Java空集合使用场景与填坑记录

List<String> list = new Arraylist<>();

if(list!=null&&!list.isEmpty()){

//走集合不为空的逻辑

}

public static final List EMPTY_LIST = new EmptyList<>();

......

public static final <T> List<T> emptyList() {
 return (List<T>) EMPTY_LIST;
}

// 继承自AbstractList抽象类
private static class EmptyList<E>
 extends AbstractList<E>
 implements RandomAccess, Serializable {
 
 private static final long serialVersionUID = 8842843931221139166L;
 public Iterator<E> iterator() {
  return emptyIterator();
 }
 public ListIterator<E> listIterator() {
  return emptyListIterator();
 }
 public int size() {return 0;}
 public boolean isEmpty() {return true;}
 public boolean contains(Object obj) {return false;}
 public boolean containsAll(Collection<?> c) { return c.isEmpty(); }
 public Object[] toArray() { return new Object[0]; }
 
 public <T> T[] toArray(T[] a) {
  if (a.length > 0)
   a[0] = null;
  return a;
 }

 public E get(int index) {
  throw new IndexOutOfBoundsException("Index: "+index);
 }

 public boolean equals(Object o) {
  return (o instanceof List) && ((List<?>)o).isEmpty();
 }

 public int hashCode() { return 1; }

 @Override
 public boolean removeIf(Predicate<? super E> filter) {
  Objects.requireNonNull(filter);
  return false;
 }
 @Override
 public void replaceAll(UnaryOperator<E> operator) {
  Objects.requireNonNull(operator);
 }
 @Override
 public void sort(Comparator<? super E> c) {}

 // Override default methods in Collection
 @Override
 public void forEach(Consumer<? super E> action) {
  Objects.requireNonNull(action);
 }

 @Override
 public Spliterator<E> spliterator() { return Spliterators.emptySpliterator(); }
 // Preserves singleton property
 private Object readResolve() {
  return EMPTY_LIST;
 }
}

public void add(int index, E element) {
 throw new UnsupportedOperationException();
}

public E remove(int index) {
 throw new UnsupportedOperationException();
}

public class RemoveIfTest {
 private static List<Object> list = Collections.emptyList();
 public static void main(String[] args) {
  
  list.add("one1");
  list.add("one2");
  list.add(1);
  list.add(2);
  list.add(new Object());
  
  System.err.println(Arrays.toString(list.toArray()));
 }
}