Object-Oriented X++: Inheritance & Interfaces

Simple explanation

OOP in X++ works like C# or Java. Classes are blueprints. Inheritance lets children get parent features. Abstract classes cannot be built directly. Interfaces define contracts. QueryBuilder creates dynamic database queries at runtime.

Classes

public class CustomerValidator
{
    private str accountNumber;
    public void new(str _acct) { accountNumber = _acct; }
    public boolean validate()
    {
        CustTable ct;
        select firstOnly ct where ct.AccountNum == accountNumber;
        return (ct.RecId != 0);
    }
    public static CustomerValidator construct(str _acct) { return new CustomerValidator(_acct); }
}

Variable Scoping

Scope	Lifetime
Class member	Object lifetime
Method local	Until method returns
Block local	Until block ends

🎓 Sophie: “Carl says keep scope tight — declare variables where they are used.”

Inheritance

public class InvoiceProcessor extends DocumentProcessor
{
    public void new(real _amount) { super(); }
    public void process() { super(); info("Invoice processed"); }
}

extends = inheritance, super() = call parent, single inheritance only.

Abstract Classes and Interfaces

public abstract class PaymentBase
{
    public abstract boolean processPayment(real _amount);
}

public interface IExportable { str exportToJson(); }

public class SalesOrder extends PaymentBase implements IExportable
{
    public boolean processPayment(real _amount) { return true; }
    public str exportToJson() { return "..."; }
}

Abstract Class vs Interface
Feature	Abstract Class	Interface
Method bodies	Mix abstract/concrete	All abstract
State	Yes	No
Inheritance	Single	Multiple
Use when	Shared base behaviour	Contract across classes

QueryBuilder

Query query = new Query();
QueryBuildDataSource qbds = query.addDataSource(tableNum(CustTable));
qbds.addRange(fieldNum(CustTable, CustGroup)).value("10");
qbds.addSortField(fieldNum(CustTable, Name), SortOrder::Ascending);
QueryRun qr = new QueryRun(query);
while (qr.next())
{
    CustTable ct = qr.get(tableNum(CustTable));
    info(ct.Name);
}

select vs QueryBuilder
	Static select	QueryBuilder
When	Compile-time fixed	Runtime dynamic
Forms	Not for form DS	Used by forms
Use	Lookups, logic	Reports, dynamic filters

Attributes

Attribute	Purpose
`[DataContractAttribute]`	Serializable class
`[DataMemberAttribute]`	Include in contract
`[SysEntryPointAttribute]`	Service entry point
`[ExtensionOf]`	CoC extension

[DataContractAttribute]
public class ReportContract
{
    private date fromDate;
    [DataMemberAttribute]
    public date parmFromDate(date _v = fromDate) { fromDate = _v; return fromDate; }
}

👨‍💻 Elena: “The parm pattern — pass a value to set, call empty to get. Every data contract uses it.”

Question

extends vs implements?

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Answer

extends = one parent (inheritance). implements = interface contract (multiple ok).

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Question

super() does what?

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Answer

Calls parent class version of current method.

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Question

QueryBuilder vs select?

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Answer

QueryBuilder for runtime dynamic queries. select for compile-time fixed.

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Question

[DataContractAttribute] purpose?

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Answer

Marks class for serialization. Used with SysOperation. Properties opt in via [DataMemberAttribute].

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Knowledge Check

Marcus needs a serializable request class. Attributes?

Knowledge Check

Dynamic filters based on user input?

Next up: Chain of Command — the most important extensibility pattern.