Call to 'substring' is redundant

Question

I've created simple code to reverse an equation, but two lines are highlighted by lint in IntelliJ IDEA. Exact message is

Call to 'substring' is redundant

final String equation = "20+3*475-2-1*4";

final int size = equation.length();
final StringBuilder sb = new StringBuilder(size);
int right = size;
for (int i = size - 1; i > -1; i--) {
    switch (equation.charAt(i)) {
        case '+': case '-': case '*':
            sb.append(equation.substring(i + 1, right));  // this line
            sb.append(equation.charAt(i));
            right = i;
    }
}
if (right != 0) {
    sb.append(equation.substring(0, right));   // and this line
}

I haven't ever faced with a situation when lint highlights something without a reason. But now have no idea why these calls are redundant.

Answer 1

You can simplify the call to this:

sb.append(equation, i + 1, right);

This avoids an explicit intermediate string construction, allowing the implementation to copy the desired part of "equation" directly.

Answer 2

That's because StringBuilder append is overlaoded with a function, that also takes start and end index like substring.

So instead of using

sb.append(equation.substring(i + 1, right)); 

you can just use

sb.append(equation, i + 1, right);