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What is SuppressWarnings ("unchecked") in Java?
(11 answers)
Closed 9 years ago.
I am learning Java. I have task to implement HashDictionary using DictionaryInterface
Hashtable<K,V> h;
Set<K> s = h.keySet();
K elements[]=(K[])Array.newInstance(KeyType, h.size());
When I have written the above statements the Eclipse IDE is showing a warning message.
#SuppressWarnings("unchecked")
If I add the above message before start of a method. The warning is disappeared. What is this means. Can anyone give the reason? Thanks in Advance
Warning in general are just that - warnings. Things that can easily go wrong, but havent technically gone wrong yet. Sometimes you are well aware of them and can simply ignore them. In those cases you might want to use #SuppressWarnings("unchecked"). That will signal the compiler to no post its warning, thus giving you an absolutely clean compile.
Note that even if you dont put that there and you do get the warning your program can still work just fine. Its just that there is an elevated chance that your program will break at that point.
If you want, you can also remove the warning entirely from eclipse by going to Windows > Preferences > General > Editors > Text Editors > Annotations > Warnings > Annotation types and then select the warnings you do/dont want.
The annotation type SuppressWarnings supports programmer control over warnings otherwise issued by the Java compiler. It contains a single element that is an array of String. If a program declaration is annotated with the annotation #SuppressWarnings(value = {someString}), then a Java compiler must not report any warning identified by one of someString if that warning would
have been generated as a result of the annotated declaration or any of its parts.
For example, Unchecked warnings are identified by the string unchecked. In your example, the below statement will emit an unchecked warning:
K elements[]=(K[])Array.newInstance(KeyType, h.size());
This statement contains an unchecked conversion, as the compiler doesn't know if the cast is safe or not; and will simply emit an unchecked conversion warning. Which can be suppressed by using #SuppressWarnings("unchecked") annotation.
In case you don't already know (you said you're a beginner), Eclipse often tells you what is causing the warning and can sometimes give you suggestions on how to fix it.
Of course, it's always much better to fix the warnings than to suppress them (unless you're doing really intricate stuff). Hope this helps.
Related
I'm new to IntelliJ IDEA. I used to use Eclipse.
By default, IntelliJ doesn't ask to give parameter type for Java Generics Type.
// Following code doesn't give any warning
List list = null;
// I want to make IntelliJ ask me to specify type parameter (say Long)
List<Long> list = null; // i.e If I want a List of Long
How can I set up IntelliJ so that it ask me to give generic type parameters while using generics java types?
I'm using JDK 8 if it matters.
IntelliJ provide an inspection name Raw use of parameterized class
After enabling that inspection.
In the Project Structure menu, under Project, check the language level setting. Setting it to 6.0 will cause the IDE to give you an error when you are missing the identifier in the diamond block.
Setting it to 7.0 or higher will suppress it.
Addition based on further exchange:
Under File -> Settings -> Inspections -> General if the box that says Unchecked warning is checked, when you run Analyze -> Inspect code, it will report the condition noted under General -> Unchecked warnings as "Unchecked call to add(E) as a member of raw type 'java.util.List'", in this example. If you declare the generic type of List explicitly, this warning will not show up in the report.
I am not aware of a way to get it to do exactly what you are asking for; however, once you would instantiate your List and then add an element to it IntelliJ will mark it with a yellow squiggly line and warn you it was unchecked. If you would put your cursor anywhere on the yellow squiggly and press Alt-Enter it will offer you "Try to generify YourClassName", press enter and it will infer the type and modify your code.
Before:
After:
I have a class called SQLProvider, it contains methods for both opening and closing a SQLite database connection. Using annotations or another approach, is it possible to flag a compiler warning if the open method is used without also calling close?
{
SQLProvider provider = new SQLProvider();
provider.open()
// display a compiler warning unless provider.close() is also invoked in this code block
}
I am not sure it would be the best approach but you can make your class implement Closeable interface . As per the Eclipse documenatation, Eclipse will display warning :
When enabled, the compiler will issue an error or a warning if a local variable holds a value of type 'java.lang.AutoCloseable' (compliance >= 1.7) or a value of type 'java.io.Closeable' (compliance <= 1.6) and if flow analysis shows that the method 'close()' is not invoked locally on that value.
There is currently no such facility in the standard Java toolchain.
In addition to the Eclipse compiler warnings (see New Idiot's answer), some static code analysers can warn you about this:
For PMD, the CloseResource design rule covers this (or at least some subcases ...)
FindBugs can also do this; e.g. FindBugs - "may fail to close stream" when using ObjectOutputStream
But the problem is these warnings are more or less heuristic. They only understand certain standard idioms for reliably closing resources. If you do it some other way (which is still provably reliable) they are likely to produce a false warning. This is not necessarily bad (because doing this a non-standard way is liable to fool / puzzle future readers!). However the possibility of false positives may tempt developers to ignore or (worse still) suppress the warnings.
There are a couple of additional issues:
These checks may give a false negative for resource objects that have a close() method without implementing Closeable or AutoCloseable.
These checks may give a false negative for Closeable / AutoCloseable resource objects where the close() operation is a no-op; for example, StringWriter.
These issues may explain why the standard Java toolchain doesn't support this.
Consider a private method which is called from JNI and not used otherwise, generating a compiler warning about an unused method:
private void someMethodCalledOnlyFromJNI() { // WARNING: method is never used
// ....
