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

import java.util.*;
public class PrintingContainers
{
  static Collection fill(Collection c) {
    c.add("dog");
    c.add("dog");
    c.add("cat");
    return c;
  }
  
  static Map fill(Map m) {
    m.put("dog","Bosco");
    m.put("dog","Spot");
    m.put("cat","Rags");
    return m;
  }

  public static void main(String[] args) {
    System.out.println(fill(new ArrayList()));
    System.out.println(fill(new HashSet()));
    System.out.println(fill(new HashMap()));
  }
};

>javac -X
  -Xlint                     Enable recommended warnings
  -Xlint:{all,deprecation,unchecked,fallthrough,path,serial,finally,-deprecation
,-unchecked,-fallthrough,-path,-serial,-finally}Enable or disable specific warni
ngs
  -Xbootclasspath/p:<path>   Prepend to the bootstrap class path
  -Xbootclasspath/a:<path>   Append to the bootstrap class path
  -Xbootclasspath:<path>     Override location of bootstrap class files
  -Djava.ext.dirs=<dirs>     Override location of installed extensions
  -Djava.endorsed.dirs=<dirs>Override location of endorsed standards path
  -Xmaxerrs <number>         Set the maximum number of errors to print
  -Xmaxwarns <number>        Set the maximum number of warnings to print
  -Xstdout <filename>        Redirect standard output

qileroro wrote:
我加了-Xlint参数，不行，看下面的错误,我用的是JDK1.5 ，请问是怎么回事呢？

PrintingContainers.java:16: warning: [unchecked] unchecked call to put(K,V) as a
member of the raw type java.util.Map
m.put("cat","Rags");
^
6 warnings