How to call child method from parent in react

const ChildComponent = forwardRef((props, ref) => {
  useImperativeHandle(ref, () => ({
    sayHello() {
      alert("Hello from Child");
    }
  }));

  return <h1>Child Component</h1>;
});

const ParentComponent = () => {
  const childRef = useRef();

  return (
    <div>
      <ChildComponent ref={childRef} />
      <button onClick={() => childRef.current.sayHello()}>Call child method</button>
    </div>
  );
};

class ChildComponent extends Component {
  sayHello() {
    alert('Hello from Child');
  }

  render() {
    return <h1>Child Component</h1>;
  }
}

class ParentComponent extends Component {
  constructor(props) {
    super(props);
    this.childRef = React.createRef();
  }

  render() {
    return (
      <div>
        <ChildComponent ref={this.childRef} />
        <button onClick={() => this.childRef.current.sayHello()}>Click</button>
      </div>
    );
  }
}