Collections.singleton(null)와 Collections.removeAll(Collections.singleton(null)) 예시

Collection에서 add나 remove를 할 때, loop 안에서 작업하면 CME 가 발생할 수 있다.

@Test
public void exceptionTest() {
 List<Member> members = Lists.newArrayList();
 members.add(new Member("1", "true"));
 members.add(null);
 members.add(new Member("3", "false"));

 for (Member member : members) {
  if (member == null) {
   members.remove(member);
   members.add(new Member("4", "true")); // 여기서 CME 발생
  }
 }

 System.out.println(members.size());
}

결과

java.util.ConcurrentModificationException

at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)

at java.util.ArrayList$Itr.next(ArrayList.java:851)

at com.google.NullElementTest.exceptionTest(NullElementTest.java:20)

Collection에서 null 인 엘리먼트를 삭제하려면, Collections.removeAll()을 사용할 수 있다.

바로 Collections.removeAll(null)을 호출하면, cme가 발생한다.

따라서, 이를 해결하려면, Collection을 하나 만들고 null 엘리먼트를 추가한 후, members.removeAll(리스트)를 호출한다.

@Test
public void avoidNpeTest() {
 List<Member> members = Lists.newArrayList();
 members.add(new Member("1", "true"));
 members.add(null);
 members.add(new Member("3", "false"));

 // members.removeAll(null); // error

 List<Member> nullList = Lists.newArrayList();
 nullList.add(null);
 members.removeAll(nullList);

 System.out.println(members);
}

결과

[com.google.NullElementTest$Member@2133c8f8[id=1,followable=true], com.google.NullElementTest$Member@3ac3fd8b[id=3,followable=false]]

조금 verbose하니, Collections.singleton(null)을 이용하면 이전 예시와 동일한 결과를 얻을 수 있다.

@Test
public void avoidElegantNpeTest() {
 List<Member> members = Lists.newArrayList();
 members.add(new Member("1", "true"));
 members.add(null);
 members.add(new Member("3", "false"));

 System.out.println(Collections.singleton(null));

 members.removeAll(Collections.singleton(null));
 System.out.println(members);
}

결과

[null]

[com.google.NullElementTest$Member@2133c8f8[id=1,followable=true], com.google.NullElementTest$Member@3ac3fd8b[id=3,followable=false]]

참고 : 코드 전문

package com.google;

import com.clearspring.analytics.util.Lists;
import org.apache.commons.lang.builder.ReflectionToStringBuilder;
import org.apache.commons.lang3.StringUtils;
import org.junit.Test;

import java.util.Collections;
import java.util.List;

public class CMETest {

 @Test
 public void exceptionTest() {
  List<Member> members = Lists.newArrayList();
  members.add(new Member("1", "true"));
  members.add(null);
  members.add(new Member("3", "false"));

  for (Member member : members) {
   if (member == null) {
    members.remove(member);
    members.add(new Member("4", "true"));
   }
  }

  System.out.println(members.size());
 }

 @Test
 public void avoidNpeTest() {
  List<Member> members = Lists.newArrayList();
  members.add(new Member("1", "true"));
  members.add(null);
  members.add(new Member("3", "false"));

  // members.removeAll(null); // error

  List<Member> nullList = Lists.newArrayList();
  nullList.add(null);
  members.removeAll(nullList);

  System.out.println(members);
 }

 @Test
 public void avoidElegantNpeTest() {
  List<Member> members = Lists.newArrayList();
  members.add(new Member("1", "true"));
  members.add(null);
  members.add(new Member("3", "false"));

  System.out.println(Collections.singleton(null));

  members.removeAll(Collections.singleton(null));
  System.out.println(members);
 }

 class Member {
  public String id;
  public String followable;

  public Member(String id, String f) {
   this.id = id;
   this.followable = f;
  }

  public String toString() {
   return ReflectionToStringBuilder.toString(this);
  }
 }
}