从集合中获取元素

#1 楼

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#3 楼

Foo getEqual(Foo sample, Set<Foo> all) {
  for (Foo one : all) {
    if (one.equals(sample)) {
      return one;
    }
  } 
  return null;
}

#4 楼

class City extends UniqueItem {

    // Somewhere in this class

    public void doSomething() {
        // Whatever
    }
}

public class GameMap {
    private MagicSet<City> cities;

    public GameMap(Collection<City> cities) {
        cities = new MagicHashSet<>(cities);
    }

    /*
     * cityId is the UUID of the city you want to retrieve.
     * If you have a copied instance of that city, you can simply 
     * call copiedCity.getId() and pass the return value to this method.
     */
    public void doSomethingInCity(UUID cityId) {
        City city = cities.getFromId(cityId);
        city.doSomething();
    }

    // Other methods can be called on a MagicSet too
}

#5 楼

Set<Foo> set = ...;
List<Foo> list = new ArrayList<Foo>(set);
Foo obj = list.get(0);

#6 楼

Foo bar = set.floor(foo); // or .ceiling
if (foo.equals(bar)) {
    // use bar…
}

#7 楼

Foo foo = set.stream().filter(item->item.equals(theItemYouAreLookingFor)).findFirst().get();

#8 楼

mySet.stream()
.filter(object -> object.property.equals(myProperty))
.findFirst().get();

#9 楼

Object objectToGet = ...
Map<Object, Object> map = new HashMap<Object, Object>(set.size());
for (Object o : set) {
    map.put(o, o);
}
Object objectFromSet = map.get(objectToGet);

#10 楼

import java.util.Iterator;
import java.util.HashSet;

public class MyClass {
 public static void main(String[ ] args) {
 HashSet<String> animals = new HashSet<String>();
animals.add("fox");
animals.add("cat");
animals.add("dog");
animals.add("rabbit");

Iterator<String> it = animals.iterator();
while(it.hasNext()) {
  String value = it.next();
  System.out.println(value);   
 }
 }
}

#11 楼

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#13 楼

Map<Integer,Foo> map = Maps.uniqueIndex(fooSet, Foo::getKey);

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#17 楼

import java.util.Arrays;
import java.util.List;
import java.util.HashSet;
import java.util.Arrays;

public class MyClass {
    public static void main(String args[]) {
        Set mySet = new HashSet();
        mySet.add("one");
        mySet.add("two");
        List list = Arrays.asList(mySet.toArray());
        Object o0 = list.get(0);
        Object o1 = list.get(1);
        System.out.println("items " + o0+","+o1);
    }
}

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#19 楼

class SelfMappingHashMap<V> extends HashMap<V, V>{
    @Override
    public String toString(){
        // otherwise you get lots of "... object1=object1, object2=object2..." stuff
        return keySet().toString();
    }

    @Override
    public V get( Object key ){
        throw new UnsupportedOperationException( "use tryToGetRealFromCandidate()");
    }

    @Override
    public V put( V key, V value ){
       // thorny issue here: if you were indavertently to `put`
       // a "candidate instance" with the element already in the `Map/Set`: 
       // these will obviously be considered equivalent 
       assert key.equals( value );
       return super.put( key, value );
    }

    public V tryToGetRealFromCandidate( V key ){
        return super.get(key);
    }
}

SelfMappingHashMap<SomeClass> selfMap = new SelfMappingHashMap<SomeClass>();
...
SomeClass candidate = new SomeClass();
if( selfMap.contains( candidate ) ){
    SomeClass realThing = selfMap.tryToGetRealFromCandidate( candidate );
    ...
    realThing.useInSomeWay()...
}

public interface NoVisibleConstructor {
    // again, this is a "nudge" technique, in the sense that there is no known method of 
    // making an interface enforce "no visible constructor" in its implementing classes 
    // - of course when Java finally implements full multiple inheritance some reflection 
    // technique might be used...
    NoVisibleConstructor addOrGetExisting( GettableSet<? extends NoVisibleConstructor> gettableSet );
};

public interface GettableSet<V extends NoVisibleConstructor> extends Set<V> {
    V getGenuineFromImpostor( V impostor ); // see below for naming
}

public class GettableHashSet<V extends NoVisibleConstructor> implements GettableSet<V> {
    private Map<V, V> map = new HashMap<V, V>();

