为什么Objects.hash()对于同一输入返回不同的值? [英] Why Objects.hash() returns different values for the same input?

问题描述

我运行了以下脚本(java)，它给了我奇怪的结果. 有人可以帮忙解释吗?

I ran the follow script(java), and it gave me the weird result. Does anyone can help to explain?

import java.util.Objects;
import org.apache.log4j.Logger;

public class CacheTester {

private static final Logger log = Logger.getLogger(CacheTester.class);

    @Test
    public void hashCodeTest() {
        for (int i = 0; i < 50; i++) {
            // if I remove the third parameter, it works fine
            log.info(Objects.hash("getDemoCache", "1", new int[]{1, 2}));
        }
    }
}

日志结果(彼此不同):

Log Result(they are different from each other):

//...
2015-04-29 17:43:20 INFO  CacheTester:42 - 1431904540
2015-04-29 17:43:20 INFO  CacheTester:42 - 1859187447
2015-04-29 17:43:20 INFO  CacheTester:42 - -2146933580
2015-04-29 17:43:20 INFO  CacheTester:42 - -2074242201
2015-04-29 17:43:20 INFO  CacheTester:42 - 1363170000
2015-04-29 17:43:20 INFO  CacheTester:42 - 1040980265
2015-04-29 17:43:20 INFO  CacheTester:42 - 1639331053
2015-04-29 17:43:20 INFO  CacheTester:42 - 570765746
2015-04-29 17:43:20 INFO  CacheTester:42 - -2023288896
2015-04-29 17:43:20 INFO  CacheTester:42 - -1892732019
2015-04-29 17:43:20 INFO  CacheTester:42 - 1464306601
2015-04-29 17:43:20 INFO  CacheTester:42 - 921799986
2015-04-29 17:43:20 INFO  CacheTester:42 - 1037804977
//...

----背景----

---- Background ----

我想将自己的keyGenrator用于@Cacheable注释(Spring& ehCache).

I wanted to used my own keyGenrator for @Cacheable annotation(Spring & ehCache).

public Object generate(Object target, Method method, Object... params) {
    int key = Objects.hashCode(method.getName(), params);
    log.info("key = " + key);
    return key;
}

在这种情况下，我发现缓存总是丢失的.

In this case, I find the cache are always missed.

然后我必须更改为:

public Object generate(Object target, Method method, Object... params) {
    int result = method.getName().hashCode() : 0;
    result = 31 * result + Objects.hashCode(params);
    return result;
}

谢谢

推荐答案

这是因为int[]的hashCode没有被覆盖.即使条目相同，也没有理由为什么int[]的两个实例应该具有相同的hashCode.

It's because hashCode for int[] is not overridden. There is no reason why two instances of int[] should have the same hashCode, even if the entries are the same.

尝试一下:

System.out.println(new int[] {1, 2}.hashCode());
System.out.println(new int[] {1, 2}.hashCode());

几乎可以肯定会看到两个不同的整数.

You will almost certainly see two different integers.

对数组使用Objects.hash的一种好方法是传递Arrays.hashCode(array)而不是实际数组.您可以这样做:

A good way to use Objects.hash with arrays is to pass Arrays.hashCode(array) instead of the actual array. In your case you could do:

Objects.hash("getDemoCache", "1", Arrays.hashCode(new int[]{1, 2}))

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