Exception handling : throw, throws and Throwable

There are 2 main types of Exceptions:
Runtime Exceptions(unchecked): eg. NullPointerException, ClassCastException,..
Checked Exceptions: eg. FileNotFoundException, CloneNotSupportedException, ..

Runtime Exceptions are exceptions that occur at runtime and the developer should not try to catch it or stop it. You only write code to avoid them or issue a command throw, when the error criteria is met. We use throw inside the method body.

public Rational(int num, int denom){
if(denom <= 0) {
  throw new IllegalArgumentException("Denominator must be positive");
}
this.num=num;
this.denom=denom;
}

However for Checked Exceptions, the JVM expects you to handle it and will give compiler error if not handled so you declare that it throws that type of exception as seen below in the clone() method.

Class Employee{
public Employee clone() throws CloneNotSupportedException{
    Employee copy = (Employee)super.clone();
    copy.hireDate = (Date)hireDate.clone();
    return copy;
}
}