java: add «2678. Number of Senior Citizens»

Signed-off-by: Matej Focko <me@mfocko.xyz>

java/number-of-senior-citizens.java

@@ -0,0 +1,15 @@
+class Solution {
+  public int countSeniors(String[] details) {
+    int count = 0;
+
+    for (String citizen : details) {
+      var age = Integer.parseInt(citizen.substring(11, 13));
+
+      if (age > 60) {
+        ++count;
+      }
+    }
+
+    return count;
+  }
+}