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根据面向对象编程的基本原则,数据应尽量的隐蔽。而ArrayList中的成员变量elementData却选择了默认可见性,也就是包可见性。源码中的注释为non-private to simplify nested class access,为了嵌套类可达。但即使外部类的成员变量为private,非静态嵌套类依然可达。
例如在内部类Itr,无论外部成员变量elementData是否为private,Itr均可访问。
ArrayList部分源码如下,
public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
transient Object[] elementData; // non-private to simplify nested class access
private int size;
public Iterator<E> iterator() {
return new Itr();
}
private class Itr implements Iterator<E> {
int cursor;
int lastRet = -1;
int expectedModCount = modCount;
public boolean hasNext() {
return cursor != size;
}
@SuppressWarnings("unchecked")
public E next() {
checkForComodification();
int i = cursor;
if (i >= size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
}
...
}
}