Ejemplo optional

import java.util.Optional;

class Main {

    int a;
    int b;

  public static void main(String[] args) {
        Main objMain = new Main();

        Arbol objArbol = new Arbol(
            null,
            new Arbol(null,null,4),
            6
        );

        Optional.ofNullable(objArbol.izq).ifPresent(x -> objMain.a = x.value);
        Optional.ofNullable(objArbol.der).ifPresent(x -> objMain.b = x.value);

        System.out.println("a " + objMain.a + " b " + objMain.b);

        }
}

class Arbol {
    Arbol izq;
    Arbol der;
    int value;
    Arbol (Arbol izq, Arbol der, int value) {
        this.izq = izq;
        this.der = der;
        this.value = value;
    }
}