Using Java 8

Default Method

Using Optional
- as return value, field (Optional
Optional.ofNullable/orElse(T value)/orElseThrow
ifPresent(Consumer consumer)/orElseThrow(XException::new)

Map + ConcurrentHashMap
ConcurrentHashMap<String, Integer> wordMap = new ConcurrentHashMap<String, Integer>(16, 0.9f, 1);

compute, computeIfAbsent, computeIfPresent

- the current (existing or computed) value associated with the specified key, or null if the computed value is null
targets.computeIfAbsent(ticket[0], k -> new PriorityQueue()).add(ticket[1]);
counters.computeIfAbsent(name, k -> new AtomicInteger()).incrementAndGet();
map.computeIfAbsent("key", k -> new LongAdder()).increment();

map.putIfAbsent(sum, i);
Instead of:
if (!map.containsKey(sum)) {
    map.put(sum, i);
}

map.getOrDefault(key, defaultValue)
wordMap.merge(word, occurrence, (key, value) -> wordMap.getOrDefault(key, 0) + value);
wordMap.reduceValues(1,(count1,count2) -> count1+count2);
concurrentHashMap.search(threshold, (k, v) -> v > 1000 ? k : null);

Lambda
iterable.forEach(x -> x.setXXX(null));
removeIf
removeIf is a default method in Collection.
Use removeIf for ArrayList O(n) instead of using Iterator.remove which is O(mn)
list.removeIf(s -> s.length() == 0);

list.replaceAll(s -> s.toUpperCase());

list.sort((x, y) -> x.length() – y.length());
Arrays.sort(intervals, (a, b) -> Integer.compare(a.start,b.start));
Arrays.sort(intervals, (a, b) -> a.start - b.start);

logger.finest(() -> complexMsg());

lineCount = Files.lines(path).count();

Collectors
toImmutable

.collect(collectingAndThen(toList(),Collections::unmodifiableList))

List to map:  
list.stream().collect(Collectors.toMap(x -> x.getId(), x -> x.getName())); - keyMapper, valueMapper
Convert to a different map - TreeMap
.collect(Collectors.toMap(keyMapper, valueMapper,(k,v) ->{ throw new RuntimeException(String.format("Duplicate key %s", k));}, TreeMap::new)); - mergeMapper, mapSupplier

stream().collect(Collectors.groupingBy(xxx));

mapStream.collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

long result=  list.stream().collect(Collectors.counting());

Collectors.joining joins the stream elements for a given delimiter, prefix and suffix. 
String result=  list.stream().collect(Collectors.joining(",","(",")"));

int result = list.stream().collect(Collectors.summingInt(i->i));
Map<String,String> map = streak.collect(Collectors.toMap(k->k, v->v+v));

stream.findFirst().ifPresent(System.out::println); 
streams cannot be reused. The stream is closed after you call any terminal operation.

Word Count:
groupBy aggregate
https://www.javacodegeeks.com/2015/03/jdk-8-streams-and-grouping.html
http://www.nurkiewicz.com/2014/07/grouping-sampling-and-batching-custom.html
Map collect = wordsList.stream().collect(groupingBy(Function.identity(), counting()));

Map map = "abcabc".chars().mapToObj(c -> (char) c).collect(Collectors.groupingBy(c -> c, Collectors.counting()));

Map countMap = Files.lines(file).flatMap(line -> SPLITTER.splitToList(line).stream())
                .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

Method and Constructor References
XClass::aMethod, XClass::new
list.sort(String::compareToIgnoreCase);
stream.forEach(System.out::println)

Stream
List to map:
list.stream().collect( Collectors.toMap(Perseon::getId, Perseon::getAge));
toString

flatMap
Combination of map and flatten. The parameter mapper function returns a list of stream, flatMap flatten them into a Stream of single value.
slices.stream().map(slice -> slice.getReplicas()).flatMap(replicas -> replicas.stream())

    .map(replica -> replica.getCoreUrl()).collect(Collectors.toList())

ArrayList.toString() 

ArrayList class extends AbstractList class, which extends AbstractCollection class which contains a toString() method

String.join(", ", list);

Troubleshooting
When compiler complains: incompatible types: cannot infer type-variable(s),try to declare an explicit type to provide enough type information to compiler: 
Map<K1, V1> to Map<K2, V2>
new HashMap<String, ValueClass>().entrySet().stream().collect(Collectors.toMap((final Entry<String, ValueClass> e) -> LocalDate.parse(e.getKey()), e -> e.getValue()));

Java 8 Time API
LocalDate/LocalTime/LocalDateTime - no time zone, good for store birthday
ZonedDateTime
date1.until(date2, ChronoUnit.DAYS),
date1.plusDays(1)

DateTimeFormatter - thread safe
-- Prefer it than SimpleDateFormat

LocalDate.parse("2016-10-06")
LocalDateTime.parse("2016-10-06", DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm"))


SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
sdf.setTimeZone(TimeZone.getTimeZone("UTC"));

Convert Date to ZonedDateTime
ZonedDateTime.ofInstant(date.toInstant(), ZoneOffset.UTC)

Month Enum
LongAdder vs AtomicLong

Related
Java Tips, Tricks and Traps