Null Pointer Exceptions In Java - What EXACTLY They Are and How to Fix Them
Summary
TLDRIn this Java tutorial, John, a lead Java software engineer, explains the common issue of NullPointerExceptions. He illustrates with examples how uninitialized or null-assigned variables can cause this exception. John advises using Java primitives over wrapper classes to avoid null values and suggests initializing variables properly. He also recommends returning empty collections instead of null to prevent NullPointerExceptions. For debugging, he highlights how Java 14's improved error messages can pinpoint the cause of the exception. Finally, John offers strategies for avoiding NullPointerExceptions, like null checks and proper initialization.
Takeaways
- đ± NullPointerException is one of the most common and troublesome exceptions in Java, often causing many bugs.
- đ NullPointerException occurs when you try to use an object reference that is not pointing to any object (null).
- đ« Calling methods or accessing fields on a null object will result in a NullPointerException.
- đšâđ» Always initialize variables properly and avoid setting them to null unless necessary.
- đ Recent versions of Java (starting from Java 14) provide detailed error messages for NullPointerExceptions, making it easier to identify the cause.
- đ Use primitive types (e.g., boolean) instead of their wrapper classes (e.g., Boolean) when possible, as they can't be set to null.
- đ When working with collections, initialize them to an empty list instead of null to avoid NullPointerExceptions during iterations.
- đ Use null checks before performing operations on variables that might be null to prevent exceptions.
- đĄ For string comparisons, use `"literal".equals(variable)` instead of `variable.equals("literal")` to avoid null-related errors.
- đ If you encounter a NullPointerException, either ensure the variable is properly initialized or use null checks to handle potential null values.
Q & A
What is a Null Pointer Exception in Java?
-A Null Pointer Exception occurs when you try to call a method or access a field on an object reference that has been set to null, meaning it points to no object in memory.
Why is the Null Pointer Exception common in Java?
-It's common because Java allows object references to be null, and developers might not always check for null before using these references, leading to attempts to perform operations on a non-existent object.
How can you cause a Null Pointer Exception with a Cat object in Java?
-You can cause a Null Pointer Exception by setting a Cat object reference to null and then trying to call a method on it, like 'myCat.makeNoise()'.
What does 'null' signify in Java?
-In Java, 'null' signifies the absence of an object reference, meaning the reference does not point to any object in memory.
How do you fix a Null Pointer Exception when it occurs?
-To fix a Null Pointer Exception, you can either initialize the null variable with an actual object or add a null check before performing operations on the variable.
What is the benefit of using Java primitives over wrapper classes when dealing with null?
-Java primitives like 'boolean' cannot be null, whereas wrapper classes like 'Boolean' can. Using primitives avoids the risk of Null Pointer Exceptions.
Why might initializing a collection to null cause a Null Pointer Exception?
-Initializing a collection to null and then trying to iterate over it or access elements can cause a Null Pointer Exception because the collection itself is null and does not contain any elements.
How do recent versions of Java help when a Null Pointer Exception occurs?
-Recent versions of Java provide more detailed error messages that specify which method or field access caused the exception, making it easier to identify and fix the problem.
What is a null check and why is it important?
-A null check is a condition that verifies if an object reference is null before performing operations on it. It's important to prevent Null Pointer Exceptions when dealing with potentially null references.
What strategies can you use to avoid Null Pointer Exceptions in your Java code?
-Avoid initializing variables to null, return empty collections instead of null from methods, and use primitive types over wrapper classes to reduce the likelihood of encountering null values.
How can you safely compare a string to a string literal in Java without causing a Null Pointer Exception?
-You can safely compare a string to a string literal by using the string literal on the left side of the equals method, so if the string is null, the literal will not cause a Null Pointer Exception.
Outlines
đŸ Understanding Null Pointer Exceptions in Java
This paragraph introduces the concept of a Null Pointer Exception (NPE) in Java, which is a common source of bugs. The speaker, John, a lead Java software engineer, explains NPEs with an example involving a 'Cat' object. He demonstrates that when a variable like 'myCat' is set to null instead of referencing an actual object, attempting to call a method on it results in an NPE. The explanation includes the difference between a variable referencing an object and being null, which means it points to nothing. John also discusses other scenarios that can lead to NPEs, such as using a Boolean wrapper class set to null or trying to access elements from a null collection.
