How to avoid unchecked cast warnings with Java Generics

Question

Somehow my old question was closed, so I open a new one:

I am using Java Generics to implement a generic bidirectional Hash Map out of an SQL Query. It should be able to map any combination of String, Integer pairs back and forth. It should be used like this:

String sql = "SELECT string_val, int_val FROM map_table";
PickMap<String, Integer> pm1 = new PickMap<String, Integer>(sql);

String key1 = "seven";
Integer value1 = pm1.getLeft2Right(key1);

Integer key2 = 7;
String value2 = pm1.getRightToLeft(key2);

Of course it should be possible to create an pm (Integer, Integer) and so on...

My implementation of Pick Map looks like this (without the getter...):

public class PickMap<L, R> {

    private final HashMap<L, R> left2Right = new HashMap<L, R>();
    private final HashMap<R, L> right2Left = new HashMap<R, L>();

    public PickMap(String sql) throws OException {
        DTable d = new DTable(sql);
        int colTypeL = d.t.getColType(1);
        int colTypeR = d.t.getColType(2);
        Extractor<L> extLeft  = (Extractor<L>) getInstance(colTypeL);
        Extractor<R> extRight = (Extractor<R>) getInstance(colTypeR);    
        int numRows = d.t.getNumRows();
        for(int i=1;i<=numRows;i++) {
            L leftVal = extLeft.extract(d, i);
            R rightVal = extRight.extract(d, i);
            this.left2Right.put(leftVal, rightVal);
            this.right2Left.put(rightVal, leftVal);
        }
    }

    private Extractor<?> getInstance(int type) {
        if(type == 1)
            return new IntExtractor();
        else
            return new StringExtractor();
    }
}

interface Extractor<E> {
    E extract(DTable source, int row);
}

class IntExtractor implements Extractor<Integer> {

    @Override
    public Integer extract(DTable source, int row) {
        int value = 5;
        return new Integer(value);
    }
}

class StringExtractor implements Extractor<String> {

    @Override
    public String extract(DTable source, int row) {
        String retVal = "hello";
        return retVal;
    }
}

I have no compiler errors and I'm pretty sure, that it will work this way. BUT I'm getting unchecked cast warnings on the "getInstance" methods Where I cast Extractor(E) to Extractor(L)...

How should I cast properly? Or what am I missing? Or should I just suppress those warnings?

score 29 · Accepted Answer · edited Feb 29 '12 at 14:13

29

You're getting warnings because what you're doing can't be proved to be safe. You're assuming that getInstance(colTypeL) will return an Extractor<L> - but that can't be verified at either compile-time or execution time.

You can use @SuppressWarnings("unchecked") as mentioned by others, but I would try to rethink the design somewhat.

edited Feb 29 '12 at 14:13

Tom Hawtin - tackline

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answered Feb 29 '12 at 13:25

Jon Skeet

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Yes yes, I understand your point. I would like to use the generic type L for instanciating my Extractor. So instead of giving the "colType" into the getInstance method, I would like to do some thing like getInstance(L) and getInstance(R)... Is that possible? I hate supressing warnings! – Sauer Feb 29 '12 at 13:38
1

@Sauer: You'd have to know `L` and `R` at execution time, which normally means having constructor parameters of type `Class extends L>` and `Class extends R>` or something similar. – Jon Skeet Feb 29 '12 at 13:39
8

One possibility is to have the constructor take the extractors it needs: `PickMap(String sql, Extractor extends L> leftExtractor, Extractor extends R> rightExtractor>)`. This not only makes the compilation safe, it also lets someone else come along and decide they want a `PickMap` without having to change the `PickMap` code (assuming `Extractor` is an interface or class they have access to). – yshavit Feb 29 '12 at 14:37

score 7 · Answer 2 · edited May 23 '17 at 10:29

7

You can use the following annotation to make the compiler not output those warnings:

@SuppressWarnings("unchecked")

See this related question which deals with the same issue. The answer there will explain everything you need to know.

edited May 23 '17 at 10:29

Community

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answered Feb 29 '12 at 13:22

Richard Walton

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score 1 · Answer 3 · answered May 17 '19 at 18:27

1

If you are using the Spring Framework, you can use CastUtils:

import static org.springframework.data.util.CastUtils.cast;

obj.setString(cast(someObject));
String bob = cast(someObject);

answered May 17 '19 at 18:27

user11058144

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How to avoid unchecked cast warnings with Java Generics

3 Answers3

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