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Parses HTTP inputs to POJOs

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package net.tokensmith.parser.validator;


import net.tokensmith.parser.validator.exception.EmptyValueError;
import net.tokensmith.parser.validator.exception.MoreThanOneItemError;
import net.tokensmith.parser.validator.exception.NoItemsError;
import net.tokensmith.parser.validator.exception.ParamIsNullError;


import java.util.List;



public class RequiredParam {
    private static String IS_NULL="parameter is null";
    private static String NO_ITEMS="parameter does not have one item";
    private static String EMPTY="parameter had no value";
    private static String TOO_MANY_VALUES="parameter has more than one item";

    /**
     * Determines if the items passes the rules for a required field. Which are:
     *  - items cannot be null
     *  - items cannot be a empty list
     *  - items first element cannot be empty
     *  - items cannot have more than one value (per OAUTH2)
     *
     * @param items the input to validate
     * @param allowMany if many items are allowed in the input
     * @return true if passes. raises exception if fails.
     * @throws EmptyValueError if items contains an empty string
     * @throws MoreThanOneItemError if items has more than one item
     * @throws NoItemsError if items has no values
     * @throws ParamIsNullError if items is null
     */
    public boolean run(List items, boolean allowMany) throws EmptyValueError, MoreThanOneItemError, NoItemsError, ParamIsNullError {

        if(items == null) {
            throw new ParamIsNullError(IS_NULL);
        }

        if (items.isEmpty()) {
            throw new NoItemsError(NO_ITEMS);
        }

        if (items.get(0).isEmpty()) {
            throw new EmptyValueError(EMPTY);
        }

        if(!allowMany && items.size() > 1) {
            throw new MoreThanOneItemError(TOO_MANY_VALUES);
        }

        return true;
    }
}