异常处理：Java 中 throw 与 throws 的区别

Question

Could someone explain me when it is useful to use the keyword throw new .. instead of using throws next to the signature of a method ?

I know that when a method throws a Checked Exception. Java forces us to deal with it by doing it directly in the method by handling the Exception into a try-catch bloc or by specifying that it will be done elsewhere with the keyword throws next to the signature.

However, I have some trouble to understand when it is useful to use the keyword throw new and why. Is it related with handling Unchecked Exceptions ?

For instance in this example. Why don't we throw new ArithmeticException() in the method compute ? Since ArithmeticException is an unchecked one ? Should we not add something like :

private static int compute(int i) { 
    if(i == 0) {
        throw new ArithmeticException();
    }
    return 1/i; 
}

.

public class Main {
  public static void main(String[] args) {
     for (int i = -2; i < 2; i++) { 
        try { 
           System.out.println(i+" -> "+compute(i)); 
        
        } catch (ArithmeticException e) {
             System.out.println(i+" -> undefined")
        }
     }
  }

   
 private static int compute(int i) { 
      return 1/i; 
  }

}

Answer 1

throws tells others that this method can throw an exception. Think of it as documentation. Checked exceptions must be part of a method's signature.

throw actually throws the exception. ( throw new Exception(); first creates a new exception instance and then throws that instance. You could use two separate statements: Exception ex = new Exception(); throw ex; ) You can throw both checked and unchecked exceptions with the throw keyword.

void checked() throws Exception { // required, Exception is a checked exception
  throw new Exception();
}

void unchecked() {
  throw new RuntimeException(); // throws not required, RuntimeException is not checked
}

void checkedCall() throws Exception {
  checked(); // throws required, because "checked" can throw a checked exception
}

void caught() {
  try {
    checked();
  } catch (final Exception ex) {
    // throws is no longer required, because the (checked) exception is handled within the method itself. It never leaves the method.
  }
}

---

Regarding your updated question: you don't throw new ArithmeticException , because it is thrown by the JVM when it tries to apply the / operator. You can throw it manually if you want to, but that's just redundant code.