How to create a deep insert structure using Olingo Client?

The following Java example using the Olingo Client works for me to post the following JSON structure:

{ 
"itemNumber": "ITEM1" 

    "lines": [ 
    
        { 
            componentNumber":   "COMPONENT1" 
        }, 
        { 
            componentNumber":   "COMPONENT2"
        } 
    ] 
    }  

Java code using OLINGO CLient

public void deepInsertExample(){
        //Initiate the ODATA client
    ODataClient client = ODataClientFactory.getClient();
        client.getConfiguration();
        getClient().getObjectFactory();
        //Initiate the Client Object Factory
        ClientObjectFactory factory = getClient().getObjectFactory();
        //Create Line Item 1
        ClientEntity lineItem1 = factory.newEntity(new FullQualifiedName("ODATA.LineItem"));
        lineItem1.getProperties()
                .add(factory.newPrimitiveProperty("componentNumber", factory.newPrimitiveValueBuilder().buildString("COMPONENT2")));
        //Create Line Item 2      
    ClientEntity   lineItem2 = factory.newEntity(new FullQualifiedName("ODATA.LineItem"));
        lineItem2.getProperties()
                .add(factory.newPrimitiveProperty("componentNumber", factory.newPrimitiveValueBuilder().buildString("COMPONENT1")));
        //Initiate the entity set
        ClientEntitySet entitySet = factory.newEntitySet();
        //Add  Line Item 1 and Line Item 2 to the Enity 
        entitySet.getEntities().add(lineItem1);
        entitySet.getEntities().add(lineItem2);
        //Create the Lines LInk  
        ClientLink linesLink = factory.newDeepInsertEntitySet("Lines", entitySet);
        ClientComplexValue complexValueCreate = factory.newComplexValue("Lines");
        complexValueCreate.getNavigationLinks().add(linesLink);
        //Create the Item object
        ClientEntity item = factory.newEntity(new FullQualifiedName("ODATA.Item"));
    item.getProperties()
                .add(factory.newPrimitiveProperty("itemNumber", factory.newPrimitiveValueBuilder().buildString("ITEM1")));
        //Add the Lines(Entity Set) link to Item Object           
        item.addLink(linesLink);
        //Post the Item  
        URI absoluteUri = client.newURIBuilder("URL").build();
        ODataEntityCreateRequest<ClientEntity> request = client.getCUDRequestFactory()
                .getEntityCreateRequest(absoluteUri, item);
        request.setAccept("application/json;odata.metadata=minimal");
        request.execute();
 }    
 

You will have to specify the the NavigationPropertyName in the deep part. So for your sample payload it would look like

{
    "itemNumber": "ITEM1",
    
    "lines": {
        "componentNumber":"COMPONENT1",
        "componentNumber":"COMPONENT2",
         
    }
}

You can refer to this post in SO to get details about n level nesting

The above answer asuumes that your NavigationProperty is named lines, you can substitute it with the right name by looking at service/$metadata

The answer assumes that you are trying to do deep inserts in a odata2 service, for OData4 the concept remains the same but syntax might vary a bit. Please refer to the payload descripted in documentation in case of OData 4