item 20 junghyunlyoo - JAVA-JIKIMI/EFFECTIVE-JAVA3 GitHub Wiki

public interface Singer {
    AudioClip sing(Song s);
}

public interface Songwriter {
    Song compose(int chartPosition);
}

public interface SingerSongwriter extends Singer, Songwriter {
    AudioClip strum();
    void actSensitive();
}

interface Entry<K, V> {
    /**
     * Returns the key corresponding to this entry.
     *
     * @return the key corresponding to this entry
     * @throws IllegalStateException implementations may, but are not
     *         required to, throw this exception if the entry has been
     *         removed from the backing map.
     */
    K getKey();

    /**
     * Returns the value corresponding to this entry.  If the mapping
     * has been removed from the backing map (by the iterator's
     * {@code remove} operation), the results of this call are undefined.
     *
     * @return the value corresponding to this entry
     * @throws IllegalStateException implementations may, but are not
     *         required to, throw this exception if the entry has been
     *         removed from the backing map.
     */
    V getValue();

    /**
     * Replaces the value corresponding to this entry with the specified
     * value (optional operation).  (Writes through to the map.)  The
     * behavior of this call is undefined if the mapping has already been
     * removed from the map (by the iterator's {@code remove} operation).
     *
     * @param value new value to be stored in this entry
     * @return old value corresponding to the entry
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by the backing map
     * @throws ClassCastException if the class of the specified value
     *         prevents it from being stored in the backing map
     * @throws NullPointerException if the backing map does not permit
     *         null values, and the specified value is null
     * @throws IllegalArgumentException if some property of this value
     *         prevents it from being stored in the backing map
     * @throws IllegalStateException implementations may, but are not
     *         required to, throw this exception if the entry has been
     *         removed from the backing map.
     */
    V setValue(V value);

    /**
     * Compares the specified object with this entry for equality.
     * Returns {@code true} if the given object is also a map entry and
     * the two entries represent the same mapping.  More formally, two
     * entries {@code e1} and {@code e2} represent the same mapping
     * if<pre>
     *     (e1.getKey()==null ?
     *      e2.getKey()==null : e1.getKey().equals(e2.getKey()))  &amp;&amp;
     *     (e1.getValue()==null ?
     *      e2.getValue()==null : e1.getValue().equals(e2.getValue()))
     * </pre>
     * This ensures that the {@code equals} method works properly across
     * different implementations of the {@code Map.Entry} interface.
     *
     * @param o object to be compared for equality with this map entry
     * @return {@code true} if the specified object is equal to this map
     *         entry
     */
    boolean equals(Object o);

    /**
     * Returns the hash code value for this map entry.  The hash code
     * of a map entry {@code e} is defined to be: <pre>
     *     (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
     *     (e.getValue()==null ? 0 : e.getValue().hashCode())
     * </pre>
     * This ensures that {@code e1.equals(e2)} implies that
     * {@code e1.hashCode()==e2.hashCode()} for any two Entries
     * {@code e1} and {@code e2}, as required by the general
     * contract of {@code Object.hashCode}.
     *
     * @return the hash code value for this map entry
     * @see Object#hashCode()
     * @see Object#equals(Object)
     * @see #equals(Object)
     */
    int hashCode();

    /**
     * Returns a comparator that compares {@link Map.Entry} in natural order on key.
     *
     * <p>The returned comparator is serializable and throws {@link
     * NullPointerException} when comparing an entry with a null key.
     *
     * @param  <K> the {@link Comparable} type of then map keys
     * @param  <V> the type of the map values
     * @return a comparator that compares {@link Map.Entry} in natural order on key.
     * @see Comparable
     * @since 1.8
     */
    public static <K extends Comparable<? super K>, V> Comparator<Map.Entry<K, V>> comparingByKey() {
        return (Comparator<Map.Entry<K, V>> & Serializable)
            (c1, c2) -> c1.getKey().compareTo(c2.getKey());
    }

    /**
     * Returns a comparator that compares {@link Map.Entry} in natural order on value.
     *
     * <p>The returned comparator is serializable and throws {@link
     * NullPointerException} when comparing an entry with null values.
     *
     * @param <K> the type of the map keys
     * @param <V> the {@link Comparable} type of the map values
     * @return a comparator that compares {@link Map.Entry} in natural order on value.
     * @see Comparable
     * @since 1.8
     */
    public static <K, V extends Comparable<? super V>> Comparator<Map.Entry<K, V>> comparingByValue() {
        return (Comparator<Map.Entry<K, V>> & Serializable)
            (c1, c2) -> c1.getValue().compareTo(c2.getValue());
    }