}
This is some legacy code in Java 1.4 - so no dice on #SuppressWarnings.
What hack would you use to suppress this compiler warning?
Edit: Obviously this is just a warning and it can easily be ignored. But, personally, I hate warnings in my code just as much as I don't want to see errors in it. AFAIC - my code should have 0 warnings, it might be an exaggeration, but I am very pedantic about this.
Just as an example, someone might see this function, not know it is used from JNI, and simply delete it.
Ignore it. It is a warning, after all - best option
use protected (and add a comment for the reason why) - bearable
Make a dummy method just above it and make the two call each other (again with comments) - ugly
configure the IDE not to show this warning at all (in eclipse it is Windows > Preferences > Java > Compiler > Errors/Warnings) - not preferable
As per your update: having 0 warnings is not a goal you should set. The number of warnings depends on the settings, so if you don't all have unified IDEs, this number will vary. And then you can add checkstyle / PMD to report warnings as well - then you'll have even more. The reasonable behaviour is to have a warnings treshold.
If you don't want anyone to delete this method, just add a comment:
// This method is used is used by JNI. (Don't delete)
Somewhere else in the class:
if(Boolean.FALSE.booleanValue())
{ // prevents warning for unused private method which is used from JNI
someMethodCalledOnlyFromJNI();
}
(can't use simple false because that results in dead code warning).
Either just ignore the warning, or declare it as protected instead. If you go for protected and want to prevent subclassing/overriding as well, then declare it final as well.
To start with, its only a warning, thus it should not be an issue for you.
You could either mod the code to remove that function thus removing the problem.
Or just call it from some where at the start/end of your code and ignore any results. As long as it is not going to try to set up any thing that will affect the rest of your program you will be fine.
you can make it public. if it's legacy code I am sure no one will complain :)
What does this error mean?
Note: Main.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
Any suggestions how to avoid that kind of error?
That's the error you get when you use Collections without specifying a type. You probably have something like:
ArrayList myList = new ArrayList(); // or some other Collection class
If that's the case, you need to change that to specify what type of objects you want to store. For example:
ArrayList<String> myList = new ArrayList<String>();
Read up on Java Generics for more information.
This is my best guess without seeing your code and the full error message. There could be other causes for that message, this is just the problem that I've seen accompany that message before.
First, recompile with -Xlint:unchecked to see what the problem is. Then fix those problems. There are a number of potential high-level causes for unchecked warnings. One is that you didn't provide type parameters where you should have. There are some situations where they are unavoidable, and then you can suppress the specific warning, but these are the exception, and care must be taken that you aren't suppressing warnings that are really important.
So recompile with -Xlint:unchecked and post additional questions if you have trouble with any of the specific issues that are revealed.
What does it mean?
Java generics allow you to write something like this:
List<String> l = ...;
String s = l.get(0); // note there is no explicit typecast.
But if the compiler tells you that your code has "unchecked or unsafe operations", it is saying that you have broken the rules for using generics safely, and your code may give runtime class cast exceptions at unexpected places; e.g. in the statement above where we left out the typecast.
There are a few things that will cause the compiler to complain about unchecked or unsafe operations, and each one requires a different remediation. Do what the compiler is telling you and run it with the -Xlint option.
I have the following code
String innerText = null;
innerText = this.getException(detail.getChildElements());
causing this warning
Type safety: The expression of type Iterator needs unchecked conversion to conform
to Iterator
The referenced method is
private String getException(Iterator<OMElementImpl> iterator) { ... }
The other method, getChildElements(), is in a JAR file that I can't touch. There are no other warnings or errors.
From Googling, it seems like the usual way to get rid of this sort of warning is
#SuppressWarnings("unchecked")
String innerText = this.getException(detail.getChildElements());
because the compiler can't guarantee safety ahead of time, but I'd prefer to avoid using SuppressWarnings if possible... is there a better way?
EDIT: getChildElements() is documented here
You can suppress the warning, but if you do so, you are relying 100% on the third-party library, and discarding the assurance of Java generic types: that any ClassCastException raised at runtime will occur right at an explicit cast.
Our coding standard is to suppress warnings only when we can prove the code is type safe—and we treat any calls outside the package as a black box, and don't rely on any comments about the content of a raw collection. So, suppression is extremely rare. Usually, if the code is type safe, the compiler can determine it, although sometimes we have to give it some help. The few exceptions involve arrays of generic type that don't "escape" from a private context.
If you don't fully trust the third-party library, create a new collection, and add the contents after casting them to OMEElementImpl. That way, if there is a bug in the library, you find out about it right away, rather than having some code far distant in time and space blow up with a ClassCastException.
For example:
Iterator<?> tmp = detail.getChildElements();
Collection<OMElementImpl> elements = new ArrayList<OMElementImpl>();
while (tmp.hasNext())
elements.add((OMElementImpl) tmp.next()); /* Any type errors found here! */
String innerText = getException(elements.iterator());
Remember, generics were not invented to make code look pretty and require less typing! The promise of generics is this: Your code is guaranteed to be type-safe if it compiles without warnings. That is it. When warnings are ignored or suppressed, code without a cast operator can mysteriously raise a ClassCastException.
Update: In this case, especially, it seems extremely risky to assume that the result of getChildElements is a iterator of OMElementImpl. At best, you might assume that they are OMElement, and that's only implied from the class, not anything on the method in particular.