Parameter validators

This page explains how to create ParameterValidators that perform checks against parameters after they are supplied by the user.

The ParameterValidator interface is a part of the commands framework that allows you to perform custom validation on command parameters. This can be particularly useful when you want to enforce specific rules or constraints on command inputs, such as validating ranges, formats, or other criteria.

Implementing a Custom ParameterValidator

Let's create an example of a custom ParameterValidator that checks if a number is within a specified range. This validator will make use of a custom annotation called @Range to specify the minimum and maximum allowed values.

Step 1: Define the @Range Annotation

First, we'll create a custom annotation @Range to specify the minimum and maximum values for our parameter:

Java

import java.lang.annotation.*;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.PARAMETER)
public @interface Range {
    double min();
    double max();
}

Kotlin

Step 2: Implement the RangeValidator

Next, we implement the RangeValidator using the ParameterValidator interface. This validator checks the parameter value against the @Range annotation's constraints:

Java

public enum RangeValidator implements ParameterValidator<CommandActor, Number> {
    INSTANCE;

    @Override
    public void validate(@NotNull CommandActor actor, Number value, @NotNull ParameterNode<CommandActor, Number> parameter, @NotNull Lamp<CommandActor> lamp) {
        Range range = parameter.getAnnotation(Range.class);
        if (range == null) return;
        double d = value.doubleValue();
        if (d < range.min())
            throw new CommandErrorException("Specified value (" + d + ") is less than minimum " + range.min());
        if (d > range.max())
            throw new CommandErrorException("Specified value (" + d + ") is greater than maximum " + range.max());
    }
}

Kotlin

Step 3: Register the Validator

To use the RangeValidator, you need to register it with your Lamp instance. This is done through the Lamp.Builder:

Java

var lamp = Lamp.builder()
    .parameterValidator(Number.class, RangeValidator.INSTANCE)
    .build();

Kotlin

Step 4: Using the @Range Annotation in Commands

Now, you can use the @Range annotation in your command methods to specify the valid range for numeric parameters:

Java

@Command("age")
public void setAge(CommandActor actor, @Range(min = 13, max = 99) int age) {
    /* ... */
}

Kotlin

In this example, the setAge command requires the age parameter to be between 18 and 99. If the user provides a value outside this range, the RangeValidator will throw an exception, and Lamp will handle it appropriately.