20201029_24 자바 해시

• 동일성, 동등성
  동등성(equality) 두 객체의 내용이 같나
  동일성(identity) 두 객체가 정확히 같은 객체인지

 

• equals메서드 : 두값이 동등한가?
  ==연산자 : 두 객체 동일한가?
  
  
  
  
  
• 해시함수
  임의의 길이로 된 데이터를 고정된 길이의 데이터로 매핑하는 함수
  h(x) = y
  -해시값
  -해시코드
  -체크섬

public class StickCoffee {
    private final String name;
    private final long price;
    public StickCoffee(String name, long price) {
        this.name = name;
        this.price = price;
    }
    // sc1.equals(sc2)
    @Override
    public boolean equals(Object o) {
        // 자기자신인가?
        if (this == o)
            return true;
        // 1. null이면 동등하지 않음
        // 2. 클래스가 동일하지 않으면 동등하지 않음
        if (o == null || getClass() != o.getClass())
            return false;
        // 값이 같으면 동등함.
        StickCoffee that = (StickCoffee) o;
        return price == that.price &&
                Objects.equals(name, that.name);
    }
    @Override
    public int hashCode() {
        return Objects.hash(name, price);
    }
    @Override
    public String toString() {
        return "StickCoffee{" +
                "name='" + name + '\'' +
                ", price=" + price +
                '}';
    }
}

 

public class Application {
    public static void main(String[] args) {
        /*final StickCoffee sc1 = new StickCoffee("maxim", 1000);
        final StickCoffee sc2 = new StickCoffee("maxim", 1000);
        System.out.println(sc1 == sc2);     // false
        System.out.println(sc1.equals("abc")); // true
        System.out.println(sc1.getClass());*/
        final StickCoffee sc1 = new StickCoffee("maxim", 1000);
        final StickCoffee sc2 = new StickCoffee("maxim", 1000);
        final StickCoffee sc3 = new StickCoffee("maxim", 1000);
        final HashMap<StickCoffee, Integer> map = new HashMap<>();
        map.put(sc1, 100);
        map.put(sc2, 200);
        map.put(sc3, 300);
        System.out.println(1081127135 % 128);
        System.out.println("++++++++++++++++");
        System.out.println(sc1 == sc2); // 동일한 객체인가?
        System.out.println(sc1.equals(sc2));    // 동등성 (값이 완전하게 같음?)
        System.out.println(sc1.hashCode());
        System.out.println(sc2.hashCode());
        System.out.println(sc3.hashCode());
        System.out.println("=================================\n\n\n\n");
        System.out.println(map.get(sc1));   //
        System.out.println(map.get(sc2));   //
        System.out.println(map.get(sc3));   //
    }
}

 

public class HashMap<K, V> {
    private static final int TABLE_SIZE = 128;
    private HashEntry<K, V>[] table;
    public HashMap() {
        table = new HashEntry[TABLE_SIZE];
    }
    // stick coffee
    public V get(K key) {
        // 95
        final int hashIndex = getHashIndex(key);
        final HashEntry<K, V> entry = table[hashIndex];
        return entry.getValue();
    }
    public void put(K key, V value) {
        // 95
        final int hashIndex = getHashIndex(key);
        final HashEntry<K, V> entry = new HashEntry<>(key, value);
        table[hashIndex] = entry;
    }
    private int getHashIndex(K key) {
        // sc1.hashCode = -1081127135
        // sc2.hashCode = -1081127135
        // sc3.hashCode = -1081127135
        // 1081127135
        return Math.abs(key.hashCode()) % TABLE_SIZE;
    }
    private static class HashEntry<K, V> {
        private final K key;
        private final V value;
        public HashEntry(K key, V value) {
            this.key = key;
            this.value = value;
        }
        public K getKey() {
            return key;
        }
        public V getValue() {
            return value;
        }
    }
}