Java: Unchecked Operations
That message that says blah.java uses unchecked or unsafe operations. Recompile with -Xlint:unchecked for details.
What do you do when you get this message after doing ant compile?
compile:
[javac] Compiling 8 source files to /Users/pconrad/github/hfj-chap15/build
[javac] Note: /Users/pconrad/github/hfj-chap15/src/VerySimpleChatServer.java uses unchecked or unsafe operations.
[javac] Note: Recompile with -Xlint:unchecked for details.
How do you Recompile with -Xlint:unchecked
The error message says to Recompile with -Xlint:unchecked but how?
- If you are using plain old
javac, you do it like this:javac -Xlint:unchecked foo.java -
If you are using Ant, you add a
<compilerarg value="-Xlint:unchecked"/>element to yourjavactask, like this:<javac srcdir="${src.dir}" destdir="${classes.dir}" classpathref="libraries"> <compilerarg value="-Xlint:unchecked"/> </javac>
Then what happens?
Here’s the output of runing via the first method (simple javac).
- The file in question is available here: VerySimpleChatServer.java
- That code comes from Chapter 15 of Head First Java
169-231-163-224:src pconrad$ javac -Xlint:unchecked VerySimpleChatServer.java
VerySimpleChatServer.java:46: warning: [unchecked] unchecked call to add(E) as a member of the raw type ArrayList
clientOutputStreams.add(writer);
^
where E is a type-variable:
E extends Object declared in class ArrayList
1 warning
169-231-163-224:src pconrad$
Here’s the same example run via an Ant build.xml:
169-231-163-224:hfj-chap15 pconrad$ ant compile
Buildfile: /Users/pconrad/github/hfj-chap15/build.xml
prepare:
compile:
[javac] Compiling 8 source files to /Users/pconrad/github/hfj-chap15/build
[javac] /Users/pconrad/github/hfj-chap15/src/VerySimpleChatServer.java:46: warning: [unchecked] unchecked call to add(E) as a member of the raw type ArrayList
[javac] clientOutputStreams.add(writer);
[javac] ^
[javac] where E is a type-variable:
[javac] E extends Object declared in class ArrayList
[javac] 1 warning
BUILD SUCCESSFUL
Total time: 0 seconds
169-231-163-224:hfj-chap15 pconrad$
What’s the underlying problem?
The underlying problem will vary dependig on your specific code.
In this particular example, the problem is that clientOutputStreams is an ArrayList when it should be an ArrayList<E> of some specific type, i.e .the type that writer is an instance of.
In particular, writer is an instance of PrintWriter. So we replace lines as follows:
- line 9: ArrayList clientOutputStreams;
+ line 9: ArrayList<PrintWriter> clientOutputStreams;
- line 40: clientOutputStreams = new ArrayList();
+ line 40: clientOutputStreams = new ArrayList<PrintWriter>();
With these changes, we get a clean compile.