How to mark a method obligatory?

Question

Suppose you create a class names Person using the builder pattern, and suppose the Builder class contains methods body(), head(), arms()

Answer 1

Why not calling body(), head(), arms() in the build()-Method if it is really mandatory and returning Person in the build() method?

[edit]

Short example:

public class Builder {

private final String bodyProp;

private final String headProp;

private final String armsProp;

private String hearProps;

public Builder(String bodyProp, String headProp, String armsProp) {
    super();
    this.bodyProp = bodyProp; // check preconditions here (eg not null)
    this.headProp = headProp;
    this.armsProp = armsProp;
}

public void addOptionalHair(String hearProps) {
    this.hearProps = hearProps;
}

public Person build() {
    Person person = new Person();

    person.setBody(buildBody());
    // ...

    return person;
}



private Body buildBody() {
    // do something with bodyProp
    return new Body();
}


public static class Person {

    public void setBody(Body buildBody) {
        // ...
    }
}

public static class Body {
}
}