How do I address unchecked cast warnings?

The obvious answer, of course, is not to do the unchecked cast.

If it's absolutely necessary, then at least try to limit the scope of the @SuppressWarnings annotation. According to its Javadocs, it can go on local variables; this way, it doesn't even affect the entire method.

Example:

@SuppressWarnings("unchecked")
Map<String, String> myMap = (Map<String, String>) deserializeMap();

There is no way to determine whether the Map really should have the generic parameters <String, String>. You must know beforehand what the parameters should be (or you'll find out when you get a ClassCastException). This is why the code generates a warning, because the compiler can't possibly know whether is safe.

Unfortunately, there are no great options here. Remember, the goal of all of this is to preserve type safety. "Java Generics" offers a solution for dealing with non-genericized legacy libraries, and there is one in particular called the "empty loop technique" in section 8.2. Basically, make the unsafe cast, and suppress the warning. Then loop through the map like this:

@SuppressWarnings("unchecked")
Map<String, Number> map = getMap();
for (String s : map.keySet());
for (Number n : map.values());

If an unexpected type is encountered, you will get a runtime ClassCastException, but at least it will happen close to the source of the problem.

Wow; I think I figured out the answer to my own question. I'm just not sure it's worth it! :)

The problem is the cast isn't checked. So, you have to check it yourself. You can't just check a parameterized type with instanceof, because the parameterized type information is unavailable at runtime, having been erased at compile time.

But, you can perform a check on each and every item in the hash, with instanceof, and in doing so, you can construct a new hash that is type-safe. And you won't provoke any warnings.

Thanks to mmyers and Esko Luontola, I've parameterized the code I originally wrote here, so it can be wrapped up in a utility class somewhere and used for any parameterized HashMap. If you want to understand it better and aren't very familiar with generics, I encourage viewing the edit history of this answer.

public static <K, V> HashMap<K, V> castHash(HashMap input,
                                            Class<K> keyClass,
                                            Class<V> valueClass) {
  HashMap<K, V> output = new HashMap<K, V>();
  if (input == null)
      return output;
  for (Object key: input.keySet().toArray()) {
    if ((key == null) || (keyClass.isAssignableFrom(key.getClass()))) {
        Object value = input.get(key);
        if ((value == null) || (valueClass.isAssignableFrom(value.getClass()))) {
            K k = keyClass.cast(key);
            V v = valueClass.cast(value);
            output.put(k, v);
        } else {
            throw new AssertionError(
                "Cannot cast to HashMap<"+ keyClass.getSimpleName()
                +", "+ valueClass.getSimpleName() +">"
                +", value "+ value +" is not a "+ valueClass.getSimpleName()
            );
        }
    } else {
        throw new AssertionError(
            "Cannot cast to HashMap<"+ keyClass.getSimpleName()
            +", "+ valueClass.getSimpleName() +">"
            +", key "+ key +" is not a " + keyClass.getSimpleName()
        );
    }
  }
  return output;
}

That's a lot of work, possibly for very little reward... I'm not sure if I'll use it or not. I'd appreciate any comments as to whether people think it's worth it or not. Also, I'd appreciate improvement suggestions: is there something better I can do besides throw AssertionErrors? Is there something better I could throw? Should I make it a checked Exception?

In Eclipse Preferences, Go to Java->Compiler->Errors/Warnings->Generic types and check the Ignore unavoidable generic type problems check-box.

This satisfies the intent of the question, i.e.

I'd like to avoid Eclipse warnings...

if not the spirit.

You can create a utility class like the following, and use it to suppress the unchecked warning.

public class Objects {

    /**
     * Helps to avoid using {@code @SuppressWarnings({"unchecked"})} when casting to a generic type.
     */
    @SuppressWarnings({"unchecked"})
    public static <T> T uncheckedCast(Object obj) {
        return (T) obj;
    }
}

You can use it as follows:

import static Objects.uncheckedCast;
...

HashMap<String, String> getItems(javax.servlet.http.HttpSession session) {
      return uncheckedCast(session.getAttribute("attributeKey"));
}

Some more discussion about this is here: http://cleveralias.blogs.com/thought_spearmints/2006/01/suppresswarning.html

This stuff is hard, but here are my current thoughts:

If your API returns Object, then there's nothing you can do -- no matter what, you will be blindly casting the object. You let Java throw ClassCastExceptions, or you can check each element yourself and throw Assertions or IllegalArgumentExceptions or some such, but these runtime checks are all equivalent. You have to suppress the compile time unchecked cast no matter what you do at runtime.

