response.getItems().forEach(item -> 
            System.out.println("Async suggestion: " + item.getText()[0])
        );
    })
    .error(throwable -> {
        // Handle error
        System.err.println("Error: " + throwable.getMessage());
    });
```

## Advanced Usage

### Index from Existing Documents

```java
import org.codelibs.fess.suggest.index.contents.document.ESSourceReader;

            final ByteArrayOutputStream outStream = new ByteArrayOutputStream();
            System.setOut(new PrintStream(outStream, true));
            final PrintStream originalErrStream = System.err;
            final ByteArrayOutputStream errStream = new ByteArrayOutputStream();
            System.setErr(new PrintStream(errStream, true));
            try {