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

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  */ @FunctionalInterface
public 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/