public static String joinSizeTagToKey(String key, String tag) {
        return new StringBuilder(key).append(SIZE_SP).append(tag).toString();
    }
public static String joinSizeTagToKey(String key, String tag) {
        return new StringBuilder(key).append(SIZE_SP).append(tag).toString();
    }
public static String joinSizeTagToKey ( String key , String tag ) { return new StringBuilder ( key ) . append ( SIZE_SP ) . append ( tag ) . toString ( ) ; }
Join the tag with the key.

@param key
@param tag
@return "$key" + "_" + "$tag"
Join the tag with the key .