đ Diagnosing and Fixing Null Pointer Exceptions
In this section, the video script discusses how to address Null Pointer Exceptions once they occur. John explains that modern Java versions provide detailed error messages that can help pinpoint the exact cause of an NPE. He suggests two approaches to fixing NPEs: initializing the null variable with a value, or performing a null check before using the variable. The paragraph provides examples of both methods, emphasizing the importance of null checks in complex method chains and the use of empty collections instead of null to avoid exceptions.
đ Strategies to Prevent Null Pointer Exceptions
The final paragraph focuses on preventative measures to avoid Null Pointer Exceptions. John advises against initializing variables to null unless absolutely necessary and suggests returning empty collections instead of null from methods. He also provides a tip for string comparisons, recommending the reversal of the comparison to avoid NPEs when dealing with potentially null strings. The paragraph concludes with a call to action for viewers to like, subscribe, and check out a full Java course for more in-depth learning.
Mindmap
Keywords
đĄNull Pointer Exception
đĄReference
đĄMemory
đĄWrapper Class
đĄPrimitive Type
đĄNull Check
đĄMethod Invocation
đĄCollection
đĄEmpty List
đĄString Comparison
Highlights
Java's null pointer exception is one of the most common and problematic bugs.
A null pointer exception occurs when a variable set to null is used to call a method.
In Java, a variable is a reference to an object in memory, not the object itself.
Setting a variable to null means it points to nothing, causing a null pointer exception when a method is called on it.
Using Java's primitive types can prevent null pointer exceptions, as they cannot be set to null.
Java's wrapper classes, like Boolean, can be set to null and may cause null pointer exceptions.
Initializing collections to empty instances instead of null can prevent null pointer exceptions.
Java 14 and later provide detailed error messages for null pointer exceptions, aiding in debugging.
To fix a null pointer exception, ensure variables are initialized before use.
Performing null checks before method calls can prevent null pointer exceptions.
Avoid initializing variables to null unless absolutely necessary.
Returning empty collections instead of null from methods can prevent null pointer exceptions.
Flipping string comparisons can avoid null pointer exceptions when dealing with potentially null strings.
Always use null checks when dealing with external code where null values might be present.
The video provides a full Java course for further learning.
The video encourages viewers to subscribe for more Java tutorials.
Transcripts
if you've been coding in java for any
length of time you've definitely run
into the dreaded null pointer exception
it's one of the most common exceptions
and probably the biggest source of bugs
in all of java in this video we'll talk
about what exactly causes a null pointer
exception how to fix it when you
encounter one and strategies for
avoiding them in the first place my name
is john and i'm a lead java software
engineer i also have a full java course
available in a link down in the
description so go check it out first
what exactly causes a null pointer
exception let's talk about it with an
example let's say you're coding along
and at some point you decide you need a
new cat object so you go ahead and
create cat my cat equals new cat and
let's say you want to make that cat make
noise so you go ahead and call it's make
noise method with my cat dot make noise
and we can see that in that method it
just prints out meow so of course if we
go back here and run our program that's
exactly what it does it just prints out
meow but what if instead of
instantiating this my cat variable to a
new cat we instead set it to null let's
go ahead and run it and see what happens
and oh no we get the dreaded null
pointer exception now why exactly is
that sure we can see that we're setting
our mycat variable to null but what
exactly does null mean and why does our
code now result in a null pointer
exception to visualize it so what we
have in our code is a mycat variable
right that my cat variable isn't a cat
object itself but instead it's a
reference to a cat object in memory so
when we instantiate our mycat variable
to a new cat that means this my cat
variable is now a reference to an actual
cat object in memory so after that when
we call my cat dot make noise it has no
problem doing that at all our my cat
variable references an actual cat object
in memory and so we're able to tell this
cat object to make noise with no problem
but what happens if instead we set it to
null well instead of my cat pointing to
an actual cat object in memory it's
instead null which essentially means
it's pointing at nothing not even like
some empty box with no cat in it it's
pointing to nothing at all this is a
null pointer so when we call the make
noise method on our my cat variable that
has been set to null there's no cat to
make noise we have a mycat variable but
it's not pointing to any actual cat
object there's nothing to meow all we
have is a null pointer and so we get a
null pointer exception in our code most
of the time when we get a null pointer
exception this is the reason we have
some variable that's been set to null
and we try to call some method on it
more generally speaking just having some
variable set to null won't give you a
null pointer exception but you will get
a null pointer exception if you try to
do pretty much anything with a null
variable for example if you try to get
