factoring out the common code in cases

Question

In

switch (ch) {
    case '0': Value v = compute(); v = invert(v); accumulate(v); break;
    case '1': v = compute(); accumulate(v); break;
    default: 
}

the compute and accumulate code pieces are common.

Java allows fall-through cases

Value v = compute();
switch (ch) {
    case '0': v = invert(v); 
    case '1': accumulate(v); break;
    default: 
}

Unfortunately, compute() is executed for ch values outside the {0,1}. Can I share both without the side effects and performance penalty?

By performance penalty I mean that I can always rewrite the switch with more flexible if-then statements.

if (ch == '-') continue; // next loop iteration
assert (ch == '1' or ch == '0');

v = compute();
if (ch == '1') v = invert(v);
accumulate(v);

However, this demands at least one branch statement more than single switch. And, every such statement adds 10x performance slowdown. Yet, I'm gonna use the construction in a loop

for (char ch : computeBits()) // ch can be '0','1','-'
    switch_block;

So, I prefer to stay with a single switch.

Answer 1

I would consider switching to if statements:

if (ch == '0' || ch == '1') {
    Value v = compute();
    if (ch == '0') {
        v = invert(v);
    }
    accumulate(v);
}

Though this is a bit denser, I find it a lot easier to see the commonality. It took me a few seconds of staring at your switches before I was positive that the only difference was that 0 executes invert and 1 does not.

Also, I tend to think that switch statements should only be used when all expected cases are covered. Considering that you're concerned about when ch is not 1 or 2, it seems that other cases are expected. From looking at your switch, I would not have thought that.

(If other values actually aren't expected, you could throw an exception in the default case to ease debugging if something slips through the cracks.)

Answer 2

if (ch != '0' || ch != '1') { return; } 
Value v = (ch == '0') ? invert(compute()) : compute();
accumulate(v);

This will take care of 0 and 1 cases.I am assuming this piece of code runs inside a method where I can have a return for invalid characters.