Invert Boolean

private double a;
...
public boolean method() {
    if (a > 15 && a < 100) {
        a = 5;
        return true;
    }
    return false;
}

private double a;
...
public boolean method() {
    if (a > 15 && a < 100) {
        a = 5;
        return false;
    }
    return true;
}

boolean b = true;
...
public double method() {
    ...
    b = false;
    ...
}

boolean b = false;
...
public double method() {
    ...
    b = true;
    ...
}