How to convert Values in a Hashmap to a List<String>

Use addAll instead of add, in order to add all the Strings of all the List<String>s to a single List<String> :

for (List<String> value : adminErrorMap.values())   
{                
     adminValues.addAll(value);
}

In Java8, you can use functional to do that:

adminErrorMap.values().forEach(adminValues::addAll);

You just need to flatten the collection. In Java8:

    final Map<String, List<String>> adminErrorMap = ImmutableMap.of(
            "a", Lists.newArrayList("first", "second"),
            "b", Lists.newArrayList("third")
    );

    final List<String> results = adminErrorMap.values().stream()
            .flatMap(Collection::stream)
            .collect(Collectors.toList());

    results.forEach(System.out::println);

It prints:

first
second
third