Java Frequently Used Utillity Methods

Objects.requireNonNull(key, message)

Objects.equals(a,b) - null safe

StringBuilder

.charAt(i) 

.setCharAt(int, char)

.setLength(0)/setLength(len-1)
- O(1) if decrease the length

.reverse()

.insert(offset,value)

.delete(start,end)

.deleteCharAt(i)

Arrays.equals(a1,a2)
.fill(long[] a, long val)
fill(long[] a, int fromIndex, int toIndex, long val)

To fill multiple dimension arrays:
int[][] target = new int[2][4];
int [] temp = new int[4];
Arrays.fill(temp, -1);

Arrays.fill(target, temp);

Collection

List

E remove(int index);

boolean remove(Object o);

ListIterator

next/hasNext/nextIndex, previous/hasPrevious/previousIndex, 

add/remove/set

ArrayList.subList
remove elements from startIndex to endIndex
list1.subList(startIndex, endIndex).clear();
Don't update original list while still need the subList.

Arrays.asList(T…a) returns a Subclass of AbstractList that doesn’t implement the add or remove method - it throws UnsupportedOperationException.

JDK 8
map.putIfAbsent(sum, i);
Instead of:
if (!map.containsKey(sum)) {
    map.put(sum, i);
}

map.getOrDefault(key, defaultValue)

computeIfAbsent

computeIfPresent

Lambda
iterable.forEach(x -> x.setXXX(null));
list.removeIf(s -> s.length() == 0);

Stream

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

list.sort((x, y) -> x.length() – y.length());

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

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

Collectors

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

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

Math:

Math.xxxExact - throws exception if overflow

addExact/subtractExact...

Base64

SecureRandom.getInstanceStrong()

Enum
SomeEnum someEnum = Enum.valueOf(SomeEnum.class, value);

// Need escape ': ''
MessageFormat.format

Initilaize 2-d array
int[][] target = new int[2][4];
int [] temp = new int[4];
Arrays.fill(temp, -1);
Arrays.fill(target, temp);

Get the inputstream from classpath:
ClassLoader.getResourceAsStream()

Guava
Preconditions
MoreObjects
firstNonNull(@Nullable T first, @Nullable T second)
joiner = Joiner.on("; ").skipNulls();
splitter = Splitter.on(',').trimResults().omitEmptyStrings();

ComparisonChain.start().compare(this.aString, that.aString)....compare(this.anEnum, that.anEnum, Ordering.natural().nullsLast()).result();

Ordering<String> ordering = = Ordering.nullsFirst().reverse();

Collections.sort(names, ordering);
Map<String, String> paramMap = Splitter.on("&").withKeyValueSeparator("=").split(params);
Joiner.on("&").withKeyValueSeparator("=").join(paramMap);

CaseFormat.LOWER_UNDERSCORE.to(CaseFormat.UPPER_CAMEL, tableName);

HtmlEscapers.htmlEscaper().escape(str)
UrlEscapers.urlFormParameterEscaper()/urlFragmentEscaper()/urlPathSegmentEscaper()/urlFormParameterEscaper().

Files.hash(file, Hashing.md5());
RateLimiter

Solr Commons
DateUtil.parseDate(dateString)
DateUtil.getThreadLocalDateFormat().format(date)

Top 16 Java Utility Classes
Apache Commons
StringUtils
CollectionUtils
ReflectionToStringBuilder.toString(this)

org.apache.commons.lang.SerializationUtils
clone
serialize

deserialize

ObjectUtils
firstNonNull(T... values)
defaultIfNull(final T object, final T defaultValue)

ArrayUtils.contains(arr,targetValue)