[BUG][JAVA] Default value of required array

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

For Java, the default value of an array is an empty ArrayList as soon as it is declared as "required" in the enclosing object. In our case, we must reject the null value with the appropriate validation: @javax.validation.constraints.NotNull. However, with this default value, we can only trigger another kind of validation: @javax.validation.constraints.NotEmpty, which can be misleading for our API users since the field is not present at all. This behavior does not seem to be consistent with the other types (string, number, integer, boolean, object) which remain at null despite the required property.

openapi-generator version

'org.openapi.generator' version '5.3.0' but reproduced in 6.0.0

OpenAPI declaration file content or url

Given :

    MyEnclosingObject:
      type: object
      required:
        - my_array
      properties:
        my_array:
          type: array
          items:
            $ref: '#/components/schemas/MyItem'

Expected generated source :

	public class MyEnclosingObject {

		@JsonProperty("my_array")
		@Valid
		private List<MyArray> myArray = null;

		public MyEnclosingObject myArray(List<MyArray> myArray) {
			this.myArray = myArray;
			return this;
		}

		public MyEnclosingObject addMyArray(MyItem myItem) {
			if (myArray == null) {
				myArray = new ArrayList<>();
                        }
			this.myArray.add(myItem);
			return this;
		}

		@ApiModelProperty(required = true)
		@NotNull @Valid
		public List<StandardPrice> getStandardPrices() {
			return standardPrices;
		}
	}

Actual generated source :

	public class MyEnclosingObject {

		@JsonProperty("my_array")
		@Valid
		private List<MyArray> myArray = new ArrayList<>();

		public MyEnclosingObject myArray(List<MyArray> myArray) {
			this.myArray = myArray;
			return this;
		}

		public MyEnclosingObject addMyArray(MyItem myItem) {
			this.myArray.add(myItem);
			return this;
		}

		@ApiModelProperty(required = true)
		@NotNull @Valid
		public List<MyItem> getMyArray() {
			return standardPrices;
		}
	}

Generation Details

Maven plugin in generate goal.
org.openapitools
openapi-generator-maven-plugin

Steps to reproduce

Define a contract of an object with a required array and call the API without that array and observe that the received array is empty instead of null.

Related issues/PRs

None found

Suggest a fix

The default value should be moved to addMyArray when we know we have at least one myItem

Start by reproducing the schema with the Maven plugin's generate goal and inspect the generated MyEnclosingObject Java model, especially the myArray field and addMyArray method. Done means a required array remains null when absent from the API request, while addMyArray initializes it when an item is added.