constructor(private route: ActivatedRoute,
  private router: Router) {}

selectedTabIndex: number;

ngOnInit(): void {
  this.selectedTabIndex = parseInt(this.route.snapshot.queryParamMap.get('tab'), 10);
  // TODO: check upper bound. Material will set the last tab as selected
  // if selectedTabIndex >= number of tabs
  if (isNaN(this.selectedTabIndex) || this.selectedTabIndex < 0) {
    this.selectedTabIndex = 0;
  }
}

onTabChange(selectedTabIndex: number): void {
  this.router.navigate([], { relativeTo: this.route, queryParams: {
    tab: selectedTabIndex
  }});
  this.selectedTabIndex = selectedTabIndex;
}