[20210320] Java Map getOrDefault(), 정규표현식, List<Integer> to int[]

[JuHyun419]

Map.getOrDefault(Object key, V defaultValue)

• Map에서 Key가 존재하면 해당 Key의 Value를, 존재하지 않으면 기본값(defaultValue)를 반환

    /**
     * Returns the value to which the specified key is mapped, or
     * {@code defaultValue} if this map contains no mapping for the key.
     *
     * @implSpec
     * The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties.
     *
     * @param key the key whose associated value is to be returned
     * @param defaultValue the default mapping of the key
     * @return the value to which the specified key is mapped, or
     * {@code defaultValue} if this map contains no mapping for the key
     * @throws ClassCastException if the key is of an inappropriate type for
     * this map
     * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified key is null and this map
     * does not permit null keys
     * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @since 1.8
     */
    default V getOrDefault(Object key, V defaultValue) {
        V v;
        return (((v = get(key)) != null) || containsKey(key))
            ? v
            : defaultValue;
    }

Map<Object, Integer> words = new HashMap<>();
// ch가 존재하면 해당 ch의 값에 + 1 을, 존재하지 않으면 1을 저장
words.put(ch, words.getOrDefault(ch, 0) + 1);

정규표현식(RegExp)

// 대문자(A-Z)가 포함되는지 판단
final String upperRegExp = ".*[A-Z].*";
Pattern.matches(upperRegExp, "AbbCd2"); // true
Pattern.matches(upperRegExp, "aeibba2"); // false

// 대소문자, 숫자, 특수문자만 포함되어있는지 확인
final String regExp = "^[A-Za-z0-9]*[~!@#$%^&*]*$";

Integer List to int Array

List<Integer> list = new ArrayList<>();
int[] answer = list.stream().mapToInt(Integer::intValue).toArray();