or set a field on that object like my
cat dot age equals three we get a null
pointer exception for essentially the
same reason there is no actual cat
object so we can't set its age to three
there are other things that can give you
a null pointer exception that you might
not expect for example what if we had a
boolean just called should print hello
and right now let's just initialize it
to true then we can have some if
statement so if should print hello then
we will print out hello so obviously if
we run that it will print out hello but
what will happen if instead of setting
this to true or false we initialize it
to null well as you may have guessed we
get a null pointer exception if you're
using a capital b boolean variable this
is the wrapper class for jabba's
lowercase b primitive boolean a
primitive boolean like all java
primitives can't be set to null as we
can see by the error we're getting here
but a capital b boolean the wrapper
class can be set to null and if you do
have a capital b boolean that is set to
null and you try to evaluate it in an if
statement it'll give you a null pointer
exception best practice in this
situation is to always use the java
primitive boolean whenever possible a
primitive boolean will never be null
it'll always be true or false another
way you might get an old pointer
exception that you may not expect is
let's say you have a collection like a
list of strings called like favorite
cheeseburgers and let's say you wanted
to use a for each loop to loop through
those cheeseburgers and just print them
all out so four string burger
in favorite cheeseburgers we're just
going to print out that burger what
happens is if this favorite
cheeseburgers list has been set to null
you get an old pointer exception so best
practice in these sorts of situations is
even if you have nothing to put in that
list instead of initializing it to null
you should just set it to be some empty
list so like new arraylist so when you
run this program with an empty list
instead of a null list which are two
very different things you don't get any
null pointer exception so all of that is
of course good to know right but what if
you're running into a null pointer
exception in your code right now how
should you go about fixing it well the
great news is with recent versions of
java starting with java 14 when you get
a null pointer exception the error
message that it prints out is enormously
helpful so if we go ahead and run this
our original code if we take a closer
look at this null pointer exception when
it's thrown it says cannot invoke cat
dot make noise because my cat is null
and it gives an exact line number in our
class in line 9. with earlier versions
of java all it would tell you is the
line number so you would basically just
see this part you'd see that there's a
null pointer exception and you could see
which line it was on but it wouldn't say
anything about which method it was
trying to call or which variable was
null that caused the problem now that
might be enough in a simple case like
this right where on line nine there's
only one thing that can be null that can
cause this problem it would have to be
when this my cat object is null and it
tries to call the make noise method on
it but there are plenty of other times
when just the line number wasn't enough
to know exactly what was going wrong for
example what if in our code we had
something like this if we got an error
and all it said was there was a null
pointer exception on line 36
we have no idea what's actually causing
the problem it could be that this cat's
variable is null and it gets a null
pointer exception when it tries to call
the get method on it or it could be that
this zeroth element in the cats list is
null and it gets a null pointer
exception when it tries to call the get
name method on it or it could be that
neither of these are null but instead
the name of that first cat in that list
is null and you get a null pointer
exception when you try to call the dot
length method on it but now with our
nice modern error messages we can scroll
over here and see okay cannot invoke
string.length because the return value
of cat.getname is null so once you know
what exactly is causing your
nullpointerexception how do you go about
fixing it you basically have two options
the first option is you can identify the
exact variable that's null that is
causing you problems and initialize it
with some value if the variable causing
your null pointer exception should never
be null this is what you'll need to do
so here i can look at the code that was
calling this method and see that okay
i'm creating this new cat object and i'm
adding it to my list but i never gave it
a name and that's why down here when
it's trying to get the name of that cat
object and then getting its length it's
causing a null pointer exception so just
back up here where i'm creating my cat
object i just need to set
the name on this my other cat so just my
other cat dot set name to jerry so now
when we run the program there are no no
pointer exceptions and it's correctly
printing out the length of the name that
we gave our cat and jerry has five
characters so it's printing out five but
what do you do if you don't have any
control over whether the variable that
you're working with is null or not or
maybe the variable being null is just a
valid situation that you have to deal
with the simplest way to fix it in that
situation is to just add a piece of code
that checks to see if that variable is
null and only performs the operation on
that variable if it's not null this is
commonly referred to just as a null
