this stream that match     * the given predicate.     *     * <p>This is an <a href="package-summary.html#StreamOps">intermediate     * operation</a>.     *     * @param predicate a <a href="package-summary.html#NonInterference">non-interfering</a>,     *                  <a href="package-summary.html#Statelessness">stateless</a>     *                  predicate to apply to each element to determine if it     *                  should be included     * @return the new stream     */    Stream<T> filter(Predicate<? super T> predicate);

对于 Predicate Interface : 它是一个 functional interface，只有一个 test method ,takes a parameter and return boolean.所以上面的 filter是这样实现的：

/** * Represents a predicate (boolean-valued function) of one argument. * * <p>This is a <a href="package-summary.html">functional interface</a> * whose functional method is {@link #test(Object)}. * * @param <T> the type of the input to the predicate * * @since 1.8 */@FunctionalInterfacepublic interface Predicate<T> {    /**     * Evaluates this predicate on the given argument.     *     * @param t the input argument     * @return {@code true} if the input argument matches the predicate,     * otherwise {@code false}     */    boolean test(T t);

Predicate<Integer> p = new Predicate<Integer>() {    @Override    public boolean test(Integer i) {        return i%5 == 0;    }};

再用 lambda expression 把 p 替换掉就成了：

System.out.println(values.stream()                         .filter(i -> i%5==0)                         .map(i -> i*2)                         .reduce(Integer::sum));

所以从总体上来看，在使用 Stream API 时，实际上变成了函数式编程，没有 focus on Object，而是 focus on functions，.stream()、.filter()、.map()、.reduce() 四个 function ，像是不用再去 create object了。

更多请看这里：http://docs.oracle.com/javase/8/docs/api/

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