我们面临着HashMap行为方式的奇怪问题。
当HashMap键实现Comparable接口但是compareTo实现与equals不一致那么HashMaps：
我创建了一个小测试来重现问题（见下文）。
import java.util.HashMap;
import java.util.Map;
public class HashMapTest {
private static final char MIN_NAME = 'A';
private static final char MAX_NAME = 'K';
private static final int EXPECTED_NUMBER_OF_ELEMENTS = MAX_NAME - MIN_NAME + 1;
private HashMap<Person, Integer> personToAgeMap;
HashMapTest(){
personToAgeMap = new HashMap();
}
public static void main(String[] args){
HashMapTest objHashMap = new HashMapTest();
System.out.println("Initial Size of Map: " + objHashMap.getPersonToAgeMap().size());
objHashMap.whenOverridingEqualElements_thenSizeOfTheMapIsStable();
objHashMap.whenGettingElementUsingPersonOfAge1_thenOverridenValuesAreReturned();
objHashMap.whenGettingElementUsingPersonOfAge100_thenOverridenValuesAreReturned();
objHashMap.whenGettingElementUsingPersonOfAge50_thenOverridenValuesAreReturned();
objHashMap.whenGettingElementUsingPersonOfAgeMinus1_thenOverridenValuesAreReturned();
}
public HashMap<Person, Integer> getPersonToAgeMap(){
return personToAgeMap;
}
public void whenOverridingEqualElements_thenSizeOfTheMapIsStable() {
System.out.println("Adding elements with age 1..");
putAllPeopleWithAge(personToAgeMap, 1);
System.out.println(personToAgeMap);
System.out.println("Expected Number Of elements: " + EXPECTED_NUMBER_OF_ELEMENTS+ "\nActual Number of elements: "+personToAgeMap.size());
System.out.println();
System.out.println("Overwriting map, with value 100..");
putAllPeopleWithAge(personToAgeMap, 100);
System.out.println(personToAgeMap);
System.out.println("Expected Number Of elements: " + EXPECTED_NUMBER_OF_ELEMENTS+ "\nActual Number of elements: "+personToAgeMap.size());
System.out.println();
}
public void whenGettingElementUsingPersonOfAge1_thenOverridenValuesAreReturned() {
useAgeToCheckAllHashMapValuesAre(1, 100);
}
public void whenGettingElementUsingPersonOfAge100_thenOverridenValuesAreReturned() {
useAgeToCheckAllHashMapValuesAre(100, 100);
}
public void whenGettingElementUsingPersonOfAge50_thenOverridenValuesAreReturned() {
useAgeToCheckAllHashMapValuesAre(50, 100);
}
public void whenGettingElementUsingPersonOfAgeMinus1_thenOverridenValuesAreReturned() {
useAgeToCheckAllHashMapValuesAre(-10, 100);
}
private void useAgeToCheckAllHashMapValuesAre(int age, Integer expectedValue) {
System.out.println("Checking the values corresponding to age = " + age);
StringBuilder sb = new StringBuilder();
int count = countAllPeopleUsingAge(personToAgeMap, age);
System.out.println("Count of People with age " + age+" =" + count);
if (EXPECTED_NUMBER_OF_ELEMENTS != count) {
sb.append("Size of the map ").append(" is wrong: ")
.append("expected <").append(EXPECTED_NUMBER_OF_ELEMENTS).append("> actual <").append(count).append(">.\n");
}
for (char name = MIN_NAME; name <= MAX_NAME; name++) {
Person key = new Person(name, age);
Integer value = personToAgeMap.get(key);
if (!expectedValue.equals(value)) {
sb.append("Unexpected value for ").append(key).append(": ")
.append("expected <").append(expectedValue).append("> actual <").append(value).append(">.\n");
}
}
if (sb.length() > 0) {
System.out.println(sb.toString());
}
}
void putAllPeopleWithAge(Map<Person, Integer> map, int age) {
for (char name = MIN_NAME; name <= MAX_NAME; name++) {
map.put(new Person(name, age), age);
}
}
int countAllPeopleUsingAge(Map<Person, Integer> map, int age) {
int counter = 0;
for (char name = MIN_NAME; name <= MAX_NAME; name++) {
if (map.containsKey(new Person(name, age))) {
counter++;
}
}
return counter;
}
String getAllPeopleUsingAge(Map<Person, Integer> map, int age) {
StringBuilder sb = new StringBuilder();
for (char name = MIN_NAME; name <= MAX_NAME; name++) {
Person key = new Person(name, age);
sb.append(key).append('=').append(map.get(key)).append('\n');
}
return sb.toString();
}
class Person implements Comparable<Person> {
char name;
int age;
public Person(char name, int age) {
this.name = name;
this.age = age;
}
//Making sure all elements end up in the very same bucket
//Nothing wrong with it except performance...
