What causes javac to issue the "uses unchecked or unsafe operations" warning

java generics warnings
toolbear picture toolbear · Oct 13, 2008 · Viewed 415.5k times · Source

For example:

javac Foo.java
Note: Foo.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

Answer

Bill the Lizard picture Bill the Lizard · Oct 13, 2008

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, just 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<>();

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