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骨格実装 (Skeletal Implementation)

骨格実装 (Skeletal Implementation) とは何か

  • 抽象クラスのうち、interface を implements し、このメソッド内容を定義したもの

目的は何か

  • 骨格実装継承クラスの interface メソッド実装定義を手助けすること

骨格実装の構築方法, 利用方法

骨格実装 提供側

骨格実装 利用側


AbstractList.java より 骨格実装の一例

jdk1.8 src.zip 展開後の java/util/(List | AbstractList | ArrayList).java より

E get(int index) メソッド(骨格実装利用者にて実装委任)

void clear() メソッド(骨格実装利用者にて実装再定義)

の内容確認


List.java (interface)
    X (implement)
    |
AbstractList.java (骨格実装)
    Y (inherit)
    |
ArrayList.java (骨格実装 利用者)

java/util/List.java (単純なインターフェース)

public interface List<E> extends Collection<E> {
  .
  .
 /** 
  * Returns the element at the specified position in this list.
  *
  * @param index index of the element to return
  * @return the element at the specified position in this list
  * @throws IndexOutOfBoundsException if the index is out of range
  *         (<tt>index &lt; 0 || index &gt;= size()</tt>)
  */
  E get(int index);
  .
  .
  /**
  * Removes all of the elements from this list (optional operation).
  * The list will be empty after this call returns.
  *
  * @throws UnsupportedOperationException if the <tt>clear</tt> operation
  *         is not supported by this list
  */
  void clear();
}

java/util/AbstractList.java (骨格実装)

public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
  .
  .
 /**
  * {@inheritDoc}
  *
  * @throws IndexOutOfBoundsException {@inheritDoc}
  */
  abstract public E get(int index);

  .
  .

 /**
  * Removes all of the elements from this list (optional operation).
  * The list will be empty after this call returns.
  *
  * <p>This implementation calls {@code removeRange(0, size())}.
  *
  * <p>Note that this implementation throws an
  * {@code UnsupportedOperationException} unless {@code remove(int
  * index)} or {@code removeRange(int fromIndex, int toIndex)} is
  * overridden.
  *
  * @throws UnsupportedOperationException if the {@code clear} operation
  *         is not supported by this list
  */
  public void clear() {
    removeRange(0, size());
  }
  
  // ドキュメンテーション省略
  protected void removeRange(int fromIndex, int toIndex) {
    ListIterator<E> it = listIterator(fromIndex);
    for (int i=0, n=toIndex-fromIndex; i<n; i++) {
        it.next();
        it.remove();
    }
  }
}

(ここまで 骨格実装提供者の領分)


(ここから 骨格実装利用者の領分)

java/util/ArrayList.java (骨格実装 利用者が実装するクラス)

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
  /**
   * Returns the element at the specified position in this list.
   *
   * @param  index index of the element to return
   * @return the element at the specified position in this list
   * @throws IndexOutOfBoundsException {@inheritDoc}
   */
  public E get(int index) {
    rangeCheck(index);
    
    return elementData(index);
  }
  
  // ドキュメンテーション省略
  private void rangeCheck(int index) {
    if (index >= size)
        throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
  }
  .
  .
  
  /**
  * Removes all of the elements from this list.  The list will
  * be empty after this call returns.
  */
  public void clear() {
    modCount++;
    
    // clear to let GC do its work
    for (int i = 0; i < size; i++)
      elementData[i] = null;
      
      size = 0;
  }

}

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