Java8-对map过滤

1、对map按值过滤返回值

public class TestMapFilter {

    public static void main(String[] args) {
        Map<Integer, String> HOSTING = new HashMap<>();
        HOSTING.put(1, "linode.com");
        HOSTING.put(2, "heroku.com");
        HOSTING.put(3, "digitalocean.com");
        HOSTING.put(4, "aws.amazon.com");

        // Before Java 8
        String result = "";
        for (Map.Entry<Integer, String> entry : HOSTING.entrySet()) {
            if ("heroku.com".equals(entry.getValue())) {
                result = entry.getValue();
            }
        }
        System.out.println("Before Java 8: " + result);

        // Map -> Stream -> Filter -> String
        result = HOSTING.entrySet().stream()
                .filter(map -> "linode.com".equals(map.getValue()))
                .map(map -> map.getValue())
                .collect(Collectors.joining());
        System.out.println("With Java 8:" + result);

        // filter more values
        result = HOSTING.entrySet().stream()
                .filter(x -> {
                    if (!x.getValue().contains("amazon") && !x.getValue().contains("digital")) {
                        return true;
                    }
                    return false;
                })
                .map(map -> map.getValue())
                .collect(Collectors.joining(","));

        System.out.println("With Java 8 : " + result);
    }
}

2、按key过滤返回map

public class TestMapFilter2 {

    public static void main(String[] args) {
        Map<Integer, String> HOSTING = new HashMap<>();
        HOSTING.put(1, "linode.com");
        HOSTING.put(2, "heroku.com");
        HOSTING.put(3, "digitalocean.com");
        HOSTING.put(4, "aws.amazon.com");

        //Map -> Stream -> Filter -> Map
        Map<Integer, String> result1 = HOSTING.entrySet().stream()
                .filter(map -> map.getKey() == 2)
                .collect(Collectors.toMap(h -> h.getKey(), h -> h.getValue()));

        System.out.println(result1);

        Map<Integer, String> result2 = HOSTING.entrySet().stream()
                .filter(map -> map.getKey() <= 4)
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
        System.out.println(result2);
    }
}

3、Predicate使用

public class TestMapFilter3 {

    public static <K, V> Map<K, V> filterByValue(Map<K, V> map, Predicate<V> predicate) {
        return map.entrySet().stream()
                .filter(x -> predicate.test(x.getValue()))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
    }

    public static void main(String[] args) {

        Map<Integer, String> HOSTING = new HashMap<>();
        HOSTING.put(1, "linode.com");
        HOSTING.put(2, "heroku.com");
        HOSTING.put(3, "digitalocean.com");
        HOSTING.put(4, "aws.amazon.com");
        HOSTING.put(5, "aws2.amazon.com");

        Map<Integer, String> result1 = filterByValue(HOSTING, x -> x.contains("heroku"));
        System.out.println(result1);

        Map<Integer, String> result2 = filterByValue(HOSTING, x -> (x.contains("aws") || x.contains("digitalocean")));
        System.out.println(result2);

        Map<Integer, String> result3 = filterByValue(HOSTING, x -> (x.contains("aws") && !x.contains("aws2")));
        System.out.println(result3);

        Map<Integer, String> result4 = filterByValue(HOSTING, x -> x.length() <= 10);
        System.out.println(result4);
    }
}