Top 10 Java Compile Time Errors (And How to Fix Them)
Top 10 Java Compile Time Errors (And How to Fix Them)
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Before your Java app can run successfully, it needs to compile — and there’s a host of errors that might get in the way. Understand these 10 most common Java compile errors, and how to address them.
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Content
4.64 -> hey i'm cameron mckenzie and i want to
6.319 -> show you the top 10 most common
8.72 -> compile-time errors that developers in
11.12 -> java run into
12.96 -> now i don't know if this is the most
14.24 -> common compile error but it's certainly
16.48 -> the most common one that new developers
18.48 -> make and it's the fact that the name of
20.32 -> the class file has to match exactly the
22.96 -> name of the file as it exists on the
24.96 -> file system i'm going to open this up in
27.76 -> my windows explorer and you'll see that
30.48 -> the file name here is compile
33.559 -> error00.java but somebody's thought it'd
35.84 -> be a good idea to remove the zero zero
38.16 -> there and make that a lowercase letter
40.879 -> well that's not going to work it has to
42.84 -> match exactly i see a lot of problems
45.2 -> with this especially when people use the
47.36 -> command window terminal window dos
49.36 -> window to write and compile code as
51.92 -> they're learning so that name
54.399 -> and the name on the file system have to
56.079 -> match exactly once they do control s
58.719 -> save the error goes away okay what is
62.16 -> another common error that we see when
64.32 -> we're developing java code well a very
67.52 -> common compile-time error is the fact
69.76 -> that well your java code is case
71.76 -> sensitive so there's an uppercase i on
74.32 -> the int there no that's supposed to be
76.64 -> lowercase there's a lowercase s
78.88 -> no that has to be uppercase i actually
81.28 -> didn't know that java had to be case
83.68 -> sensitive that java was case sensitive
85.439 -> when i first started programming and
88.479 -> i actually gave up on it i gave up on it
90.56 -> for a long time because i didn't know
93.36 -> was case sensitive and i had nobody
94.799 -> there to guide me so this can be very
97.04 -> very frustrating to new developers
98.64 -> especially javascript developers where
101.04 -> you're working with a language that's
102.24 -> not case sensitive but java is this
104.479 -> expresses a lot of meaning about your
106 -> code how things are cased camel case
108.24 -> drama dairy case snake case
110.72 -> so just be careful casing is important
112.96 -> in java
114.64 -> the third most common type of compile
117.36 -> error that i see is missing brackets you
120 -> can see right here well there should be
121.84 -> a round bracket there to match this
123.84 -> round bracket there you always need to
126.24 -> have matching brackets every time you
128.16 -> open a bracket you have to have a
129.52 -> matching close bracket the problem with
131.76 -> this error is that the error messages
133.28 -> aren't always great so look system out
135.44 -> cannot be resolved it's like system out
137.76 -> can't be resolved like that's a
139.12 -> fundamental class then the last one
141.44 -> their duplicate local variable x that's
143.92 -> not helpful at all right those aren't
146.239 -> the errors the error is just the fact
148.4 -> that i don't have that round brace there
150.72 -> and those errors go away but with missed
153.68 -> brackets sometimes you get really weird
155.76 -> errors that are difficult to
157.04 -> troubleshoot the furthermore sometimes
159.12 -> you get error messages in a weird spot
161.12 -> so here the error message says hey add a
164.239 -> bracket to complete the class body on
166.72 -> line 12 but you know the bracket should
168.879 -> be added up on line 10. so even just
171.36 -> where the errors go can throw you off a
173.12 -> little bit that can be frustrating for a
174.64 -> new developer so sometimes if you just
176.959 -> right click and you say source format
179.28 -> code
180.239 -> that can help you identify errors online
182.8 -> linting tools are good as well but yeah
185.2 -> be careful always match your braces
188.239 -> here's another common error
190.08 -> the rule that i always say about java is
192.08 -> that every line of code in java ends
194.64 -> with a semicolon except for the lines of
197.12 -> code that don't end with a semicolon and
199.44 -> that's sort of a tautological statement
201.44 -> but those three statements there did not
204 -> have semicolons on them and that's why
206 -> we had the compile error
208.08 -> but if you add those semicolons in those
210.56 -> errors go away now always joke that
212.56 -> every line of code is a semicolon except
