What causes javac to issue the "uses unchecked or unsafe operations" warning
For Android Studio, you need to add:
allprojects {
gradle.projectsEvaluated {
tasks.withType(JavaCompile) {
options.compilerArgs << "-Xlint:unchecked"
}
}
// ...
}
in your project's build.gradle file to know where this error is produced.
If you do what it suggests and recompile with the "-Xlint:unchecked" switch, it will give you more detailed information.
As well as the use of raw types (as described by the other answers), an unchecked cast can also cause the warning.
Once you've compiled with -Xlint, you should be able to rework your code to avoid the warning. This is not always possible, particularly if you are integrating with legacy code that cannot be changed. In this situation, you may decide to suppress the warning in places where you know that the code is correct:
@SuppressWarnings("unchecked")
public void myMethod()
{
//...
}
This warning means that your code operates on a raw type, recompile the example with the
-Xlint:unchecked
to get the details
like this:
javac YourFile.java -Xlint:unchecked
Main.java:7: warning: [unchecked] unchecked cast
clone.mylist = (ArrayList<String>)this.mylist.clone();
^
required: ArrayList<String>
found: Object
1 warning
docs.oracle.com talks about it here: http://docs.oracle.com/javase/tutorial/java/generics/rawTypes.html
This comes up in Java 5 and later if you're using collections without type specifiers (e.g., Arraylist()
instead of ArrayList<String>()
). It means that the compiler can't check that you're using the collection in a type-safe way, using generics.
To get rid of the warning, you need to be specific about what type of objects you're storing in the collection. So, instead of
List myList = new ArrayList();
use
List<String> myList = new ArrayList<String>();
In Java 7 you can shorten generic instantiation by using Type Inference.
List<String> myList = new ArrayList<>();