帮助删除 Java 列表中的空引用？
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Helper to remove null references in a Java List?
提问by Arthur Ronald
Given the following List:
鉴于以下列表：
List<String> list = new ArrayList<String>();
list.add("s1");
list.add("s2");
list.add(null);
list.add("s3");
list.add(null);
list.add("s4");
I need a helper class that removes null references. Something like:
我需要一个帮助程序类来删除空引用。就像是：
SomeHelper.removeNullReference(list);
SomeHelper.removeNullReference(list);
such that the list only contains "s1", "s2", "s4", "s4" (non-null references).
这样列表只包含“s1”、“s2”、“s4”、“s4”（非空引用）。
What should I use to fulfill this requirement?
我应该用什么来满足这个要求？
采纳答案by Michael Borgwardt
list.removeAll(Collections.singleton(null));
回答by Dónal
If you can control the instantiation of the Listthen you could prevent the nulls being added by writing a custom Listimplementation such as:
如果您可以控制 的实例化，List那么您可以通过编写自定义List实现来防止添加空值，例如：
public class NoNullsList<E> extends ArrayList<E> {
public void add(int index, E element) {
if (element != null) {
super.add(index, element);
}
}
public boolean add(E e) {
return e == null ? false : super.add(e);
}
}
AFAIK, you don't need to override addAllbecause the ArrayListimplementation thereof calls add.
AFAIK，您不需要覆盖，addAll因为其ArrayList实现调用add.
List<String> list = new NoNullsList<String>();
list.add("s1");
list.add("s2");
list.add(null);
list.add("s3");
list.add(null);
list.add("s4");
回答by debaser
A drawback of Don's approach is that it extends from ArrayList. That may be good enough for now, but what happens when you want to use a different Listimplementation? You could make a NoNullsLinkedListand a NoNullsCopyOnWriteArrayList, etc, but then you wind up with a bunch of little classes that differ only in their extendsclause. It might be better to create a Listwrapper that doesn't accept null values. For example:
Don 方法的一个缺点是它从ArrayList. 现在这可能已经足够了，但是当您想使用不同的List实现时会发生什么？您可以创建 aNoNullsLinkedList和 aNoNullsCopyOnWriteArrayList等，但是您最终会得到一堆仅在extends子句上有所不同的小类。最好创建一个List不接受空值的包装器。例如：
public class NonNullList<E> extends AbstractList<E> {
private final List<E> delegate;
public NonNullList(List<E> delegate) {
this.delegate = delegate;
}
@Override
public E get(int index) {
return delegate.get( index );
}
@Override
public int size() {
return delegate.size();
}
@Override
public E set(int index, E element) {
return delegate.set( index, element );
}
@Override
public void add(int index, E element) {
if( element != null ) {
delegate.add( index, element );
}
}
@Override
public E remove(int index) {
return delegate.remove( index );
}
}
It's more code, but now you have the flexibility of choosing a Listimplementation when you create the object.
这是更多的代码，但现在您可以在List创建对象时灵活地选择实现。
A possible problem is that you can still insert nulls into the underlying Listobject. Don's approach doesn't have the same limitation.
一个可能的问题是您仍然可以将nulls 插入到底层List对象中。Don 的方法没有相同的限制。
回答by Jeffrey Bosboom
Java 8 added Collection.removeIf(Predicate)that removes all elements matching the predicate, so you can remove all occurrences of nullfrom a list (or any collection) with
Java 8 添加Collection.removeIf(Predicate)了删除与谓词匹配的所有元素，因此您可以null从列表（或任何集合）中删除所有出现的
list.removeIf(Objects::isNull);
using java.util.Objects.isNullas a Predicate.
使用java.util.Objects.isNull的Predicate。
You can also use .removeIf(x -> x == null)if you prefer; the difference is very minor.
.removeIf(x -> x == null)如果您愿意，也可以使用；差别很小。
回答by omt66
I really like the Java 8's new concise solution, I believe you should use that whenever possible.
我真的很喜欢 Java 8 新的简洁解决方案，我相信你应该尽可能使用它。
list.removeIf(Objects::isNull);
On the other side, if you are looking for a simple implementation to learn how it could be done, such as a helper class that returns new non-null list, you can take a look at the following code. It can also be made to accept generic types, for the sake of simplicity I used the same collection type (as in the question) in the helper class.
另一方面，如果您正在寻找一个简单的实现来了解它是如何完成的，例如返回新的非空列表的帮助类，您可以查看以下代码。它也可以接受泛型类型，为了简单起见，我在 helper 类中使用了相同的集合类型（如问题中所示）。
package removenullrefdemo;
import java.util.ArrayList;
import java.util.List;
import java.util.Objects;
class ListHelper {
public static List<String> removeNullReference(List<String> list) {
List<String> newList = new ArrayList<>();
list.forEach(item -> {
if (item != null)
newList.add(item);
});
return newList;
}
}
public class Main {
public static void main(String[] args) {
List<String> list = new ArrayList<String>();
list.add("s1");
list.add("s2");
list.add(null);
list.add("s3");
list.add(null);
list.add("s4");
println("Items:");
list.forEach(item -> println(item));
// Remove null refs
println("Items (after nulls removed):");
boolean useJava8removeIf = false;
if (useJava8removeIf) {
println("Using Java8 removeIf method with predicate.");
list.removeIf(Objects::isNull);
}
else {
println("Using our own helper.");
list = ListHelper.removeNullReference(list);
}
list.forEach(item -> println(item));
}
static void println(String msg) {
System.out.println(msg);
}
}
