user.service.ts

import { Injectable } from '@angular/core';
import { HttpClient, HttpHeaders , HttpParams } from '@angular/common/http';

import { User } from '../models/user';
import { tap, map, shareReplay, share, publishReplay, refCount } from 'rxjs/operators';
import { Observable } from 'rxjs';

@Injectable({ providedIn: 'root' })
export class UserService {

    headers = new HttpHeaders({
        'Content-Type':  'application/json',
        'Authorization': 'application/json'
    });

    host = 'http://localhost:8080/';

    constructor(private http: HttpClient) { }

    getUserById(userId:number){

        let params = new HttpParams().set( 'userId', String(userId) );
        let options = { headers: this.headers, params: params };

        return this.http.get(this.host + 'getUserById', options)
            .pipe(
                map((user:User) => {
                    return user
                })
            )
    }

}