How to validate nested objects for Null values

<br/>
I am working on a service which receives a request from the upstream system.<br/>

• The request can have about 300 fields.
• My task is to process the order, but before processing the order, I have to perform data validation.
• Data validation mainly involves null & empty checks. I have to check at-least whether all the mandatory fields are neither empty nor null.

Order DTO:
<br/>

public class Order{
   private String orderId;
   private String sourceOrder;
   private String destinationSystem;
   private String orderingTimeStamp;
   private List<Mapper> properties;
          ....
}

I don't want to check null on each step, like
if(sourceOrder!=null && !sourceOrder.isEmpty()), as I would end up writing a lot of boilerplate code.<br/>
So, I thought before processing the order, let's validate the order for null & empty checks. If all mandatory values are present, then process the order else reject the order.

<br/>
I am aware of @NotNull annotation, but that won't work for nested objects for example private List<Mapper> properties, will have a list of objects. Out of those elements I want to validate only selected and not all. <br/>

<br/>
For example, I have to validate only "firstName", "lastName", "Address" in the List<Mapper> properties, not all.
<br/>
Is there any way to achieve it, or do we have any design pattern/framework to do this?<br/>
Please help.

What I would do is define a validate function on all nested properties.

First, you want to define a seperate class which has the different flags to control validation behaviour.

public class OrderValidationOptions {
  // Your options
}

Then or our nested type:

public class Mapper {
 // Other Code
 
 public boolean validate(OrderValidationOptions options) {
   // validation logic
 }
}

And similarly in the Order class:

public class Order {
 // Other Code
 private List<Mapper> properties;
 public boolean validate(OrderValidationOptions options) {
   // Other logic
   if (this.properties != null) {
     for(Mapper property: this.properties) {
       property.validate(options)
     }
   }
 }
}

All Lists and Sets extend the Interface Collection. I make use of that. When you have any Object example of any type, you can call validate(example); and the right result will return.
If example implements Collection, the first method will be called, otherwise the second.

public boolean validate(Collection c) {
   if(c == null) return false;
   if(c.isEmpty()) return false;
   return true;
}
public boolean validate(Object obj) {
   return obj != null;
}