child2.component.ts

import { Component,Input,OnInit } from '@angular/core';

@Component({
    selector: 'appchild',
    template: `<h2>Hi {{message}}</h2>`
})
export class ChildComponent  implements OnInit{

    _message: string; 
    constructor(){

    }   
    ngOnInit(){

    }
    @Input()
    set message(message : string ){
        this._message = (message && message.trim()) || "I am default message";
    }
    get message(){
        return this._message;
    }
}