@Override
public int hashCode() {
return 0;
}
//equals is only by name
@Override
public boolean equals(Object other) {
Person otherPerson = (Person)other;
return this.name == otherPerson.name;
}
public String toString() {
return name + "[age=" + age + "]";
}
//comparing by age
//NOTE: compareTo is inconsistent with equals which should be OK in non-sorted collections
//https://docs.oracle.com/javase/8/docs/api/java/lang/Comparable.html
@Override
public int compareTo(Person other) {
return this.age - other.age;
}
}
}
预期输出
Initial Size of Map: 0
Adding elements with age 1..
{K[age=1]=1, J[age=1]=1, I[age=1]=1, H[age=1]=1, G[age=1]=1, F[age=1]=1, E[age=1]=1, D[age=1]=1, C[age=1]=1, B[age=1]=1, A[age=1]=1}
Expected Number Of elements: 11
Actual Number of elements: 11
Overwriting map, with value 100..
{K[age=1]=100, J[age=1]=100, I[age=1]=100, H[age=1]=100, G[age=1]=100, F[age=1]=100, E[age=1]=100, D[age=1]=100, C[age=1]=100, B[age=1]=100, A[age=1]=100}
Expected Number Of elements: 11
Actual Number of elements: 11
Checking the values corresponding to age = 1
Count of People with age 1 =11
Checking the values corresponding to age = 100
Count of People with age 100 =11
Checking the values corresponding to age = 50
Count of People with age 50 =11
Checking the values corresponding to age = -10
Count of People with age -10 =11
实际OutPut
Initial Size of Map: 0
Adding elements with age 1..
{I[age=1]=1, A[age=1]=1, B[age=1]=1, C[age=1]=1, D[age=1]=1, E[age=1]=1, F[age=1]=1, G[age=1]=1, H[age=1]=1, J[age=1]=1, K[age=1]=1}
Expected Number Of elements: 11
Actual Number of elements: 11
Overwriting map, with value 100..
{I[age=1]=100, A[age=1]=1, B[age=1]=100, C[age=1]=1, D[age=1]=100, A[age=100]=100, C[age=100]=100, E[age=100]=100, H[age=100]=100, J[age=100]=100, K[age=100]=100, F[age=100]=100, G[age=100]=100, E[age=1]=1, F[age=1]=1, G[age=1]=1, H[age=1]=1, J[age=1]=1, K[age=1]=1}
Expected Number Of elements: 11
Actual Number of elements: 19
Checking the values corresponding to age = 1
Count of People with age 1 =11
Unexpected value for E[age=1]: expected <100> actual <1>.
Unexpected value for F[age=1]: expected <100> actual <1>.
Unexpected value for G[age=1]: expected <100> actual <1>.
Unexpected value for J[age=1]: expected <100> actual <1>.
Unexpected value for K[age=1]: expected <100> actual <1>.
Checking the values corresponding to age = 100
Count of People with age 100 =10
Size of the map is wrong: expected <11> actual <10>.
Unexpected value for B[age=100]: expected <100> actual <null>.
Checking the values corresponding to age = 50
Count of People with age 50 =5
Size of the map is wrong: expected <11> actual <5>.
Unexpected value for B[age=50]: expected <100> actual <null>.
Unexpected value for E[age=50]: expected <100> actual <null>.
Unexpected value for F[age=50]: expected <100> actual <null>.
Unexpected value for G[age=50]: expected <100> actual <null>.
Unexpected value for J[age=50]: expected <100> actual <null>.
Unexpected value for K[age=50]: expected <100> actual <null>.
Checking the values corresponding to age = -10
Count of People with age -10 =4
Size of the map is wrong: expected <11> actual <4>.
Unexpected value for A[age=-10]: expected <100> actual <1>.
Unexpected value for B[age=-10]: expected <100> actual <null>.
Unexpected value for C[age=-10]: expected <100> actual <1>.
Unexpected value for D[age=-10]: expected <100> actual <null>.
Unexpected value for E[age=-10]: expected <100> actual <1>.
Unexpected value for F[age=-10]: expected <100> actual <null>.
Unexpected value for G[age=-10]: expected <100> actual <null>.
Unexpected value for H[age=-10]: expected <100> actual <null>.
Unexpected value for J[age=-10]: expected <100> actual <null>.
Unexpected value for K[age=-10]: expected <100> actual <null>.
答案 0 :(得分：3)
从Java 8开始，HashMap添加了一项优化，以便在密钥为Comparable时处理冲突：
为了改善影响，当密钥为
java.lang.Comparable时，此类可以使用密钥之间的比较顺序来帮助打破关系。
如果您的密钥具有与equals()不一致的自然顺序，则可能会发生任何奇怪的事情，在这种情况下，看起来地图正试图使用自然顺序跟踪密钥。 / p>
答案 1 :(得分：1)
我重现了你意想不到的结果。然后我从Comparable中删除了Person并且所有检查都已通过。然后我恢复了Comparable，切换到OpenJDK 7，并且所有检查都通过了。可以肯定的是，我切换回Oracle Java 8，并再次获得了意想不到的结果。显然，甲骨文已经做了一些事情，并且聪明地做了很多事情。在Java 8中使用HashMap。保持良好的工作，Oracle。