javalist初始值null_关于list集合存储null的问题

工作中&＃xff0c;遇到list集合存储null的问题&＃xff0c;不确定list能否存储null值。于是写一些demo测试list&＃xff0c;set,table,及map存储null的问题。

1.list之arraylist

1 public static voidmain(String[] args) {2 String string&＃61; null;3 List list&＃61; new ArrayList();4 System.out.println("size:"&＃43;list.size());5 list.add(string);6 System.out.println("size:"&＃43; list.size()&＃43; ",list(i):"&＃43;list.get(0));7 }

run&＃xff1a;

size:0

size:1,list(i):null

总结&＃xff1a;arraylist集合是能存储null的。

2.list之linkedlist

1 public static voidmain(String[] args) {2 List list&＃61; new LinkedList();3 System.out.println("list.size():"&＃43;list.size());4 String string&＃61; null;5 list.add(string);6 System.out.println("list.size():"&＃43;list.size()&＃43;", list.get(0):"&＃43;list.get(0));7 }

run:

list.size():0

list.size():1, list.get(0):null

结论&＃xff1a;linkedlist能存储null

3.set之hashset:

1 public static voidmain(String[] args) {2 //TODO Auto-generated method stub

3 Set set&＃61; new HashSet();4 System.out.println("set.size():"&＃43;set.size());5 String string&＃61; null;6 String string2&＃61; null;7 set.add(string);8 for (Iterator it&＃61;set.iterator() ; it.hasNext();) {9 string2 &＃61;it.next();10 }11 System.out.println("set.size():"&＃43;set.size()&＃43;", set.iterator.next():"&＃43;string2);12 }

run:

set.size():0

set.size():1,  set.iterator.next():null

结论&＃xff1a;hashset能存储null

4.set之treeset (1):

1 public static voidmain(String[] args) {2 Set set&＃61; new TreeSet();3 System.out.println("set.size():"&＃43;set.size());4 String string&＃61; null;5 String string2&＃61; null;6 set.add(string);7 for (Iterator it&＃61;set.iterator() ; it.hasNext();) {8 string2 &＃61;it.next();9 }10 System.out.println("set.size():"&＃43;set.size()&＃43;", set.iterator.next():"&＃43;string2);11 }

run:

set.size():0

Exception in thread "main" java.lang.NullPointerException

at java.util.TreeMap.compare(TreeMap.java:1188)

at java.util.TreeMap.put(TreeMap.java:531)

at java.util.TreeSet.add(TreeSet.java:255)

at com.westward.Demo3.main(Demo3.java:15)

4.set之treeset (2):

public static voidmain(String[] args) {

Set set&＃61; new TreeSet();

System.out.println("set.size():"&＃43;set.size());

String string&＃61; "1";

String string2&＃61; null;

set.add(string);for (Iterator it&＃61;set.iterator() ; it.hasNext();) {

string2&＃61;it.next();

}

System.out.println("set.size():"&＃43;set.size()&＃43;", set.iterator.next():"&＃43;string2);

}

run:

set.size():0

set.size():1,  set.iterator.next():1

结论&＃xff1a;treeset不能存储null

还有map&＃xff0c;及table&＃xff0c;未完待续...