java中常用的方法

StringUtils(org.apache.commons.lang3)

isEmpty(str)

判断字符串是否为 “” ， null

// Empty checks
    //-----------------------------------------------------------------------
    /**
     * <p>Checks if a CharSequence is empty ("") or null.</p>
     *
     * <pre>
     * StringUtils.isEmpty(null)      = true
     * StringUtils.isEmpty("")        = true
     * StringUtils.isEmpty(" ")       = false
     * StringUtils.isEmpty("bob")     = false
     * StringUtils.isEmpty("  bob  ") = false
     * </pre>
     *
     * <p>NOTE: This method changed in Lang version 2.0.
     * It no longer trims the CharSequence.
     * That functionality is available in isBlank().</p>
     *
     * @param cs  the CharSequence to check, may be null
     * @return {@code true} if the CharSequence is empty or null
     * @since 3.0 Changed signature from isEmpty(String) to isEmpty(CharSequence)
     */
    public static boolean isEmpty(final CharSequence cs) {
        return cs == null || cs.length() == 0;
    }

isBlank(str)

判断字符串是否为 “” ，null ， “ ”

 /**
     * <p>Checks if a CharSequence is empty (""), null or whitespace only.</p>
     *
     * <p>Whitespace is defined by {@link Character#isWhitespace(char)}.</p>
     *
     * <pre>
     * StringUtils.isBlank(null)      = true
     * StringUtils.isBlank("")        = true
     * StringUtils.isBlank(" ")       = true
     * StringUtils.isBlank("bob")     = false
     * StringUtils.isBlank("  bob  ") = false
     * </pre>
     *
     * @param cs  the CharSequence to check, may be null
     * @return {@code true} if the CharSequence is null, empty or whitespace only
     * @since 2.0
     * @since 3.0 Changed signature from isBlank(String) to isBlank(CharSequence)
     */
    public static boolean isBlank(final CharSequence cs) {
        int strLen;
        if (cs == null || (strLen = cs.length()) == 0) {
            return true;
        }
        for (int i = 0; i < strLen; i++) {
            if (!Character.isWhitespace(cs.charAt(i))) {
                return false;
            }
        }
        return true;
    }

isAnyBlank(str)

判断多个字符串中是否存在 null , " " , "’

/**
     * <p>Checks if any of the CharSequences are empty ("") or null or whitespace only.</p>
     *
     * <p>Whitespace is defined by {@link Character#isWhitespace(char)}.</p>
     *
     * <pre>
     * StringUtils.isAnyBlank((String) null)    = true
     * StringUtils.isAnyBlank((String[]) null)  = false
     * StringUtils.isAnyBlank(null, "foo")      = true
     * StringUtils.isAnyBlank(null, null)       = true
     * StringUtils.isAnyBlank("", "bar")        = true
     * StringUtils.isAnyBlank("bob", "")        = true
     * StringUtils.isAnyBlank("  bob  ", null)  = true
     * StringUtils.isAnyBlank(" ", "bar")       = true
     * StringUtils.isAnyBlank(new String[] {})  = false
     * StringUtils.isAnyBlank(new String[]{""}) = true
     * StringUtils.isAnyBlank("foo", "bar")     = false
     * </pre>
     *
     * @param css  the CharSequences to check, may be null or empty
     * @return {@code true} if any of the CharSequences are empty or null or whitespace only
     * @since 3.2
     */
    public static boolean isAnyBlank(final CharSequence... css) {
      if (ArrayUtils.isEmpty(css)) {
        return false;
      }
      for (final CharSequence cs : css) {
        if (isBlank(cs)) {
          return true;
        }
      }
      return false;
    }

Map

putIfAbsent

如果map中不存在该 key 或 key所对应的val为null ，则 put ，方法返回 null
否则返回key对应的val值

    /**
     * If the specified key is not already associated with a value (or is mapped
     * to {@code null}) associates it with the given value and returns
     * {@code null}, else returns the current value.
     *
     * @implSpec
     * The default implementation is equivalent to, for this {@code
     * map}:
     *
     * <pre> {@code
     * V v = map.get(key);
     * if (v == null)
     *     v = map.put(key, value);
     *
     * return v;
     * }</pre>
     *
     * <p>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 key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with the specified key, or
     *         {@code null} if there was no mapping for the key.
     *         (A {@code null} return can also indicate that the map
     *         previously associated {@code null} with the key,
     *         if the implementation supports null values.)
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if the key or value 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 or value is null,
     *         and this map does not permit null keys or values
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws IllegalArgumentException if some property of the specified key
     *         or value prevents it from being stored in this map
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @since 1.8
     */
    default V putIfAbsent(K key, V value) {
        V v = get(key);
        if (v == null) {
            v = put(key, value);
        }

        return v;
    }