4. The throw-statement

The throw-statement can be used to throw an object when a program detects an error. The use of the throw-statement allows a program to react to errors that it detects just the way the Java runtime system and all the predefined classes react. This allows a certain uniformity that would not be possible otherwise. Click the icon to see how to throw an exception! Here's the modified script.

 

The throw-statement has the form:

throw <expression> ;

where the expression yields an instance of (a subclass of) Throwable . Its execution causes the instance to be thrown, as explained in the video above.

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Throwing an object while another is being handled or propagated

Nothing prevents a catch-block or finally-block from throwing another object a1 (say) ---by mistake or otherwise--- while a previously thrown object a0 is being handled. If this happens, the previously thrown object a0 is ignored and the newly thrown object a1 is propagated, as described earlier.

Try this out yourself. Write a method main like the following and execute it to see what happens.

public static void main(String[] pars) {
    try {
       System.out.println("try-block 0");
       throw new ArithmeticException("fake exception 1");
    } catch (ArithmeticException ae) {
       System.out.println("catch-block 0");
       System.out.println(ae);
       throw new ArithmeticException("fake exception 2");
    }
}

Catching and throwing an exception further

See why it may be useful to catch an exception and then throw it on to someone else. Here's the modified script.

 

 

 

©This material is from the CD ProgramLive by David Gries and Paul Gries