[BUG][JAVA] Problem generating model where default value is specified for date property

Description

Model properties that are dates generate invalid Java code where a default value is specified.

openapi-generator version

v5.0.0-beta

OpenAPI declaration file content or url

{
    "openapi": "3.0.1",
    "info": {
        "title": "Test",
        "version": "1.0"
      },
    "paths": {},
    "components": {
        "schemas": {
            "Request": {
                "type": "object",
                "properties": {
                    "endDate": {
                        "type": "string",
                        "format": "date",
                        "default": "1900-01-01"
                    },
                    "startDate": {
                        "type": "string",
                        "format": "date"
                    }
                }
            }
        }
    }
}

Command line used for generation

java -jar openapi-generator-cli-5.0.0-beta.jar generate -i test.json -g java -o .\output --additional-properties dateLibrary=java8-localdatetime

Steps to reproduce

Run the command above;
View generated Request.java which contains the following invalid variable initialization:

@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", date = "2020-07-24T10:22:22.238+10:00[Australia/Sydney]")
public class Request {
  public static final String SERIALIZED_NAME_END_DATE = "endDate";
  @SerializedName(SERIALIZED_NAME_END_DATE)
  private LocalDate endDate = 1900-01-01; // <--- Type mismatch: cannot convert from int to LocalDate

  public static final String SERIALIZED_NAME_START_DATE = "startDate";
  @SerializedName(SERIALIZED_NAME_START_DATE)
  private LocalDate startDate;
  ...

Related issues/PRs

None known.

Suggest a fix

In the toDefaultValue(Schema) method of org.openapitools.codegen.languages.AbstractJavaCodegen the default value for a date is returned as a String representation of the date, but should probably be a call to LocalDate.parse or similar.

Start in org.openapitools.codegen.languages.AbstractJavaCodegen, specifically the toDefaultValue(Schema) method, and reproduce the issue with the supplied OpenAPI declaration and Java generation command. Inspect the generated Request.java and verify that a date default produces valid Java for LocalDate rather than the shown invalid initialization.