    /**
     * Returns a comparator that compares {@link Map.Entry} by key using the given
     * {@link Comparator}.
     *
     * <p>The returned comparator is serializable if the specified comparator
     * is also serializable.
     *
     * @param  <K> the type of the map keys
     * @param  <V> the type of the map values
     * @param  cmp the key {@link Comparator}
     * @return a comparator that compares {@link Map.Entry} by the key.
     * @since 1.8
     */
    public static <K, V> Comparator<Map.Entry<K, V>> comparingByKey(Comparator<? super K> cmp) {
        Objects.requireNonNull(cmp);
        return (Comparator<Map.Entry<K, V>> & Serializable)
            (c1, c2) -> cmp.compare(c1.getKey(), c2.getKey());
    }

    /**
     * Returns a comparator that compares {@link Map.Entry} by value using the given
     * {@link Comparator}.
     *
     * <p>The returned comparator is serializable if the specified comparator
     * is also serializable.
     *
     * @param  <K> the type of the map keys
     * @param  <V> the type of the map values
     * @param  cmp the value {@link Comparator}
     * @return a comparator that compares {@link Map.Entry} by the value.
     * @since 1.8
     */
    public static <K, V> Comparator<Map.Entry<K, V>> comparingByValue(Comparator<? super V> cmp) {
        Objects.requireNonNull(cmp);
        return (Comparator<Map.Entry<K, V>> & Serializable)
            (c1, c2) -> cmp.compare(c1.getValue(), c2.getValue());
    }
}

import java.util.*;

// 코드 20-2 골격 구현 클래스 (134-135쪽)
public abstract class AbstractMapEntry<K, V>
        implements Map.Entry<K, V> {
    // 변경 가능한 엔트리는 이 메서드를 반드시 재정의해야 한다.
    @Override
    public V setValue(V value) {
        throw new UnsupportedOperationException();
    }

    // Map.Entry.equals의 일반 규약을 구현한다.
    @Override
    public boolean equals(Object o) {
        if (o == this)
            return true;
        if (!(o instanceof Map.Entry))
            return false;
        Map.Entry<?, ?> e = (Map.Entry) o;
        return Objects.equals(e.getKey(), getKey())
                && Objects.equals(e.getValue(), getValue());
    }

    // Map.Entry.hashCode의 일반 규약을 구현한다.
    @Override
    public int hashCode() {
        return Objects.hashCode(getKey())
                ^ Objects.hashCode(getValue());
    }

    @Override
    public String toString() {
        return getKey() + "=" + getValue();
    }
}

public static class SimpleEntry<K, V> implements Entry<K, V>, Serializable {
    private static final long serialVersionUID = -8499721149061103585L;
    private final K key;
    private V value;

    public SimpleEntry(K key, V value) {
        this.key = key;
        this.value = value;
    }

    public SimpleEntry(Entry<? extends K, ? extends V> entry) {
        this.key = entry.getKey();
        this.value = entry.getValue();
    }

    public K getKey() {
        return this.key;
    }

    public V getValue() {
        return this.value;
    }

    public V setValue(V value) {
        V oldValue = this.value;
        this.value = value;
        return oldValue;
    }

    public boolean equals(Object o) {
        if (!(o instanceof Entry)) {
            return false;
        } else {
            Entry<?, ?> e = (Entry)o;
            return AbstractMap.eq(this.key, e.getKey()) && AbstractMap.eq(this.value, e.getValue());
        }
    }

    public int hashCode() {
        return (this.key == null ? 0 : this.key.hashCode()) ^ (this.value == null ? 0 : this.value.hashCode());
    }

    public String toString() {
        return this.key + "=" + this.value;
    }
}

static List<Integer> intArrayAsList(int[] a) {
    Objects.requireNonNull(a);

    // 다이아몬드 연산자를 이렇게 사용하는 건 자바 9부터 가능하다.
    // 더 낮은 버전을 사용한다면 <Integer>로 수정하자.
    return new AbstractList<>() {
        @Override public Integer get(int i) {
            return a[i];  // 오토박싱(아이템 6)
        }

        @Override public Integer set(int i, Integer val) {
            int oldVal = a[i];
            a[i] = val;     // 오토언박싱
            return oldVal;  // 오토박싱
        }

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