Reference types - Data Structures and Problem Solving Using Java - page 51

2.5.4 the throw and throws clauses

The programmer can generate an exception by use of the throw clause. For
instance, we can create and then throw an ArithmeticException object by

The throw clause is
used to throw an
exception.

throw new ArithmeticException( "Divide by zero" );

Since the intent is to signal to the caller that there is a problem, you
should never throw an exception only to catch it a few lines later in the same
scope. In other words, do not place a throw clause in a try block and then han-
dle it immediately in the corresponding catch block. Instead, let it leave
unhandled, and pass the exception up to the caller. Otherwise, you are using
exceptions as a cheap go to statement, which is not good programming and is
certainly not what an exception—signaling an exceptional occurrence—is to
be used for.
Java allows programmers to create their own exception types. Details on
creating and throwing user-defined exceptions are provided in Chapter 4.

As mentioned earlier, standard checked exceptions must either be caught or
explicitly propagated to the calling routine, but they should, as a last resort, even-
tually be handled in main . To do the latter, the method that is unwilling to catch
the exception must indicate, via a throws clause, which exceptions it may propa-
gate. The throws clause is attached at the end of the method header. Figure 2.14
illustrates a method that propagates any IOException s that it encounters; these
must eventually be caught in main (since we will not place a throws clause in
main ).

The throws clause
indicates propa-
gated exceptions.

input and output

2.6

Input and output (I/O) in Java is achieved through the use of the java.io pack-
age. The types in the I/O package are all prefixed with java.io , including, as
we have seen, java.io.IOException . The import directive allows you to avoid
using complete names. For instance, with

import java.io.IOException;

you can use IOException as a shorthand for java.io.IOException at the top of
your code. (Many common types, such as String and Math , do not require
import directives, as they are automatically visible by the shorthands by virtue
of being in java.lang .)