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Eliminate "[unchecked] unchecked call ..." compiler warning Tag(s): Language


import java.util.*;

public class ArrayListGeneric {
  public static void main(String[] args) {
    ArrayList data = new ArrayList();
    data.add("hello");
    data.add("world");


    Iterator<String> it = data.iterator();
    while (it.hasNext()) {
      String s = it.next();
      System.out.println(s);
    }
  }
}
When compiling the above class, the compiler (jdk1.5) emits the following warnings : 
C:\ArrayListGeneric.java:21:
 warning: [unchecked] unchecked call to add(E) as a member of the raw type
  java.util.ArrayList
    data.add("hello");
            ^
C:\ArrayListGeneric.java:22:
 warning: [unchecked] unchecked call to add(E) as a member of the raw type
      java.util.ArrayList
    data.add("world");
            ^
C:\ArrayListGeneric.java:25: warning: [unchecked] unchecked conversion
found   : java.util.Iterator
required: java.util.Iterator
    Iterator it = data.iterator();
                                       ^
3 WARNINGS
Since there are only warnings, your class is ready to run but ... it's not bad idea to eliminate the warnings in production code.

Simply add the expected type (between < and >) after the class. 

import java.util.*;

public class ArrayListGeneric {
  public static void main(String[] args) {
    ArrayList<String> data = new ArrayList<String>();
    data.add("hello");
    data.add("world");


    Iterator<String> it = data.iterator();
    while (it.hasNext()) {
      String s = it.next();
      System.out.println(s);
    }
  }
}

In JDK 1.6, it will be possible to insert a special annotation to suppress this kind of warning, something like : 

import java.util.*;

public class ArrayListGeneric {

  @SuppressWarnings("unchecked")
  public static void main(String[] args) {
    ArrayList data = new ArrayList();
    data.add("hello");
    data.add("world");


    Iterator it = data.iterator();
    while (it.hasNext()) {
      String s = it.next();
      System.out.println(s);
    }
  }
}

A complete list of possible @SuppressWarnings parameters can be found at http://mindprod.com/jgloss/annotations.html.

ANT
When compiling with ANT, you see the following output : 
[javac] Note: Some input files use unchecked or unsafe operations.
[javac] Note: Recompile with -Xlint:unchecked for details.
but it's not easy to see the details of those unsafe operations. To add the -Xlint, you need to use the compilerarg tag in the javac task definition.

Something like 

<target name="compile" description="Compile code">
       <mkdir dir="${bin}"/>
       <mkdir dir="${lib}"/>
       <javac srcdir="${src}" destdir="${bin}"
              includeAntRuntime="no"
              classpathref="lib.path"
              debug="${compile.debug}">
           <compilerarg value="-Xlint:unchecked"/>
       </javac>
</target>