LaunchDarkly, Create and Add UserSegmentRules with the Java API based on examples given here

LaunchdarlyUserSegmentAPI.java

package com.moshe.ldclient;

import com.launchdarkly.api.ApiClient;
import com.launchdarkly.api.ApiException;
import com.launchdarkly.api.Configuration;
import com.launchdarkly.api.api.UserSegmentsApi;
import com.launchdarkly.api.auth.ApiKeyAuth;
import com.launchdarkly.api.model.*;
import com.launchdarkly.sdk.LDUser;
import com.launchdarkly.sdk.server.LDClient;
import com.launchdarkly.sdk.server.LDConfig;
import com.launchdarkly.sdk.server.interfaces.LDClientInterface;

import java.util.List;

/**
 * Created by: moshee
 * Date: 2021-05-13 2:49 p.m.
 **/

public class FeatureToggleClientImpl {
    private UserSegmentsApi apiInstance;

    public FeatureToggleClientImpl() {
        final ApiClient apiClient = Configuration.getDefaultApiClient();
        apiClient.setDebugging(true);
        ApiKeyAuth Token = (ApiKeyAuth) apiClient.getAuthentication("Token");
        Token.setApiKey("launch-darkly-api-key");
        this.apiInstance = new UserSegmentsApi(apiClient); // initiate UserSegmentAPI client with debugging
    }

    public FeatureToggleClientImpl(LDClientInterface ldClient, UserSegmentsApi apiInstance) {
        this.apiInstance = apiInstance;
    }


    public String createSegment(String key, String description) {

        String projectKey = "default"; // String | The project key, used to tie the flags together under one project so they can be managed together.
        String environmentKey = "production"; // String | The environment key, used to tie together flag configuration and users under one environment so they can be managed together.
        UserSegmentBody userSegmentBody = new UserSegmentBody(); // UserSegmentBody | Create a new user segment.

        userSegmentBody.setName(key);
        userSegmentBody.setUnbounded(false);
        userSegmentBody.setKey(key);
        userSegmentBody.setDescription(description);
        userSegmentBody.setTags(List.of("api", "generated")); // set relevant tags

        


        PatchOperation patchOperation = new PatchOperation();
        patchOperation.setOp("add");// OP - add/replace/remove
        patchOperation.setPath("/rules/0"); // rules are numbered - got to start from 0

        Clause clause = new Clause(); // a clause is the rule filter.
        clause.setAttribute("attribute"); // use any attribute in the UserContext
        clause.setOp("in"); // OP - in/endsWith/startsWith/matches/contains/greaterThanOrEqual
        clause.setValues(List.of(16)); // casts to the relevant type in LaunchDarkly
        clause.setNegate(false); // use negate to get opposite of `op` endsWith + negate = true, means does NOT endWith
        
        patchOperation.setValue(new Clauses(List.of(clause))); // include clause in rule


        UserSegment result;
        String segmentKey = "";
        
        try {
            // You have to first create the User Segment, only later can you "Patch" it to add properties such as rules
            result = apiInstance.getUserSegment(projectKey, environmentKey, key);
            System.out.println(result);

            
            // use the PATCH command to add/update values internal to the object, represented as PatchOperation(s)
            UserSegment userSegment = apiInstance.patchUserSegment(projectKey, environmentKey, key, List.of(patchOperation));
            System.out.println(userSegment);

        } catch (ApiException e) {
            System.err.println("Exception when calling UserSegmentsApi#postUserSegment");
            e.printStackTrace();
        }

        return "";
    }
  
    // This structure adds the required `clauses` section is required for the API call to work
    public class Clauses {
        private List<Clause> clauses = null;

        public Clauses(List<Clause> clauses) {
            this.clauses = clauses;
        }

        @ApiModelProperty(required = true, value = "")
        public List<Clause> getClauses() {
            return clauses;
        }
    }
}

Source for structure is here
launchdarkly/ld-openapi#87 (comment)