### Question:
PlaceHolderHelper { public String replacePlaceholders(String value, final Properties properties) { MoreObjects.notNull(properties, "'properties' must not be null"); MoreObjects.notNull(value, "'value' must not be null"); return parseStringValue(value, properties::getProperty, new HashSet<>()); } PlaceHolderHelper(String placeholderPrefix, String placeholderSuffix);  PlaceHolderHelper(String placeholderPrefix, String placeholderSuffix,
      String valueSeparator, boolean ignoreUnresolvablePlaceholders); static PlaceHolderHelper defaultInstance(); String replacePlaceholders(String value, final Properties properties); String replacePlaceholders(String value, UnaryOperator<String> operator); static final String DEFAULT_PLACEHOLDER_PREFIX; static final String DEFAULT_PLACEHOLDER_SUFFIX; static final String DEFAULT_VALUE_SEPARATOR; static final PlaceHolderHelper DEFAULT_HELPER; }### Answer:
@Test public void testHelper() { assertEquals("a", helper.replacePlaceholders(props.getProperty("a"), props)); assertEquals("a", helper.replacePlaceholders(props.getProperty("b"), props)); assertEquals("a", helper.replacePlaceholders(props.getProperty("c"), props)); assertEquals("a", helper.replacePlaceholders(props.getProperty("d"), props)); assertEquals("a", helper.replacePlaceholders(props.getProperty("e"), props)); }