import {Http} from '@angular/http';
@Injectable()
export class StudentService{
    constructor(public http: Http){}
    getAllStudents(): Observable<Students[]>{
        return this.http.get('assets/students.json')
        .map(res => res.json().data)     
        }
    }

notice the constructor now again if we want to use this service method we will go to our view/page and :

import {StudentService} from './student.service';
import { SocialSharing } from '@ionic-native/social-sharing';
export class HomePage implements OnInit {

  constructor(public _studentService: StudentService, public socialSharing: SocialSharing) {
   }

again notice the constructor here, we are creating an instance of StudentService in constructor and one more thing, we are using socialSharing plugin so to use that we are creating instance of that in constructor as well.

OnInit: this is really amazing thing in ionic2 or we can say in AngularJs2. With the same above example we can see what is ngOnInit is. So you are ready with the service method, now in your view/page you want that student list data available as soon as your view is going to appear, this should be the first operation happend automatically on load, because as the view load the student list should be visible. So the class implements OnInit and you define ngOnInit. Example:

export class HomePage implements OnInit {
...
....
constructor(....){}

ngOnInit(){
    this._studentService.getAllStudents().subscribe(
     (students: Students[]) => this.students = students, 
    )