    @Override
    public V getGenuineFromImpostor(V impostor ) {
        return map.get( impostor );
    }

    @Override
    public int size() {
        return map.size();
    }

    @Override
    public boolean contains(Object o) {
        return map.containsKey( o );
    }

    @Override
    public boolean add(V e) {
        assert e != null;
        V result = map.put( e,  e );
        return result != null;
    }

    @Override
    public boolean remove(Object o) {
        V result = map.remove( o );
        return result != null;
    }

    @Override
    public boolean addAll(Collection<? extends V> c) {
        // for example:
        throw new UnsupportedOperationException();
    }

    @Override
    public void clear() {
        map.clear();
    }

    // implement the other methods from Set ...
}

class SomeClass implements NoVisibleConstructor {

    private SomeClass( Object param1, Object param2 ){
        // ...
    }

    static SomeClass getOrCreate( GettableSet<SomeClass> gettableSet, Object param1, Object param2 ) {
        SomeClass candidate = new SomeClass( param1, param2 );
        if (gettableSet.contains(candidate)) {
            // obviously this then means that the candidate "fails" (or is revealed
            // to be an "impostor" if you will).  Return the existing element:
            return gettableSet.getGenuineFromImpostor(candidate);
        }
        gettableSet.add( candidate );
        return candidate;
    }

    @Override
    public NoVisibleConstructor addOrGetExisting( GettableSet<? extends NoVisibleConstructor> gettableSet ){
       // more elegant implementation-hiding: see below
    }
}

protected SomeClass(){
    throw new UnsupportedOperationException();
}

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#21 楼

public static <E> E get(NavigableSet<E> set, E key) {
    return set.tailSet(key, true).floor(key);
}

import java.util.*;
import java.lang.reflect.Field;
import java.lang.reflect.Method;

public class Test {
    private static final Field mapField;
    private static final Method hashMethod;
    private static final Method getNodeMethod;
    private static final Field keyField;
    static {
        try {
            mapField = HashSet.class.getDeclaredField("map");
            mapField.setAccessible(true);
            hashMethod = HashMap.class.getDeclaredMethod("hash", Object.class);
            hashMethod.setAccessible(true);
            getNodeMethod = HashMap.class.getDeclaredMethod("getNode",
                    Integer.TYPE, Object.class);
            getNodeMethod.setAccessible(true);
            keyField = Class.forName("java.util.HashMap$Node").getDeclaredField("key");
            keyField.setAccessible(true);
        } catch (ReflectiveOperationException e) {
            throw new RuntimeException(e);
        }
    }

    public static <E> E get(HashSet<E> set, E key) {
        try {
            Object map = mapField.get(set);
            Object hash = hashMethod.invoke(null, key);
            Object node = getNodeMethod.invoke(map, hash, key);
            if (node == null)
                return null;
            @SuppressWarnings("unchecked")
            E result = (E)keyField.get(node);
            return result;
        } catch (ReflectiveOperationException e) {
            throw new RuntimeException(e);
        }
    }

    public static <E> E get(NavigableSet<E> set, E key) {
        return set.tailSet(key, true).floor(key);
    }

    public static void main(String[] args) {
        HashSet<Integer> s = new HashSet<>();
//      HashSet<Integer> s = new LinkedHashSet<>();
//      TreeSet<Integer> s = new TreeSet<>();
        for (int i = 0; i < 100_000; i++)
            s.add(i);
        Integer key = java.awt.event.KeyEvent.VK_FIND;
        Integer hidden = get(s, key);
        System.out.println(key);
        System.out.println(hidden);
        System.out.println(key.equals(hidden));
        System.out.println(key == hidden);
    }
}

#22 楼

ObjectClass[] arrayName = SetOfObjects.toArray(new ObjectClass[setOfObjects.size()]);

#23 楼

<T> T onlyItem(Collection<T> items) {
    if (items.size() != 1)
        throw new IllegalArgumentException("Collection must have single item; instead it has " + items.size());

    return items.iterator().next();
}

#24 楼

   SharedPreferences se_get = getSharedPreferences("points",MODE_PRIVATE);
   Set<String> main = se_get.getStringSet("mydata",null);
   for(int jk = 0 ; jk < main.size();jk++)
   {
      Log.i("data",String.valueOf(main.toArray()[jk]));
   }