I'd just prefer to blind cast and let the JVM perform its runtime check for me since we "know" what the API should return, and are usually willing to assume that the API works. Use generics everywhere above the cast, if you need them. You aren't really buying anything there since you still have the single blind cast, but at least you can use generics from there on up so the JVM can help you avoid blind casts in other pieces of your code.

In this particular case, presumably you can see the call to SetAttribute and see the type is going in, so just blind-casting the type to same on the way out is not immoral. Add a comment referencing the SetAttribute and be done with it.

Here is a shortened example that avoids the "unchecked cast" warning by employing two strategies mentioned in other answers.

Pass down the Class of the type of interest as a parameter at runtime (Class inputElementClazz). Then you can use: inputElementClazz.cast(anyObject); For type casting of a Collection, use the wildcard ? instead of a generic type T to acknowledge that you indeed do not know what kind of objects to expect from the legacy code (Collection unknownTypeCollection). After all, this is what the "unchecked cast" warning wants to tell us: We cannot be sure that we get a Collection, so the honest thing to do is to use a Collection. If absolutely needed, a collection of a known type can still be built (Collection knownTypeCollection).

The legacy code interfaced in the example below has an attribute "input" in the StructuredViewer (StructuredViewer is a tree or table widget, "input" is the data model behind it). This "input" could be any kind of Java Collection.

public void dragFinished(StructuredViewer structuredViewer, Class<T> inputElementClazz) {
    IStructuredSelection selection = (IStructuredSelection) structuredViewer.getSelection();
    // legacy code returns an Object from getFirstElement,
    // the developer knows/hopes it is of type inputElementClazz, but the compiler cannot know
    T firstElement = inputElementClazz.cast(selection.getFirstElement());

    // legacy code returns an object from getInput, so we deal with it as a Collection<?>
    Collection<?> unknownTypeCollection = (Collection<?>) structuredViewer.getInput();

    // for some operations we do not even need a collection with known types
    unknownTypeCollection.remove(firstElement);

    // nothing prevents us from building a Collection of a known type, should we really need one
    Collection<T> knownTypeCollection = new ArrayList<T>();
    for (Object object : unknownTypeCollection) {
        T aT = inputElementClazz.cast(object);
        knownTypeCollection.add(aT);
        System.out.println(aT.getClass());
    }

    structuredViewer.refresh();
}

Naturally, the code above can give runtime errors if we use the legacy code with the wrong data types (e.g. if we set an array as the "input" of the StructuredViewer instead of a Java Collection).

Example of calling the method:

dragFinishedStrategy.dragFinished(viewer, Product.class);

In the HTTP Session world you can't really avoid the cast, since the API is written that way (takes and returns only Object).

With a little bit of work you can easily avoid the unchecked cast, 'though. This means that it will turn into a traditional cast giving a ClassCastException right there in the event of an error). An unchecked exception could turn into a CCE at any point later on instead of the point of the cast (that's the reason why it's a separate warning).

Replace the HashMap with a dedicated class:

import java.util.AbstractMap;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class Attributes extends AbstractMap<String, String> {
    final Map<String, String> content = new HashMap<String, String>();

    @Override
    public Set<Map.Entry<String, String>> entrySet() {
        return content.entrySet();
    }

    @Override
    public Set<String> keySet() {
        return content.keySet();
    }

    @Override
    public Collection<String> values() {
        return content.values();
    }

    @Override
    public String put(final String key, final String value) {
        return content.put(key, value);
    }
}

Then cast to that class instead of Map<String,String> and everything will be checked at the exact place where you write your code. No unexpected ClassCastExceptions later on.

In Android Studio if you want to disable inspection you can use:

//noinspection unchecked
Map<String, String> myMap = (Map<String, String>) deserializeMap();

In this particular case, I would not store Maps into the HttpSession directly, but instead an instance of my own class, which in turn contains a Map (an implementation detail of the class). Then you can be sure that the elements in the map are of the right type.

But if you anyways want to check that the contents of the Map are of right type, you could use a code like this:

public static void main(String[] args) {
    Map<String, Integer> map = new HashMap<String, Integer>();
    map.put("a", 1);
    map.put("b", 2);
    Object obj = map;

    Map<String, Integer> ok = safeCastMap(obj, String.class, Integer.class);
    Map<String, String> error = safeCastMap(obj, String.class, String.class);
}

@SuppressWarnings({"unchecked"})
public static <K, V> Map<K, V> safeCastMap(Object map, Class<K> keyType, Class<V> valueType) {
    checkMap(map);
    checkMapContents(keyType, valueType, (Map<?, ?>) map);
    return (Map<K, V>) map;
}

private static void checkMap(Object map) {
    checkType(Map.class, map);
}

private static <K, V> void checkMapContents(Class<K> keyType, Class<V> valueType, Map<?, ?> map) {
    for (Map.Entry<?, ?> entry : map.entrySet()) {
        checkType(keyType, entry.getKey());
        checkType(valueType, entry.getValue());
    }
}

private static <K> void checkType(Class<K> expectedType, Object obj) {
    if (!expectedType.isInstance(obj)) {
        throw new IllegalArgumentException("Expected " + expectedType + " but was " + obj.getClass() + ": " + obj);
    }
}

The Objects.Unchecked utility function in the answer above by Esko Luontola is a great way to avoid program clutter.

