How to check between two subclasses which one has been called when creating an instance?

I'm trying to create a "bank account" with its operations. Some of them are:

1. bank addition : class Ajout
2. bank withdrawal : class Retrait

There are the bank balance variable named solde and amount named montant

The AjoutOuRetrait class is the mother class (means AdditionOrWithdrawal)

Now what I expect in my main class is the following:

c1.operation(new Ajout(750, new Date(01,01,2017)));
c1.operation(new Retrait(50, new Date(05,03,2017)));
System.out.println(c1.getSolde());          // result -> 700

public class AjoutOuRetrait {
	public int montant;
	public Date date;

	public AjoutOuRetrait(int montant, Date d) {
		this.montant = montant;
		this.date = d;
	}

public class CompteBancaire {
	
	private String id;
	private Banque banque;
	private int solde;	
	
	public CompteBancaire(String id, Banque b) {
		this.id = id;
		this.banque = b;
	}
	
	public void operation(AjoutOuRetrait aor){
		this.solde = aor.montant;
}

more the getters and setters that I omitted.

public class Retrait extends AjoutOuRetrait {

	public Retrait(int montant, Date d) {
		super(montant, d);
	}
}

public class Ajout extends AjoutOuRetrait{
		
	public Ajout(int montant, Date d) {
		super(montant, d);
	}

I was thinking on a way to differentiate (with conditional statement) which child class I call in argument of operation() method , whether it is Retrait (WithDrawal) -- or Ajout(Additional) ++

You might check the child type in operation like.

public void operation(AjoutOuRetrait aor){
    if(aor instanceof Ajout) {
        this.solde += aor.montant;
    } else if (aor instanceof Retrait) {
        this.solde -= aor.montant;
    }
}

or split them

public void operation(Ajout a){
     this.solde += a.montant;
}
public void operation(Retrait r){
     this.solde -= r.montant;
}

Also you might get the value used as right operand of your operation like

public class AjoutOuRetrait {
    public int getValue() {
        return this.montant;
    }
}

public class Retrait {
    @Override
    public int getValue() {
        return -this.montant;
    }
}

public void operation(AjoutOrRetrait aor) {
    this.solde += aor.getValue();
}