Class 'ListItemFactory' incorrectly implements interface 'IFactory'. Types of property 'getItems' are incompatible

I am having the following error:

[ts] Class 'ListItemFactory' incorrectly implements interface 'IFactory'. Types of property 'getItems' are incompatible. Type '(requester: SPHttpClient, siteUrl: string, listName: string) => IListItem[]' is not assignable to type '() => IListItem[]'

I am trying to implement a simple factory method design pattern, the relevant code is like this:

export  interface IListItem {
    [key: string]: any;
    id: string;
    title: string;
    modified: Date;
    created: Date;
    modifiedby: string;
    createdby: string;    
}

import {IListItem} from './IListItem';

export interface IDirectoryListItem extends IListItem {
        firstName: string;
        lastName: string;
        mobileNumber: string;
        internalNumber: string;  
}

import {IListItem} from './IListItem';

export interface  IAnnouncementListItem extends IListItem {
    announcementBody: string;
    expiryDate: Date;  
}


import {IListItem} from './IListItem';

export interface  INewsListItem extends IListItem {
    newsheader: string;
    newsbody: string;
    expiryDate: Date;
}

import { IListItem } from './models/IListItem';

export  interface IFactory{
    getItems(): IListItem[]
}

import { SPHttpClient, SPHttpClientResponse } from '@microsoft/sp-http';
import { IWebPartContext } from '@microsoft/sp-webpart-base';
import { IListItem} from './models/IListItem';
import { IFactory } from './IFactory';
import { INewsListItem } from './models/INewsListItem';
import { IDirectoryListItem } from './models/IDirectoryListItem';
import { IAnnouncementListItem } from './models/IAnnouncementListItem';

export class ListItemFactory implements IFactory{   
    getItems(requester: SPHttpClient, siteUrl: string, listName: string): IListItem[] {
        switch(listName) {
            case 'List':
                let items: IListItem[];
                requester.get(`${siteUrl}/_api/web/lists/getbytitle('${listName}')/items?$select=Title,Id`,
                SPHttpClient.configurations.v1,
                {
                    headers: {
                    'Accept': 'application/json;odata=nometadata',
                    'odata-version': ''
                    }
                })
                .then((response: SPHttpClientResponse): Promise<{ value: IListItem[] }> => {
                    return response.json();
                })
                .then((response: { value: IListItem[] }): void => {
                    items= response.value;
                });
                return items;
            case 'News':
                let newsitems: INewsListItem[];
                requester.get(`${siteUrl}/_api/web/lists/getbytitle('${listName}')/items?$select=Title,Id`,
                SPHttpClient.configurations.v1,
                {
                    headers: {
                    'Accept': 'application/json;odata=nometadata',
                    'odata-version': ''
                    }
                })
                .then((response: SPHttpClientResponse): Promise<{ value: INewsListItem[] }> => {
                    return response.json();
                })
                .then((response: { value: INewsListItem[] }): void => {
                    newsitems= response.value;
                });
                return newsitems;
            case 'Announcements':
                let announcementitems: IAnnouncementListItem[];
                requester.get(`${siteUrl}/_api/web/lists/getbytitle('${listName}')/items?$select=Title,Id`,
                SPHttpClient.configurations.v1,
                {
                    headers: {
                    'Accept': 'application/json;odata=nometadata',
                    'odata-version': ''
                    }
                })
                .then((response: SPHttpClientResponse): Promise<{ value: IAnnouncementListItem[] }> => {
                    return response.json();
                })
                .then((response: { value: IAnnouncementListItem[] }): void => {
                    announcementitems= response.value;
                });
                return announcementitems;
            case 'Directory':
                let directoryitems: IDirectoryListItem[];
                requester.get(`${siteUrl}/_api/web/lists/getbytitle('${listName}')/items?$select=Title,Id`,
                SPHttpClient.configurations.v1,
                {
                    headers: {
                    'Accept': 'application/json;odata=nometadata',
                    'odata-version': ''
                    }
                })
                .then((response: SPHttpClientResponse): Promise<{ value: IDirectoryListItem[] }> => {
                    return response.json();
                })
                .then((response: { value: IDirectoryListItem[] }): void => {
                    items= response.value;
                });
                return directoryitems;
            default:
                return null;
        }
      }
} 

Error is in the last class ListItemFactory, but dont know why