inline if statement java, why is not working

The ternary operator ? : is to return a value, don't use it when you want to use if for flow control.

if (compareChar(curChar, toChar("0"))) getButtons().get(i).setText("§");

would work good enough.

https://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html

Syntax is Shown below:

"your condition"? "step if true":"step if condition fails"