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GraphqL Java
There is a newer version: 230521-nf-execution
package graphql.language;

import graphql.GraphQLException;
import graphql.Internal;

import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.stream.Collectors;

import static java.util.function.Function.identity;

/**
 * Helper class for working with {@link Node}s
 */
@Internal
public class NodeUtil {

    public static Map directivesByName(List directives) {
        return getByName(directives, Directive::getName);
    }

    public static Map argumentsByName(List arguments) {
        return getByName(arguments, Argument::getName);
    }

    private static  Map getByName(List namedObjects, Function nameFn) {
        return namedObjects.stream().collect(Collectors.toMap(
                nameFn,
                identity(),
                mergeFirst())
        );
    }

    private static  BinaryOperator mergeFirst() {
        return (o1, o2) -> o1;
    }


    public static class GetOperationResult {
        public OperationDefinition operationDefinition;
        public Map fragmentsByName;
    }

    public static GetOperationResult getOperation(Document document, String operationName) {


        Map fragmentsByName = new LinkedHashMap<>();
        Map operationsByName = new LinkedHashMap<>();

        for (Definition definition : document.getDefinitions()) {
            if (definition instanceof OperationDefinition) {
                OperationDefinition operationDefinition = (OperationDefinition) definition;
                operationsByName.put(operationDefinition.getName(), operationDefinition);
            }
            if (definition instanceof FragmentDefinition) {
                FragmentDefinition fragmentDefinition = (FragmentDefinition) definition;
                fragmentsByName.put(fragmentDefinition.getName(), fragmentDefinition);
            }
        }
        if (operationName == null && operationsByName.size() > 1) {
            throw new GraphQLException("missing operation name");
        }
        OperationDefinition operation;

        if (operationName == null) {
            operation = operationsByName.values().iterator().next();
        } else {
            operation = operationsByName.get(operationName);
        }
        if (operation == null) {
            throw new GraphQLException("no operation found");
        }
        GetOperationResult result = new GetOperationResult();
        result.fragmentsByName = fragmentsByName;
        result.operationDefinition = operation;
        return result;
    }
}