In this method:
public boolean aplicaDescontoDe(double porcentagem) {
if(porcentagem > 0.3) {
return false;
} else {
this.valor -= valor * porcentagem;
return true;
}
}
What does the ! operator mean in the if code:
if(!livro.aplicaDescontoDe(0.1)) {
System.out.println("Desconto nao pode ser maior que 30%");
} else {
System.out.println("Valor com desconto: " + livro.valor);
}
In Java, the ! operator is a unary operator used to invert the value of a Boolean expression. Thus, if a Boolean expression evaluates to True, the ! operator, if applied to it, will invert the value to False.
In the question example, it will invert the value returned by the aplicaDescontoDe function. If the function returns True, then the ! operator will reverse the value to False.
One way to read the question code is: "If no applies the discount of 0.1, then ...". (That's exactly how I read this kind of code)
It's the same thing to do:
if (livro.aplicaDescontoDe(0.1) == false)...
The ! operator is for deny.
That is, if the line: livro.aplicaDescontoDe(0.1) returns true , when it precedes the ! operator, it becomes falsa and vice versa.
public class HelloWorld{
public static void main(String []args){
boolean verdade = true;
System.out.println(verdade); // imprime true
System.out.println(!verdade); // imprime false
}
}
! This is a negation, ie it is denying (or reversing the result) method aplicaDescontoDe