junit testing - assertEquals for exception

Question

How can I use assertEquals to see if the exception message is correct? The test passes but I don't know if it hits the correct error or not.

The test I am running.

@Test
public void testTC3()
{
    try {
    assertEquals("Legal Values: Package Type must be P or R", Shipping.shippingCost('P', -5));
    } 
    catch (Exception e) {
    }        
}

The method being tested.

public static int shippingCost(char packageType, int weight) throws Exception
{
    String e1 = "Legal Values: Package Type must be P or R";
    String e2 = "Legal Values: Weight < 0";
    int cost = 0;
        if((packageType != 'P')&&(packageType != 'R'))
        {
             throw new Exception(e1);
        }

        if(weight < 0)
        {
             throw new Exception(e2);
        }        
         if(packageType == 'P')
         {
             cost += 10;
         }
         if(weight <= 25)
         {   
             cost += 10;
         }
         else
         {
            cost += 25;
         }
         return cost;       
}

}

Thanks for the help.

Answer 1

try {
    assertEquals("Legal Values: Package Type must be P or R", Shipping.shippingCost('P', -5));
    Assert.fail( "Should have thrown an exception" );
} 
catch (Exception e) {
    String expectedMessage = "this is the message I expect to get";
    Assert.assertEquals( "Exception message must be correct", expectedMessage, e.getMessage() );
}   

Answer 2

The assertEquals in your example would be comparing the return value of the method call to the expected value, which isn't what you want, and of course there isn't going to be a return value if the expected exception occurs. Move the assertEquals to the catch block:

@Test
public void testTC3()
{
    try {
        Shipping.shippingCost('P', -5);
        fail(); // if we got here, no exception was thrown, which is bad
    } 
    catch (Exception e) {
        final String expected = "Legal Values: Package Type must be P or R";
        assertEquals( expected, e.getMessage());
    }        
}