It looks like Walnut Leaf Gall Mite damage. The mites (Aceria erinea) attacks the leaves and causes raised hollow bumps on upper leaf surface. Generally they won't harm tree unless if there is high infestation.
Management:
1. Remove and burn the infected leaves.
2. Provide proper care and nutrition to plants to increase its growth and to with stand mite damage.
3. Spray strong jet of water occasionally to remove mites from leaves.