# Lab Logic Toolkit FAQ

## What Java/Groovy methods can I use in a Lab Logic Toolkit (LLTK) Expression?

You can use all methods included in the Java and Groovy base packages.

Commonly used Java classes are:

* String: <http://docs.oracle.com/javase/8/docs/api/java/lang/String.html>
* Math: <https://docs.oracle.com/javase/8/docs/api/java/lang/Math.html>

For information on Groovy closures, see [Groovy Closures](https://groovy-lang.org/closures.html).

## How do I handle attempts to access custom fields that do not exist or do not have a value set?

Use the *hasValue* function to check for nonexistent fields and fields that do not have a value set - including toggle switches in LIMS v5 and later.

See [Non-UDF Custom Field Properties](/clarity-lims/integration-toolkits/lltk/lltk-working-with-the-script/lltk-non-udf-custom-field-properties.md).

## How do I prevent a user from moving to the next screen if certain criteria are not met?

Use the *hasValue* function together with the *fail()* method to check for step UDF / custom field values when the Record Details screen is exited, and prevent the user from advancing to the next screen if they have not entered values for these fields.

See [Failing a Script](/clarity-lims/integration-toolkits/lltk/lltk-working-with-the-script/lltk-failing-a-script.md).

## How do I control the number of decimal places for a custom field?

Use the following expression:

1. Use Groovy syntax to format the following string:

   <pre class="language-markup" data-overflow="wrap"><code class="lang-markup">String.format(::%.0f::, step.::counter::)
   </code></pre>
2. Replace *step.::counter::* with the field you want to apply the formatting to.
3. Replace *%.0f* with the number of decimal places needed. For example, for three decimal places, use *%.3f*.

**Example:**

{% code overflow="wrap" %}

```markup
String.format(::%.2f::, Concentration::).
```

{% endcode %}

## How do I round to the closest integer below a value using Math.floor?

Use the following expression:

{% code overflow="wrap" %}

```markup
-exp 'output.::NTP Uses:: = Math.floor((input.::NTP Volume (uL):: - step.::NTP Dead Volume (uL)::)/ step.::NTP Volume Per Lane (uL)::)'
```

{% endcode %}

## How do I use LLTK with required custom fields?

Custom fields are set as a bulk group in the API. The API checks all required fields.

When LLTK updates a field, all required fields must be set or an error will occur.

**Workaround:**

1. Set the required fields with a value such "Please fill in this value".
2. Use another LLTK expression triggered on step completion to check whether this value has been updated.
   * If the field value has been updated, allow the step to proceed.
   * If the field value has not been updated, log an error and halt progress of the step.


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