Many classes are prone to NPEs

[manticore-projects]

When implementing the Statement Classifications I looked into a few classes, which can easily throw NPEs because they do not enforce proper initiation:

Testing TimezoneExpression

ExpressionVisitorAdapter expressionVisitorAdapter0 = new ExpressionVisitorAdapter();
TimezoneExpression timezoneExpression1 = TimezoneExpression();
// throw an exception of type java.lang.NullPointerException
expressionVisitorAdapter0.visit(timezoneExpression1);

Example TimezoneExpression

    private Expression leftExpression;
    private ArrayList<String> timezoneExpressions = new ArrayList<>();

    public Expression getLeftExpression() {
        return leftExpression;
    }

    public void setLeftExpression(Expression expression) {
        leftExpression = expression;
    }

    @Override
    public void accept(ExpressionVisitor expressionVisitor) {
        expressionVisitor.visit(this);
    }

    public List<String> getTimezoneExpressions() {
        return timezoneExpressions;
    }

    public void addTimezoneExpression(String timezoneExpr) {
        this.timezoneExpressions.add(timezoneExpr);
    }

    @Override
    public String toString() {
        String returnValue = getLeftExpression().toString();
        for (String expr : timezoneExpressions) {
            returnValue += " AT TIME ZONE " + expr;
        }

        return returnValue;
    }

We do not enforce setting leftExpression and we also do not check if it is set in toString() or in the Deparser. Of course I do understand that this is unlikely to happen because the Parser will call setLeftExpression() during parsing. Nevertheless I do not think that this approach is clean.

Instead I would like to suggest a refactoring into something like

    private final Expression leftExpression;
    private final ArrayList<String> timezoneExpressions = new ArrayList<>();
    
    public TimezoneExpression(Expression leftExpression) {
        this.leftExpression = Objects.requireNonNull(leftExpression, "The Left Side Expression of the Timezone Expression must not be null.");
    }

    public Expression getLeftExpression() {
        return leftExpression;
    }
    

    @Override
    public void accept(ExpressionVisitor expressionVisitor) {
        expressionVisitor.visit(this);
    }

    public List<String> getTimezoneExpressions() {
        return timezoneExpressions;
    }

    public void addTimezoneExpression(String timezoneExpr) {
        this.timezoneExpressions.add(timezoneExpr);
    }

    @Override
    public String toString() {
        String returnValue = getLeftExpression().toString();
        for (String expr : timezoneExpressions) {
            returnValue += " AT TIME ZONE " + expr;
        }

        return returnValue;
    }

What do you think about please?
We have many of similar classes, please see the attached Test Case. I would like to recommend and volunteer refactoring those -- but of course only, when there is no counter argument and the effort was welcome.

Cheers.
ErrorTest0.zip

[wumpz]

This works for constructor initialization only classes. For the other ones make this in setters is not enough, since there are many combinations of parameters including some set to null. This one should go into the validators as well, I think.