1 Introduction

Evil nullhas plagued programmers for a long time, and also brought many find it difficult to cause serious damage NullPointerException. We need to avoid it as much as possible, there is a simple way is to deal with before it enters the next round, we'll strangle it in the cradle.

This article describes the four types of methods, namely Listthe method interface Stream, Guavaand Apache Commons Collectionsto delete an Listinside nullvalue. I hope readers can learn by analogy, to get more inspiration.

2 four categories Methods

2.1 List comes methods

ListThere are many removemethods you can use, we are able to meet the following three requirements:

List.remove(Object o): Delete an element, success is returned true; it only deletes a note;
List.removeAll(Collection<?> c): Delete all cases there is a collection of c, pay attention to the Senate is not an element;
List.removeIf(Predicate<? super E> filter): Delete all elements to meet the conditions, to the Senate for the Lambda expressions.

code show as below:

@Test
public void listRemove() {
  List<String> list = Lists.newArrayList("Cup", null, "Apple", null, "Desk");
  List<String> expected = Lists.newArrayList("Cup", "Apple", "Desk");
  //remove
  while (list.remove(null));//巧妙利用循环删除
  assertEquals(expected, list);
  //removeAll
  list = Lists.newArrayList("Cup", null, "Apple", null, "Desk");
  list.removeAll(Collections.singletonList(null));
  assertEquals(expected, list);
  //removeIf
  list = Lists.newArrayList("Cup", null, "Apple", null, "Desk");
  list.removeIf(Objects::isNull);
  assertEquals(expected, list);
}

2.2 Stream way

StreamThe method is easily understood, is to add a filter, the filter is non-empty condition, the specific code is as follows:

@Test
public void stream() {
  List<String> list = Lists.newArrayList("Cup", null, "Apple", null, "Desk");
  List<String> expected = Lists.newArrayList("Cup", "Apple", "Desk");
  List<String> result = list.parallelStream()
    .filter(Objects::nonNull)
    .collect(Collectors.toList());
  assertEquals(expected, result);
}

2.3 Guava library

GuavaIs a very good Java library that provides a lot of good way to deal with a set of classes, this time using a Iterablesclass processing code is as follows:

@Test
public void guava() {
  //改变原有List
  List<String> list = Lists.newArrayList("Cup", null, "Apple", null, "Desk");
  List<String> expected = Lists.newArrayList("Cup", "Apple", "Desk");
  Iterables.removeIf(list, Objects::isNull);
  assertEquals(expected, list);
  //保留原有List
  list = Lists.newArrayList("Cup", null, "Apple", null, "Desk");
  List<String> result = Lists.newArrayList(Iterables.filter(list, Objects::nonNull));
  assertEquals(expected, result);
}

This article provides two methods, one can change the original List, and the other does not.

2.4 Apache Commons library

Apache Commons CollectionsAlso provides a very convenient method, the specific code is as follows:

@Test
public void apacheCommonsCollections() {
  List<String> list = Lists.newArrayList("Cup", null, "Apple", null, "Desk");
  List<String> expected = Lists.newArrayList("Cup", "Apple", "Desk");
  CollectionUtils.filter(list, Objects::nonNull);
  assertEquals(expected, list);
}

3 summary

Although this article talking about the Listdeletion nullprocess, but slightly modified, can be extended to collections remove the value of certain conditions, it is still very useful.

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[Java] four types of compulsory method removes all null values inside the List