以下是ArrayList泛型类的实现。为避免与类库中的类混淆命名为MyArrayLIst,主要细节有:
- 成员变量包含基础数组,数组容量,以及存储在MyArrayList中的当前项数。
- 提供一种机制以改变基础数组的容量。通过获得一个新数组,将老数组复制到新数组来改变新数组的容量,允许虚拟机回收老数组。
- 提供get()和set()的实现。
- 提供基本的操作,如size(),isEmpty()和clear(),还提供remove(idx),以及add(x)和add(idx,x)的操作。如果数组大小和容量相同,那么这两个add操作将扩大容量。
- 提供一个实现Iterator接口的类。这个类将存储迭代序列中的下一项的下标,并提供next(),hasNext()和remove()等方法的实现。迭代器方法直接返回实现Iterator接口的该类新实例。
import java.util.Iterator;
import java.util.NoSuchElementException;
public class MyArrayList<AnyType> implements Iterable<AnyType> {
private static final int DEFAULT_CAPACITY = 10;
private int theSize;
private AnyType[] theItems;
public MyArrayList() {
clear();
}
public void clear() {
theSize = 0;
ensureCapacity(DEFAULT_CAPACITY);
}
public int size() {
return theSize;
}
public boolean isEmpty() {
return size() == 0;
}
public void trumToSize() {
ensureCapacity(size());
}
public AnyType get(int idx) {
if (idx < 0 || idx >= size()) {
throw new ArrayIndexOutOfBoundsException();
}
return theItems[idx];
}
public AnyType set(int idx, AnyType newVal) {
if (idx < 0 || idx >= size()) {
throw new ArrayIndexOutOfBoundsException();
}
AnyType old = theItems[idx];
theItems[idx] = newVal;
return old;
}
@SuppressWarnings("unchecked")
public void ensureCapacity(int newCapacity) {
if (newCapacity < size()) {
return;
}
AnyType[] old = theItems;
theItems = (AnyType[]) new Object[newCapacity];
for (int i = 0; i < size(); i++) {
theItems[i] = old[i];
}
}
public void add(AnyType x) {
add(size(), x);
}
public void add(int idx, AnyType x) {
if (theItems.length == size()) {
ensureCapacity(size() * 2 + 1);
}
for (int i = size(); i > idx; i--) {
theItems[i] = theItems[i - 1];
}
theItems[idx] = x;
theSize++;
}
public AnyType remove(int idx) {
AnyType removedItem = theItems[idx];
for (int i = idx; i < size() - 1; i++) {
theItems[i] = theItems[i + 1];
}
theSize--;
return removedItem;
}
public Iterator<AnyType> iterator() {
return new ArrayListIterator();
}
private class ArrayListIterator implements Iterator<AnyType> {
private int current = 0;
public boolean hasNext() {
return current < size();
}
public AnyType next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
return theItems[current++];
}
public void remove() {
MyArrayList.this.remove(--current);
}
}
}