214.319 -> the lines of code that don't have
215.36 -> semicolons because that doesn't have a
216.959 -> semicolon and that doesn't have a
218.72 -> semicolon
220 -> for loops don't have semicolons and
221.84 -> while loops don't have semicolons it's
223.84 -> because these are statements right these
225.84 -> are actual statements to execute and
228.72 -> these are
230.159 -> class code structure comments and even
232.879 -> while loops for loops they're flow
234.4 -> control so those elements don't have
236.72 -> semicolons in them because they don't
238.239 -> terminate a statement they're more
239.84 -> structured but statements like that
241.599 -> always have to have a semicolon
244.959 -> the fifth error here
247.36 -> let's take a look at this code oh this
249.28 -> code looks all good to me what's the
251.12 -> error well left hand side assignment
253.76 -> variable oh boy all sorts of weird
256.239 -> errors here um the error is actually
258.799 -> somebody's put round brackets after a
261.199 -> variable i see that quite often people
264.32 -> that aren't familiar of where to put the
266.08 -> brackets right you only put these round
267.6 -> brackets when you're calling a method or
269.68 -> defining a method sometimes people put
271.759 -> them next to a variable and that can
273.52 -> generate a whole bunch of really
275.919 -> weird errors
277.84 -> that one gets fixed just by
279.919 -> getting rid of those semicolons
282.4 -> also down here you'll see this method
284.08 -> the method x is undefined for the type
286.479 -> compile error 4 like right over here
290.56 -> this here somebody's actually calling a
292.56 -> method just with the wrong name in this
294.639 -> case so this here can sometimes trigger
297.28 -> this error which is the method undefined
301.28 -> but down here
302.88 -> you're definitely got a method undefined
304.479 -> because they're calling the method x and
306.4 -> it should be
307.84 -> x spelled eks
310 -> so sometimes it's literally just a
312 -> method being called incorrectly other
314.32 -> times it's just a piece of code like
316.32 -> that where somebody's actually put the
318.32 -> round brackets on a variable
322.4 -> okay that code is fixed let's go over
325.12 -> here to compile error number five this
327.84 -> one is a duplicate variable declaration
330.32 -> you can't do that once a variable has
332.24 -> been declared and typed you can't
334.56 -> declare and type it again even when i
336.56 -> say type it that's like give it a data
339.12 -> type even if it's the same data type
341.6 -> right so index equals 10 equal intex
343.919 -> equals 20. it's like no no no you
345.919 -> already declared it as an x just leave
347.84 -> it alone right there x equals 20. so
351.52 -> declaring variables a second time is a
354.16 -> very common compile error the duplicate
356.8 -> local variable compile error comes a lot
359.6 -> up a lot so if you do that just find out
361.68 -> where you originally declared that
362.96 -> variable and keep track of it and don't
366.08 -> re-declare that variable in your code
367.68 -> and if you do want to re-declare the
369.919 -> variable we'll just
371.44 -> give it a different variable name and
373.759 -> that will work as well
376.479 -> okay compile error 7
379.52 -> oh look at this one we've got a class
382.16 -> variable and a local variable here and
384.4 -> we're trying to print out the local
385.68 -> variable that all looks good to me
387.36 -> what's the error
388.72 -> boom the local variable may not have
390.8 -> been initialized well anytime you have a
392.8 -> local variable if you ever want to use
394.479 -> it in your code you have to initialize
396.479 -> it so it's an int i can actually just
398.16 -> assign it to zero
400.319 -> and the code goes away right so you just
402.8 -> have to initialize it here it's saying
404.639 -> the local variable may not have been
406.16 -> initialized and just giving it a default
408.479 -> value
410.479 -> fixes it now you might be saying well i
412.639 -> thought all primitive types got
414.96 -> initialized to zero or the equivalent of
417.44 -> zero and i thought all class variables
419.68 -> got initialized to null why is it doing
421.68 -> that well it's because this is a local
423.84 -> variable if you actually have a class
425.44 -> variable those are initialized to zero
427.919 -> so if i change this from local variable
430 -> to
430.96 -> class variable
432.72 -> notice i don't have the error right it
435.199 -> the error actually just goes away so
437.68 -> well here it says you can't make a
439.12 -> static reference to that so i'll make
440.88 -> that variable static
442.96 -> there we go problem goes away
445.84 -> but yeah so it's kind of interesting
447.759 -> class variables instance variables and