check so from our first example we know
that if this mycat variable is null if
we try to call the make noise method on
it we're going to get a null pointer
exception so what we can do is add just
a simple null check so we can say if my
cat is not equal to null
then we can call the make noise method
on it you'll see this kind of coding
pattern all over for the rest of your
java coding career it might seem overly
simplistic but there's really no need to
complicate things if a simple piece of
code like this will do the trick now if
you have a long chain of method calls
like this one and any one of these
values might be null you might need
multiple null checks which can get a
little bit nasty looking so to make sure
we can call this.length method at the
end first we need to make sure that this
getname method won't return null but to
make sure that we can even call this
getname method we have to make sure that
this get method won't return null and to
make sure that that won't cause a null
point or exception we first need to make
sure that cats is not null so that means
that we have to have a pretty large
series of null checks so we can say if
cats is not equal to null and
cats.get zero is not equal to null and
cats.get
zero
dot get name is not equal to null
then we know that we can successfully
return this value as the length of that
name otherwise maybe we'll just return
zero so this can definitely get kind of
ugly but sometimes you don't really have
a good way around it and the only way
you can make sure that you won't get a
null pointer exception is just having a
large series of null checks so now we
know how we can fix null pointer
exceptions when we get them but what are
some strategies for avoiding them in the
first place so in situations where you
have just a local variable like this
where you're declaring it and
instantiating it right in the same
method that you're using it you have
complete and full control in general
just try to avoid initializing variables
like that to null it's almost always
best to just initialize it properly
right there when you declare it there
are some very specific situations where
you might have to initialize a variable
to null just take very good care to make
sure that it is always getting
initialized to some actual object
somewhere down the line before you use
it also try to avoid returning null as
the return value in methods that you
write if at all possible for example if
you have some method that returns like a
collection right so this method here git
cats returns some list of cats so what
if at the end of this method ah you
don't have any cats that you want to
return you might think well it's a good
idea to just return null well that's
okay but then in whatever piece of code
that's calling this method it has to
deal with the possibility of the result
of this method being null what you can
do instead is just return an empty
collection so here we're using a list so
instead of returning null we can just
return a new arraylist as we saw earlier
an empty list won't give you any null
pointer exceptions when you try to loop
through it but a null list will another
small tip often you'll find yourself
having to do some string comparisons
where you need to take some string that
you have and compare it to some constant
value or some string literal for example
let's say we just wanted to take the
name of this my other cat object and we
wanted to see if it was equal to some
string literal which is just a string
that you write out literally in quotes
in the code like this so if we wanted to
see if this name was equal to newman so
we could write something like this so we
can say if
my other cat.getname
then we print out hello newman so this
code will work fine right but what will
happen if instead of setting the name to
jerry we have it set to null well if we
try and run that we're going to get a
null pointer exception and we can see
that it says we can't invoke string dot
equals because the return value of
cat.getname
is null so what's happening here is
get myothercat.getname is returning null
because that cat's name has been set to
null so when we try to call the dot
equals method on that null name we get a
null pointer exception so what you can
do instead to eliminate the possibility
of a null pointer exception in this
scenario is just flip this comparison
around so instead of saying my other cat
dot get name dot equals newman we can
say newman as a string literal in quotes
dot equals and pass in my other cat dot
get name so now when we run our code
just by flipping around that comparison
we no longer get a null pointer
exception this string literal newman is
of course not null we can see the string
value right there so there's no chance
of a null pointer exception when we call
dot equals on it and if this my other
cat dot get name does return null here
it doesn't cause any problems this
equals method just compares newman to
null and sees that they're not equal so
this equals method just ends up
returning false and everything just
works as it should with no exceptions
sometimes though you just don't have
complete control over the code that
you're having to deal with in those
cases it's best to just do a simple null
check before doing anything with that
variable if you enjoyed this video or
learned something please let me know by
leaving a like and be sure to subscribe
so you don't miss each new java tutorial
and be sure to check out the full java
course in the link down in the
description thank you so much for
watching i really do appreciate you
being here with me i'll see you next
time
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