map.put(key,value);

map.put(key,map.get(key)+1);

map.getOrDefault(key,value);

package tech.luxsun.interview.luxinterviewstarter.collection;
?
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import java.util.HashMap;
?
/**
?* @author Lux Sun
?* @date 2021/4/22
?*/
public class MapDemo0 {
?
? ? public static void main(String[] args) {
? ? ? ? HashMap<Object, Object> map = new HashMap<>();
?
? ? ? ? // Person Case
? ? ? ? Person p = new Person("Bob", 12);
? ? ? ? map.put(p, "person");
? ? ? ? System.out.println(p.hashCode());
? ? ? ? System.out.println(map.get(p));
?
? ? ? ? p.setAge(13);
? ? ? ? System.out.println(p.hashCode());
? ? ? ? System.out.println(map.get(p));
?
? ? ? ? // Student Case
? ? ? ? Student stu = new Student("Bob", 12);
? ? ? ? map.put(stu, "student");
? ? ? ? System.out.println(stu.hashCode());
? ? ? ? System.out.println(map.get(stu));
?
? ? ? ? stu.setAge(13);
? ? ? ? System.out.println(stu.hashCode());
? ? ? ? System.out.println(map.get(stu));
? ? }
}
?
@Data
@AllArgsConstructor
@NoArgsConstructor
class Person {
? ? private String name;
? ? private Integer age;
?
? ? public int hashCode() {
? ? ? ? return 123456;
? ? }
}
?
@Data
@AllArgsConstructor
@NoArgsConstructor
class Student {
? ? private String name;
? ? private Integer age;
}

public int hashCode() {
? ? int PRIME = true;
? ? int result = 1;
? ? Object $age = this.getAge();
? ? int result = result * 59 + ($age == null ? 43 : $age.hashCode());
? ? Object $name = this.getName();
? ? result = result * 59 + ($name == null ? 43 : $name.hashCode());
? ? return result;
}

/**
?* Returns the value to which the specified key is mapped,
?* or {@code null} if this map contains no mapping for the key.
?*
?* <p>More formally, if this map contains a mapping from a key
?* {@code k} to a value {@code v} such that {@code (key==null ? k==null :
?* key.equals(k))}, then this method returns {@code v}; otherwise
?* it returns {@code null}. ?(There can be at most one such mapping.)
?*
?* <p>A return value of {@code null} does not <i>necessarily</i>
?* indicate that the map contains no mapping for the key; it's also
?* possible that the map explicitly maps the key to {@code null}.
?* The {@link #containsKey containsKey} operation may be used to
?* distinguish these two cases.
?*
?* @see #put(Object, Object)
?*/
public V get(Object key) {
? ? Node<K,V> e;
? ? return (e = getNode(hash(key), key)) == null ? null : e.value;
}
?
?
/**
?* Computes key.hashCode() and spreads (XORs) higher bits of hash
?* to lower. ?Because the table uses power-of-two masking, sets of
?* hashes that vary only in bits above the current mask will
?* always collide. (Among known examples are sets of Float keys
?* holding consecutive whole numbers in small tables.) ?So we
?* apply a transform that spreads the impact of higher bits
?* downward. There is a tradeoff between speed, utility, and
?* quality of bit-spreading. Because many common sets of hashes
?* are already reasonably distributed (so don't benefit from
?* spreading), and because we use trees to handle large sets of
?* collisions in bins, we just XOR some shifted bits in the
?* cheapest possible way to reduce systematic lossage, as well as
?* to incorporate impact of the highest bits that would otherwise
?* never be used in index calculations because of table bounds.
?*/
static final int hash(Object key) {
? ? int h;
? ? return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
?
/**
?* Implements Map.get and related methods
?*
?* @param hash hash for key
?* @param key the key
?* @return the node, or null if none
?*/
final Node<K,V> getNode(int hash, Object key) {
? ? Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
? ? if ((tab = table) != null && (n = tab.length) > 0 &&
? ? ? ? (first = tab[(n - 1) & hash]) != null) {
? ? ? ? if (first.hash == hash && // always check first node
? ? ? ? ? ? ((k = first.key) == key || (key != null && key.equals(k))))
? ? ? ? ? ? return first;
? ? ? ? if ((e = first.next) != null) {
? ? ? ? ? ? if (first instanceof TreeNode)
? ? ? ? ? ? ? ? return ((TreeNode<K,V>)first).getTreeNode(hash, key);
? ? ? ? ? ? do {
? ? ? ? ? ? ? ? if (e.hash == hash &&
? ? ? ? ? ? ? ? ? ? ((k = e.key) == key || (key != null && key.equals(k))))
? ? ? ? ? ? ? ? ? ? return e;
? ? ? ? ? ? } while ((e = e.next) != null);
? ? ? ? }
? ? }
? ? return null;
}