If you don't want the SuppressWarnings on an entire method, Java forces you to put it on a local. If you need a cast on a member it can lead to code like this:

@SuppressWarnings("unchecked")
Vector<String> watchedSymbolsClone = (Vector<String>) watchedSymbols.clone();
this.watchedSymbols = watchedSymbolsClone;

Using the utility is much cleaner, and it's still obvious what you are doing:

this.watchedSymbols = Objects.uncheckedCast(watchedSymbols.clone());

NOTE: I feel its important to add that sometimes the warning really means you are doing something wrong like :

ArrayList<Integer> intList = new ArrayList<Integer>();
intList.add(1);
Object intListObject = intList; 

 // this line gives an unchecked warning - but no runtime error
ArrayList<String> stringList  = (ArrayList<String>) intListObject;
System.out.println(stringList.get(0)); // cast exception will be given here

What the compiler is telling you is that this cast will NOT be checked at runtime, so no runtime error will be raised until you try to access the data in the generic container.

Warning suppression is not a solution. You should not be doing two level casting in one statement.

HashMap<String, String> getItems(javax.servlet.http.HttpSession session) {

    // first, cast the returned Object to generic HashMap<?,?>
    HashMap<?, ?> theHash = (HashMap<?, ?>)session.getAttribute("attributeKey");

    // next, cast every entry of the HashMap to the required type <String, String>
    HashMap<String, String> returingHash = new HashMap<>();
    for (Entry<?, ?> entry : theHash.entrySet()) {
        returingHash.put((String) entry.getKey(), (String) entry.getValue());
    }
    return returingHash;
}

I may have misunderstood the question(an example and a couple of surrounding lines would be nice), but why don't you always use an appropriate interface (and Java5+)? I see no reason why you would ever want to cast to a HashMap instead of a Map<KeyType,ValueType>. In fact, I can't imagine any reason to set the type of a variable to HashMap instead of Map.

And why is the source an Object? Is it a parameter type of a legacy collection? If so, use generics and specify the type you want.

If I have to use an API that doesn't support Generics.. I try and isolate those calls in wrapper routines with as few lines as possible. I then use the SuppressWarnings annotation and also add the type-safety casts at the same time.

This is just a personal preference to keep things as neat as possible.

Take this one, it's much faster than creating a new HashMap, if it's already one, but still secure, as each element is checked against it's type...

@SuppressWarnings("unchecked")
public static <K, V> HashMap<K, V> toHashMap(Object input, Class<K> key, Class<V> value) {
       assert input instanceof Map : input;

       for (Map.Entry<?, ?> e : ((HashMap<?, ?>) input).entrySet()) {
           assert key.isAssignableFrom(e.getKey().getClass()) : "Map contains invalid keys";
           assert value.isAssignableFrom(e.getValue().getClass()) : "Map contains invalid values";
       }

       if (input instanceof HashMap)
           return (HashMap<K, V>) input;
       return new HashMap<K, V>((Map<K, V>) input);
    }

A quick guess if you post your code can say for sure but you might have done something along the lines of

HashMap<String, Object> test = new HashMap();

which will produce the warning when you need to do

HashMap<String, Object> test = new HashMap<String, Object>();

it might be worth looking at

Generics in the Java Programming Language

if your unfamiliar with what needs to be done.

Almost every problem in Computer Science can be solved by adding a level of indirection*, or something.

So introduce a non-generic object that is of a higher-level that a Map. With no context it isn't going to look very convincing, but anyway:

public final class Items implements java.io.Serializable {
    private static final long serialVersionUID = 1L;
    private Map<String,String> map;
    public Items(Map<String,String> map) {
        this.map = New.immutableMap(map);
    }
    public Map<String,String> getMap() {
        return map;
    }
    @Override public String toString() {
        return map.toString();
    }
}

public final class New {
    public static <K,V> Map<K,V> immutableMap(
        Map<? extends K, ? extends V> original
    ) {
        // ... optimise as you wish...
        return Collections.unmodifiableMap(
            new HashMap<String,String>(original)
        );
    }
}

static Map<String, String> getItems(HttpSession session) {
    Items items = (Items)
        session.getAttribute("attributeKey");
    return items.getMap();
}

*Except too many levels of indirection.

