But first, let's check some small concepts.

## In a hurry? { #in-a-hurry }

If you don't care about any of these terms and you just need to add security with authentication based on username and password *right now*, skip to the next chapters.

## OAuth2 { #oauth2 }

OAuth2 is a specification that defines several ways to handle authentication and authorization.

It is quite an extensive specification and covers several complex use cases.

    private final ReferenceEntry<K, V> next;

    public DummyEntry(K key, int hash, ReferenceEntry<K, V> next) {
      this.key = key;
      this.hash = hash;
      this.next = next;
    }

    public static <K, V> DummyEntry<K, V> create(
        K key, int hash, @Nullable ReferenceEntry<K, V> next) {
      return new DummyEntry<>(key, hash, next);
    }

    public void clearKey() {

    private final ReferenceEntry<K, V> next;

    public DummyEntry(K key, int hash, ReferenceEntry<K, V> next) {
      this.key = key;
      this.hash = hash;
      this.next = next;
    }

    public static <K, V> DummyEntry<K, V> create(
        K key, int hash, @Nullable ReferenceEntry<K, V> next) {
      return new DummyEntry<>(key, hash, next);
    }

    public void clearKey() {

      String nextSeparator = "";
      StringBuilder builder = new StringBuilder(32).append(className).append('{');
      for (ValueHolder valueHolder = holderHead.next;
          valueHolder != null;
          valueHolder = valueHolder.next) {
        Object value = valueHolder.value;
        if (valueHolder instanceof UnconditionalValueHolder
            || (value == null
                ? !omitNullValuesSnapshot

     */
    void add(Problem.Severity severity, String message, int line, int column, Exception cause);

    /**
     * The next messages will be bound to this source. When calling this method again, previous messages keep
     * their source, but the next messages will use the new source.
     *
     * @param source a source
     */
    void setSource(String source);

    /**
     *

                return true;
            }
        }
        return false;
    }

    @Override
    public E next() {
        if (!hasNext()) {
            throw new NoSuchElementException();
        }
        return iterators[index].next();
    }

    @Override
    public void remove() {
        throw new ClUnsupportedOperationException("remove");
    }

    if (comparator == null) {
      comparator = NATURAL_ORDER;
    }
    Iterator<Entry<K, V>> entryItr = navigableMap.entrySet().iterator();
    Entry<K, V> prevEntry = entryItr.next();
    while (entryItr.hasNext()) {
      Entry<K, V> nextEntry = entryItr.next();
      assertTrue(comparator.compare(prevEntry.getKey(), nextEntry.getKey()) < 0);
      prevEntry = nextEntry;
    }
  }

    private String generateRegistrationId() {
        return "REG-" + System.currentTimeMillis() + "-" + Integer.toHexString(System.identityHashCode(this));
    }

    /**
     * Gets the next sequence number for this registration.
     *
     * @return the next sequence number
     */
    public long getNextSequenceNumber() {
        return sequenceNumber.incrementAndGet();
    }

    /**

              "Overflow adding %s occurrences to a count of %s",
              occurrences,
              current);
          int next = current + occurrences;
          if (countMap.replace(element, current, next)) {
            return current;
          }
        }
        // If we're still here, there was a race, so just try again.
      }
    }

    /**

      protected @Nullable C computeNext() {
        while (merged.hasNext()) {
          C next = merged.next();
          boolean duplicate = lastValue != null && comparator.compare(next, lastValue) == 0;

          // Keep looping till we find a non-duplicate value.
          if (!duplicate) {
            lastValue = next;
            return lastValue;
          }
        }