如何使用 Java 泛型避免未经检查的强制转换警告 [英] How to avoid unchecked cast warnings with Java Generics

问题描述

不知何故，我的旧问题被关闭了，所以我打开了一个新问题:

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

我正在使用 Java 泛型从 SQL 查询中实现通用的双向哈希映射.它应该能够来回映射字符串、整数对的任意组合.它应该像这样使用:

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);

当然应该可以创建一个 pm (Integer, Integer) 等等...

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

我的 Pick Map 实现看起来像这样(没有 getter...):

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;
    }
}

我没有编译器错误，而且我很确定它会以这种方式工作.但是我在getInstance"方法中收到未经检查的强制转换警告，其中我将 Extractor(E) 转换为 Extractor(L)...

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)...

我应该如何正确投射?或者我错过了什么?或者我应该压制这些警告?

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

推荐答案

您收到警告是因为无法证明您的操作是安全的.您假设 getInstance(colTypeL) 将返回一个 Extractor - 但在任何一个编译时都无法验证或执行时间.

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.

您可以使用其他人提到的 @SuppressWarnings("unchecked")，但我会尝试重新考虑设计.

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

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