Builder: validation in build() catches an invalid combination

Individual setters can look fine on their own even when the combination they leave behind is not, so build() is where the whole object gets checked before it is allowed to exist. Here a lower bound greater than the upper bound is rejected only once build() runs.

Code

class Range {
    final int lo, hi;
    private Range(int lo, int hi) { this.lo = lo; this.hi = hi; }
    static class Builder {
        private int lo, hi;
        Builder lo(int v) { lo = v; return this; }
        Builder hi(int v) { hi = v; return this; }
        Range build() {
            if (lo > hi) throw new IllegalStateException("lo must not exceed hi");
            return new Range(lo, hi);
        }
    }
}
Range.Builder builder = new Range.Builder().lo(10).hi(5);
String outcome;
try {
    builder.build();
    outcome = "built";
} catch (IllegalStateException e) {
    outcome = "rejected: " + e.getMessage();
}
System.out.println(outcome);

Output

rejected: lo must not exceed hi