47. Common Exceptions

In the following sections we’ll throw some errors and exceptions (on purpose) so that we can check out some of the most common children of java.lang.Throwable you’re likely to see. As a bonus we’ll also discover a few approaches to avoiding them.

java.lang.NullPointerException

The good old NullPointerException (NPE) will haunt your debugging sessions for years to come. Basically it means that you’ve tried to call a method or access a property on an object that isn’t there (i.e. the variable is null). Let’s cause an NPE - it’s easy:

def tmp = null
tmp.class

To avoid these, check for null by:

• Using the Safe Navigation operator (tmp?.class)
• Checking the variable with an if statement before trying to access it

java.lang.AssertionError

This is an Error, not an Exception and occurs when your assert expression evaluates to false:

assert true == false : 'This cannot be'

When your Groovy assertions fail you actually seem to receive a org.codehaus.groovy.runtime.­powerassert.PowerAssertionError - a subclass of java.lang.AssertionError.

The assert statement is usually seen in tests and small scripts. It’s usually better to signal an incorrect state/situation using throws - more about them in a later chapter.

java.lang.NumberFormatException

Groovy can be used as a dynamic language so there are times where you may try to do something to a value that just can’t be done. Trying to convince something that it can be a number when it really can’t be will give you a NumberFormatException:

'Kitten' as Integer

If you really need a variable to be a specific type you could use the class check feature of the switch statement. In the code below I check if value is of a type within the Number family tree before I try to convert it to an Integer. If it isn’t, I could throw and exception or handle it in some other way:

def value = 'kitten'

switch (value) {
    case Number:
        value = value as Integer
        break;
    default:
        println 'I should throw an exception'
}

groovy.lang.MissingPropertyException

This exception happens when you try to access an object’s property but it doesn’t have that property. In the example below, my Person class doesn’t have a name property:

class Person {}
def jim = new Person()
jim.name

Using try-catch around this sort of exception can let you duck type - an approach that uses an object’s properties and methods to determine if something is possible.

I reckon that the hasProperty() method available on all Groovy objects is a cleaner approach than the try-catch option. We can check the object has the property before trying to use it and without causing an exception:

class Person {}
def jim = new Person()

if (jim.hasProperty('name')) {
    jim.name
}

groovy.lang.MissingMethodException

This exception appears when you call a method on an object that doesn’t support the method. The lack of a getName() method on the Person class will cause an exception for poor old jim:

class Person {}
def jim = new Person()
jim.getName()

There’s no hasMethod() method - it’s called respondsTo():

class Person {}
def jim = new Person()

if (jim.respondsTo('getName')) {
    jim.getName()
}

The respondsTo() method just checks if the method exists but we may want to be certain that the methods exists and has the parameter list we’re after. To achieve this we need to call respondsTo() with a second parameter, a list of the method argument types we expect - respondsTo(String name, Object[] argTypes).

java.lang.IndexOutOfBoundsException

These appear when you attempt to get() an index from a list that isn’t there. The code below attempts to get the 5th element from a 3-element list:

def list = [0, 1, 2]
println list.get(5)

Note that if we’d written that code in a slightly different way, we’d get null returned rather than an exception raised:

def list = [0, 1, 2]
println list[5]

Checking list.size() (or the length property) before trying to access is another option:

def list = [0, 1, 2]

if (list.size() >= 5) {
    println list.get(5)
}

Of course the for-in loop will iterate through the list and not try to give you an element that isn’t there.

def list = [0, 1, 2]

for (item in list) {
    println item
}

java.lang.ArrayIndexOutOfBoundsException

If you just had to use an array (instead of a list) then you’ll get an ArrayIndexOutOfBoundsException if you attempt to use an array index that isn’t there:

Integer[] nums = [0, 1, 2]
nums[5]

We can use the length property to make sure we don’t try to access an element that isn’t there:

Integer[] nums = [0, 1, 2]

if (nums.length >= 5) {
    nums[5]
}

The for-in loop is also handy for staying within the bounds:

Integer[] nums = [0, 1, 2]

for (item in nums) {
    println item
}