內容來自《碼出高效》:
Arrays.asList方法可以將數組轉換成集合List,之後這個list可以set值,但是注意 不能修改元素的個數,因爲Arrays.asList返回的是Arrays的內部類,並沒有實現集合個數修改的相關方法,只有set方法。
正常情況(只set元素):
public class ArrayAsListTest {
public static void main(String[] args) {
String[] stringArray=new String[3];
stringArray[0]="one";
stringArray[1]="two";
stringArray[2]="three";
//array to list
List<String> stringList= Arrays.asList(stringArray);
stringList.set(0,"oneList");
System.out.println(stringArray[0]);
System.out.println(stringList);
//可以看到數組第一個元素的值也被修改爲了oneList
for (String s : stringArray) {
System.out.println(s);
}
//stringList.add("four");
}
}
輸出:
異常情況(修改了元素個數):
public class ArrayAsListTest {
public static void main(String[] args) {
String[] stringArray=new String[3];
stringArray[0]="one";
stringArray[1]="two";
stringArray[2]="three";
//array to list
List<String> stringList= Arrays.asList(stringArray);
stringList.set(0,"oneList");
//stringList.add("four");
stringList.remove(2);
stringList.clear();
}
}
報錯輸出:
Arrays.asList返回的List是Arrays的內部類 代碼如下:
private static class ArrayList<E> extends AbstractList<E>
implements RandomAccess, java.io.Serializable
{
private static final long serialVersionUID = -2764017481108945198L;
private final E[] a;
ArrayList(E[] array) {
a = Objects.requireNonNull(array);
}
@Override
public int size() {
return a.length;
}
@Override
public Object[] toArray() {
return a.clone();
}
@Override
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
int size = size();
if (a.length < size)
return Arrays.copyOf(this.a, size,
(Class<? extends T[]>) a.getClass());
System.arraycopy(this.a, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
@Override
public E get(int index) {
return a[index];
}
@Override
public E set(int index, E element) {
E oldValue = a[index];
a[index] = element;
return oldValue;
}
@Override
public int indexOf(Object o) {
E[] a = this.a;
if (o == null) {
for (int i = 0; i < a.length; i++)
if (a[i] == null)
return i;
} else {
for (int i = 0; i < a.length; i++)
if (o.equals(a[i]))
return i;
}
return -1;
}
@Override
public boolean contains(Object o) {
return indexOf(o) != -1;
}
@Override
public Spliterator<E> spliterator() {
return Spliterators.spliterator(a, Spliterator.ORDERED);
}
@Override
public void forEach(Consumer<? super E> action) {
Objects.requireNonNull(action);
for (E e : a) {
action.accept(e);
}
}
@Override
public void replaceAll(UnaryOperator<E> operator) {
Objects.requireNonNull(operator);
E[] a = this.a;
for (int i = 0; i < a.length; i++) {
a[i] = operator.apply(a[i]);
}
}
@Override
public void sort(Comparator<? super E> c) {
Arrays.sort(a, c);
}
}
這個不是集合中的那個ArrayList,是Arrays的內部類,有set get 但是並無修改元素個數的方法。
異常是由Arrays的內部類ArrayList的父類AbstractList的remove方法拋出的(Arrays的ArrayList並沒有override父類的這個方法)
最後介紹下Arrays的ArrayList的replaceAll怎麼使用的:
replaceAll其中的參數UnaryOperator<E> operator可以自定義
示例代碼如下:
public class ArrayListExample
{
public static void main(String[] args) throws CloneNotSupportedException
{
ArrayList<String> alphabets = new ArrayList<>(Arrays.asList("A", "B", "C", "D", "E"));
System.out.println(alphabets);
alphabets.replaceAll( new MyOperator() );
System.out.println(alphabets);
}
}
class MyOperator implements UnaryOperator<String>
{
@Override
public String apply(String t) {
return t.toLowerCase();
}
}