FlexibleConstructor.java

class Y {
    int value;
    Y(int value) {
        this.value = value;
        display();
    }

    public void display() {
        IO.println("Value: " + value);
    }
}

class X extends Y {
    int start, end;
    String greet = "Hello!";
    X(int start, int end) {
        // Prologue
        if (start > end) {
            throw new IllegalArgumentException("start > end");
        }
        IO.println(this); // Error - You can't observe current instance in prologue
        this.start = start; // OK - assignment to an uninitialized field
        this.end = end; // OK
        greet = "Hi!"; //Error - assignment to an initialized field
        this.toString(); // Error - refers to instance method
        
        
        super(end - start); // Or, this(<with optional args>)
        
        // Epilogue
        // OK - Call a method of the current instance in epilogue
        this.display();
    }

    @Override
    public void display() {
        IO.println("Start: " + start + ", End: " + end + ", Greet: " + greet);
    }
}