if(invItems.length != this.material.length) return null;
for(int i = 0;i < this.material.length;i++){
if(this.material[i].itemID != invItems[i].itemID) return null;
if(invItems.length != this.material.length) return null;
for(int i = 0;i < this.material.length;i++){
if(this.material[i].itemID != invItems[i].itemID) return null;