449.44 -> class variables static variables those
451.759 -> are initialized by default even if you
453.68 -> don't give them a value they're
454.639 -> initialized to zero
456.24 -> local variables are not so be careful
459.039 -> with that one that's a tricky little
460.88 -> rule in java
463.599 -> over here we've got a variable declared
466.72 -> int x equals 10 and then string square
469.44 -> equals x times x that should be the
471.68 -> square of x but of course this is a
474.4 -> string so i'm getting an error message
476.08 -> here that says type mismatch cannot
478.319 -> convert from into string
480.319 -> and that's because yeah this is an int
483.599 -> multiply an int by an int and you get an
486.319 -> int and we're trying to hold it as a
488.08 -> string so there's an easy way to fix
490.24 -> this you just make your square
493.28 -> the data type that gets returned from
495.12 -> multiplying two ends which is an int
498.24 -> and now everything goes swimmingly
501.12 -> there's actually even another little
502.479 -> trick if you did want this as a string
504.08 -> you could actually go plus x times x and
508.56 -> that would actually cause the int to get
511.52 -> concatenated added to a string and that
514.399 -> would work as well but that's a little
516.8 -> trick
517.919 -> if you want to take those ins and very
519.919 -> quickly convert them into a string but
522.24 -> really
523.2 -> this is the key here if you've got a
525.04 -> variable
526.56 -> you can't change the variable type
528.72 -> dynamically and if you've got a string
531.6 -> well you can't store an int in a string
534.24 -> you'll end up getting that compile error
537.519 -> now over here
538.959 -> point of no return it looks like we've
541.2 -> got a method that returns a string and
542.88 -> then we've got a string right here a b c
545.76 -> d e f u and your anyways um right there
550.08 -> the method must return a result of
551.92 -> string well there's a string there's a
553.519 -> string but aha
555.279 -> there's no return statement
557.519 -> right so this says this method is going
559.36 -> to return a string well we declared a
562 -> string but we didn't explicitly return
564.16 -> it so if your method says that it's
567.2 -> going to return an instance of a certain
569.04 -> type you better have a return statement
571.12 -> in it that actually returns an instance
572.959 -> of that type
574.32 -> alternatively if you don't want to do
575.68 -> that
576.56 -> make it void so actually let me just see
578.64 -> what that error message was before i did
580.56 -> that
582.16 -> the method must return a result of type
584 -> string yep
585.519 -> so optionally if you don't want to
587.04 -> return anything you just make it void
588.8 -> and then the error goes away as well of
590.64 -> course we get a little warning there
592.24 -> because it's going to say when you
593.6 -> declared a variable you didn't do
595.12 -> anything with it but i don't know i feel
597.6 -> that's kind of judgy and
599.92 -> i don't feel like i need to be judged
601.36 -> today
602.32 -> okay and then finally number ten this
605.12 -> only goes up to number nine i thought i
606.8 -> had 10 here my counting must be off
609.2 -> anyways
610.24 -> here's a little bit of logic okay all
612.24 -> looks good we've got a variable do
613.68 -> something if it's less than 10 otherwise
615.76 -> do something else and then when we're
617.68 -> done print out done
620.32 -> then it says unreachable code well of
622.16 -> course that code's unreachable right so
624.64 -> we said if x is less than 10 return true
627.68 -> otherwise return false
629.76 -> well there's no other condition right
631.279 -> either it does this
632.88 -> and if it doesn't do that it does this
635.12 -> either way
636.8 -> you know we've returned we've exited the
639.04 -> method
640.16 -> so this will never get encountered so
642.24 -> yeah we end up getting this message here
643.839 -> unreachable code yeah because that code
645.839 -> will never run
647.519 -> so
649.04 -> move it before the return statements
651.839 -> now
653.2 -> that's actually going to get executed so
655.44 -> click control s and that error goes away
658.399 -> and there you go those are the 10 most
661.44 -> common compile errors that new
663.76 -> developers
664.959 -> and let's face it even experienced
667.2 -> developers often run into
671.12 -> and there you go those are the top 10
672.959 -> compile time errors that java
674.56 -> programmers run into if you enjoyed that
676.56 -> tutorial head over to the server side
678.48 -> dot com we got lots of great tutorials
680.56 -> on java devops and enterprise
682.399 -> programming in general and please
685.519 -> subscribe on youtube
Source: https://www.youtube.com/watch?v=eEnSQV0YY0A