Here's one way I handle this when I override the equals() operation.

public abstract class Section<T extends Section> extends Element<Section<T>> {
    Object attr1;

    /**
    * Compare one section object to another.
    *
    * @param obj the object being compared with this section object
    * @return true if this section and the other section are of the same
    * sub-class of section and their component fields are the same, false
    * otherwise
    */       
    @Override
    public boolean equals(Object obj) {
        if (obj == null) {
            // this exists, but obj doesn't, so they can't be equal!
            return false;
        }

        // prepare to cast...
        Section<?> other;

        if (getClass() != obj.getClass()) {
            // looks like we're comparing apples to oranges
            return false;
        } else {
            // it must be safe to make that cast!
            other = (Section<?>) obj;
        }

        // and then I compare attributes between this and other
        return this.attr1.equals(other.attr1);
    }
}

This seems to work in Java 8 (even compiled with -Xlint:unchecked)

Just typecheck it before you cast it.

Object someObject = session.getAttribute("attributeKey");
if(someObject instanceof HashMap)
HashMap<String, String> theHash = (HashMap<String, String>)someObject;  

And for anyone asking, it's quite common to receive objects where you aren't sure of the type. Plenty of legacy "SOA" implementations pass around various objects that you shouldn't always trust. (The horrors!)

EDIT Changed the example code once to match the poster's updates, and following some comments I see that instanceof doesn't play nicely with generics. However changing the check to validate the outer object seems to play well with the commandline compiler. Revised example now posted.

If you are sure that the type returned by session.getAttribute() is HashMap then you can not typecast to that exact type, but rely on only checking the generic HashMap

HashMap<?,?> getItems(javax.servlet.http.HttpSession session) {  
    HashMap<?,?> theHash = (HashMap<?,?>)session.getAttribute("attributeKey");
    return theHash;
} 

Eclipse will then surprise warnings, but of course this can lead to runtime errors that can be hard to debug. I use this approach in not operation-critical contexts only.

Two ways, one which avoids the tag completely, the other using a naughty but nice utility method. The problem is pre-genericised Collections... I believe the rule of thumb is: "cast objects one thing at a time" - what this means when trying to use raw classes in a genericised world is that because you don't know what is in this Map (and indeed the JVM might even find that it isn't even a Map!), it obvious when you think about it that you can't cast it. If you had a Map map2 then HashSet keys = (HashSet)map2.keySet() does not give you a warning, despite this being an "act of faith" for the compiler (because it might turn out to be a TreeSet)... but it is only a single act of faith. PS to the objection that iterating as in my first way "is boring" and "takes time", the answer is "no pain no gain": a genericised collection is guaranteed to contain Map.Entrys, and nothing else. You have to pay for this guarantee. When using generics systematically this payment, beautifully, takes the form of coding compliance, not machine time! One school of thought might say that you should set Eclipse's settings to make such unchecked casts errors, rather than warnings. In that case you would have to use my first way.

package scratchpad;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Vector;

public class YellowMouse {

    // First way

    Map<String, String> getHashMapStudiouslyAvoidingSuppressTag(HttpSession session) {
      Map<?, ?> theHash = (Map<?, ?>)session.getAttribute("attributeKey");

      Map<String, String> yellowMouse = new HashMap<String, String>();
      for( Map.Entry<?, ?> entry : theHash.entrySet() ){
        yellowMouse.put( (String)entry.getKey(), (String)entry.getValue() );
      }

      return yellowMouse;
    }


    // Second way

    Map<String, String> getHashMapUsingNaughtyButNiceUtilityMethod(HttpSession session) {
      return uncheckedCast( session.getAttribute("attributeKey") );
    }


    // NB this is a utility method which should be kept in your utility library. If you do that it will
    // be the *only* time in your entire life that you will have to use this particular tag!!

    @SuppressWarnings({ "unchecked" })
    public static synchronized <T> T uncheckedCast(Object obj) {
        return (T) obj;
    }


}

This makes the warnings go away...

 static Map<String, String> getItems(HttpSession session) {
        HashMap<?, ?> theHash1 = (HashMap<String,String>)session.getAttribute("attributeKey");
        HashMap<String,String> theHash = (HashMap<String,String>)theHash1;
    return theHash;
}

Solution: Disable this warning in Eclipse. Don't @SuppressWarnings it, just disable it completely.

Several of the "solutions" presented above are way out of line, making code unreadable for the sake of suppressing a silly warning.