我有以下代码:

public enum Continent {ASIA, EUROPE}

public class Country {      
   private String name;
   private Continent region;

    public Country (String na, Continent reg) { 
        this.name = na;
        this.region = reg;
    }
    public String getName () {return name;} 
    public Continent getRegion () {return region;}
    @Override
    public String toString() {
        return "Country [name=" + name + ", region=" + region + "]";
    }
}

public static void main(String[] args) throws IOException {
        List<Country> couList = Arrays.asList(
            new Country ("Japan", Continent.ASIA), 
            new Country ("Sweden", Continent.EUROPE), 
            new Country ("Norway", Continent.EUROPE));
        Map<Continent, List<String>> regionNames = couList
                .stream()
                //.peek(System.out::println)
                .collect(Collectors.groupingBy(Country::getRegion, Collectors.mapping(Country::getName, Collectors.toList())));
        System.out.println(regionNames);
}

{EUROPE=[Sweden, Norway], ASIA=[Japan]}

Country [name=Japan, region=ASIA]
Country [name=Sweden, region=EUROPE]
Country [name=Norway, region=EUROPE]
{ASIA=[Japan], EUROPE=[Sweden, Norway]}

最佳答案

enum的hashCode实现使用Object提供的默认值。该方法的The documentation提到:

Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.

 // Marsaglia's xor-shift scheme with thread-specific state
 // This is probably the best overall implementation -- we'll
 // likely make this the default in future releases.
 unsigned t = Self->_hashStateX ;
 t ^= (t << 11) ;
 Self->_hashStateX = Self->_hashStateY ;
 Self->_hashStateY = Self->_hashStateZ ;
 Self->_hashStateZ = Self->_hashStateW ;
 unsigned v = Self->_hashStateW ;
 v = (v ^ (v >> 19)) ^ (t ^ (t >> 8)) ;
 Self->_hashStateW = v ;
 value = v ;

_hashStateX = os::random() ;
_hashStateY = 842502087 ;
_hashStateZ = 0x8767 ;    // (int)(3579807591LL & 0xffff) ;
_hashStateW = 273326509 ;

next_rand = (16807*seed) mod (2**31-1)

PS $> grep -r init_random
os/bsd/vm/os_bsd.cpp:  init_random(1234567);
os/linux/vm/os_linux.cpp:  init_random(1234567);
os/solaris/vm/os_solaris.cpp:  init_random(1234567);
os/windows/vm/os_windows.cpp:  init_random(1234567);
... test methods

Map<Continent, List<String>> regionNames = couList
        .stream()
        //.peek(System.out::println) // Does this call Object.hashCode?
        .collect(Collectors.groupingBy(Country::getRegion,
            Collectors.mapping(Country::getName, Collectors.toList()))); // hash code for constant generated here

Object.hashCode() line: not available [native method]   
HashMap<K,V>.hash(Object) line: 338 
HashMap<K,V>.put(K, V) line: 611    
HashSet<E>.add(E) line: 219 
Collections$SynchronizedSet<E>(Collections$SynchronizedCollection<E>).add(E) line: 2035 
Launcher$AppClassLoader(ClassLoader).checkPackageAccess(Class<?>, ProtectionDomain) line: 508   
Main.main(String...) line: 19   

Continent.ASIA.hashCode();
Continent.EUROPE.hashCode();

关于java - 为什么Stream <T> collect方法返回不同的键顺序？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/44066380/