throws exception

[abalfazl hossein]

There are times when it makes sense for a program to deliberately throw an exception. This is the case when the program discovers some sort of exceptional or error condition, but there is no reasonable way to handle the error at the point where the problem is discovered. The program can throw an exception in the hope that some other part of the program will catch and handle the exception.

http://www.faqs.org/docs/javap/c9/s3.html

May someone give me simple example of such case?

[David Newton]

Just about anything. A pre-condition not being met, wrapping an underlying exception into something more application specific, validation issues, ...

[Rob Spoor]

public class Person
{
    private String name;

    public Person(String name)
    {
        if (name.length() == 0) // will throw NullPointerException if name == null
        {
            throw new IllegalArgumentException("must provide a name");
        }
    }
}

[Campbell Ritchie]

Rob Prime wrote: . . . will throw NullPointerException if name == null . . .

It's usually a good idea to throw a NullPointerException for null inputs.

[David Newton]

And it will? (Unless your point was to make it explicit.)

[Rob Spoor]

I think Campbell means this: if an empty name is allowed but a null name isn't then manually throwing an NPE is a good practice:

public Person(String name)
{
    if (name == null) throw new NullPointerException("name cannot be null");
    this.name = name;
}

[Campbell Ritchie]

Rob Prime wrote:I think Campbell means this: . . .

Yes, he does

[abalfazl hossein]

All right,

From what I understand, We use try/catch while we deal with a code that may throw exception

But We use throw when we want the code throws exception, right now!


and there is a catch clause for this exception that throws by "throw keyword" explicitly .

I saw these codes in wikipedia:

 catch (NullPointerException e) {
    // --- I caught a Nullpointer ex. ---
    log_it( e );
    // --- I don't know how to handle this, so I throw it again ---
    throw e;
}
catch (Exception e) {
    // --- I caught all application but not Runtime exceptions ---
    log_it( e );
    // --- I don't know how to handle this, so I throw it again ---
    throw e;
}
catch (RuntimeException e) {  // -- COMPILATION ERROR, THIS CODE WILL NEVER BE EXECUTED ---
    // --- I caught runtime ex. but not NullPointer ---
    log_it( e );
    // --- I don't know how to handle this, so I throw it again ---
    throw e;
}
   

As you, the throw keyword is used now in catch clause.
How the exception throws in this caught?

When is it necessary to use throw in a catch clause?



This is what I understand:

You have a code that might throw exception, You put this code in try clause, Then exception caught by catch clause.

But there are sometimes that you need throw exception explicitly,then you use throw key word, and there is a catch clause that waaits to process this exception.Then, the throw key word must not be in a catch, It must be out of catch.

Please correct me If I'm wrong

[Campbell Ritchie]

Be careful to distinguish the keywords throw and throws. They are used in different contexts, however; the compiler will notice the difference.

It is usually a bad idea to throw and catch an exception in the same place. So you have a method which might suffer something going wrong, and it will include something like

 . . .
throw new XYZException();

In order that any methods using that method can handle that Exception, you mark the method with

throws XYZException

Now any method using it (if it is a checked Exception) must use try-catch, or another throws clause.

By the way: it is usually a bad idea to log and throw the same Exception.
Unchecked Exceptions are better handled with @throws tags in their documentation comments than with the throws keyword.

[abalfazl hossein]

1. public Person(String name)  
   2. {  
   3.     if (name == null) throw new NullPointerException("name cannot be null");  
   4.     this.name = name;  
   5. }  

In this code, You didn't use any catch, Does JVM catch this exception?

Could you put a catch in your code to show how catch this exception by catch clause?

[David Newton]

The JVM catches any exception that isn't caught by your own code. Yes, you can catch a NullPointerException.

[abalfazl hossein]

public class ExceptionDemo
 {
     int i;

     public ExceptionDemo(int i)
     {
         this.i = i;
    }//end constructor

  void divideByZeroAndPrint()throws ArithmeticException  {
      if(i==0)
          throw new ArithmeticException("Integer division by 0");
      System.out.println(12 / i);
    }//end divide . . .
}

public class Main {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
      // try
        // {
             ExceptionDemo ed = new ExceptionDemo(0);
            ed.divideByZeroAndPrint();

}
}

java.lang.ArithmeticException: / by zero
at exceptionthrows.ExceptionDemo.divideByZeroAndPrint(ExceptionDemo.java:22)
at exceptionthrows.Main.main(Main.java:19)
java.lang.ArithmeticException: / by zero
BUILD SUCCESSFUL (total time: 0 seconds)

It must print : "Integer division by 0"

But it does not!Why?

[Campbell Ritchie]

public class ExceptionDemo
 {
     int i;

     public ExceptionDemo(int i)
     {
         this.i = i;
    }//end constructor

  void divideByZeroAndPrint()throws ArithmeticException  {
      if(i==0)
          throw new ArithmeticException("Integer division by 0");
      System.out.println(12 / i);
    }//end divide . . .



    public static void main(String[] args) {

             ExceptionDemo ed = new ExceptionDemo(0);
            ed.divideByZeroAndPrint();

    }
}

Your code is incorrectly indented, still.

java ExceptionDemo
Exception in thread "main" java.lang.ArithmeticException: Integer division by 0
at ExceptionDemo.divideByZeroAndPrint(ExceptionDemo.java:12)
at ExceptionDemo.main(ExceptionDemo.java:21)

I can't see what is wrong with it. Have you saved and recompiled your Java code before execution?

[abalfazl hossein]

[b]throw new ArithmeticException("Integer division by 0"); [/b]

This code must print: Integer division by 0

But it does not, Why?

It is said the statement you use in throw keyword will be printed

[David Newton]

If something catches and logs the stack trace, it will, and Campbell provided output that shows this happening.

[abalfazl hossein]

What do you mean by " logs the stack trace"

Again, I don't understand why this code does not print what is wrote in throw clause?

[David Newton]

It does.