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);
     }
 }