Java之HashMap案例详解

    /**
     * 建立键值对应关系，如果之前已经存在对应的key，
     * 返回之前存储的value，之前如果不存在对应的key，返回null
     */
    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

public class MyKey {
    private int id;
    
    @Override
    public int hashCode() {
        System.out.println("调用 hashCode()");
        return id;
    }

    // constructor, setters and getters 
}

   @Test
    public void mapKeyTest(){
        HashMap<MyKey,String> map = new HashMap<MyKey, String>();
        String retV = map.put(new MyKey(1),"value1");
    }

调用 hashCode()

static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

    @Test
    public void mapKeyTest(){
        HashMap<MyKey,String> map = new HashMap<MyKey, String>();
        MyKey key1 = new MyKey(1);
        map.put(key1,"value1");
        String retV =  map.get(key1);
    }

调用 hashCode()
调用 hashCode()

Set<K> keys = map.keySet()
Collection<V> values = map.values()
Set<Entry<K, V>> entries = map.entrySet();

@Test
public void givenIterator_whenFailsFastOnModification_thenCorrect() {
    Map<String, String> map = new HashMap<>();
    map.put("name", "baeldung");
    map.put("type", "blog");
 
    Set<String> keys = map.keySet();
    Iterator<String> it = keys.iterator();
    map.remove("type");
    while (it.hasNext()) {
        String key = it.next();
    }
}

// 会抛出java.util.ConcurrentModificationException异常

public void givenIterator_whenRemoveWorks_thenCorrect() {
    Map<String, String> map = new HashMap<>();
    map.put("name", "baeldung");
    map.put("type", "blog");
 
    Set<String> keys = map.keySet();
    Iterator<String> it = keys.iterator();
 
    while (it.hasNext()) {
        it.next();
        it.remove();
    }
 
    assertEquals(0, map.size());
}

    int threshold;
    final float loadFactor;

Map<String,String> hashMapWithCapacity=new HashMap<>(32);
Map<String,String> hashMapWithCapacityAndLF=new HashMap<>(32, 0.5f);