Java Unchecked Compiler Warnings

When you work with a team of Java developers of varying experience levels, you will often see compiler warnings that are ignored by Java 5 — and thus just as often ignored by the developers who figure that if it isn’t important to Java, it isn’t important for them to fix either. I find the warnings quite annoying, and finally reached the point today where I decided to fix them (sending the developers a message as well).

Java 5.0 (aka “1.5”, aka “Tiger”) introduces “generics” — which provide a way for you to communicate the type of a collection to the compiler, so that it can be checked. Once the compiler knows the element type of the collection, the compiler can check that you have used the collection consistently and can insert the correct casts on values being taken out of the collection. Generics are a good thing. Omitting them will cause these ignorable, yet annoying and avoidable compiler warnings.

Problem #1:

warning: [unchecked] unchecked call to add(E) as a member of the raw type java.util.ArrayList

ArrayList arrUserInfo = new ArrayList(10);
UserInfo userInfo = null;
userInfo = createUserInfo();

The fix below tells the compiler that arrUserInfo should expect only UserInfo objects.

ArrayList<UserInfo> arrUserInfo = new ArrayList<UserInfo>(m_pageSize);

Problem #2:

warning: [unchecked] unchecked call to put(K,V) as a member of the raw type java.util.Hashtable

private Hashtable env = new Hashtable(10);
env.put("java.naming.ldap.factory.socket", "com.foobar.SSLSocketFactory");

Again, tell the compiler what type of objects to expect:

private Hashtable<String, String> env = new Hashtable<String, String>(10);

Problem #3:

Two different warnings about the same code. “warning: [unchecked] unchecked conversion” and “warning: [unchecked] unchecked method invocation: sort(java.util.List,java.util.Comparator) in java.util.Collections…

public class FooBarOptionComparator implements Comparator {}

public static void sortOptions(ArrayList<OptionItem> list) {
    Collections.sort(list, new FooBarOptionComparator());


public class FooBarOptionComparator implements Comparator<Object> {}

It’s really nice to have fewer compile-time warnings!

If you’re still stuck, feel free to read more information about Java generics on Sun’s official documentation website.

3 Responses to “Java Unchecked Compiler Warnings”

  1. James Butler

    Your solution for Problem #2 above does not work with my code since I am not importing the java.util.Hashtable class but my class extends java.util.Hashtbale and uses the super(4); to initialize the Hashtable for four items. Code below:

    import java.applet.AudioClip;

    import javax.swing.*;



    * Loads and holds audio files whose location are

    * specified relative to a fixed base URL.


    public class SoundList extends java.util.Hashtable


    JApplet applet;

    URL baseURL;

    public SoundList(URL baseURL)


    super(4); // Initialize Hashtable with capacity of 4 entries this.baseURL = baseURL;


    public void startLoading(String relativeURL)


    new SoundLoader(this, baseURL, relativeURL);


    public AudioClip getClip(String relativeURL)


    return (AudioClip)get(relativeURL);


    public void putClip(AudioClip clip, String relativeURL)


    put(relativeURL, clip);


    }So what should I do to initialize the super hash table so that it knows the types I will pass to it?


    James Butler

  2. Cod`

    Dude, thankyou so much for posting this. I was going crazy trying to get rid of error messages and all I needed was thanks a bunch.


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