title: What is the difference between ngOnInit(), ngAfterViewInit(), ngafterContentInit(), ngAfterViewChecked() and a constructor()?
body: <p>What is the difference between <strong>ngOnInit</strong>(), <strong>ngAfterViewInit</strong>(), <strong>ngafterContentInit</strong>(), <strong>ngAfterViewChecked</strong>() and a <strong>constructor</strong>()? How do we implement them in the Angular 2? 
  What are their purposes and usages? Where all will it be useful for implementing them?</p>
  
  <p>Thanks. </p>