java 将list按照指定数量分成小list

阳光穿透心脏的1/2处 2021-09-14 00:48 422阅读 0赞

**1、自己编写相关代码：**

**2、使用guava：**

import java.util.ArrayList;  
    import java.util.List;  
    import com.google.common.collect.Lists;  
      
      
    public class Test4 {  
      
        public static void main(String[] args) {  
            List<Long> list = new ArrayList<>();  
            list.add(1L);  
            list.add(2L);  
            list.add(3L);  
            list.add(4L);  
            list.add(5L);  
            list.add(6L);  
            list.add(7L);  
            list.add(8L);  
            list.add(9L);  
              
            //test1(list);  
            test2(list);  
        }  
          
        private static void test1(List<Long> list) {  
            int size = 2;  
            List<List<Long>> listArr = new ArrayList<>();    
              
            int arrSize = list.size()%size==0?list.size()/size:list.size()/size+1;    
            for(int i=0;i<arrSize;i++) {    
                List<Long>  sub = new ArrayList<>();    
                for(int j=i*size;j<=size*(i+1)-1;j++) {    
                    if(j<=list.size()-1) {    
                        sub.add(list.get(j));    
                    }    
                }    
                listArr.add(sub);    
            }    
            System.out.println(listArr.toString());  
        }  
          
          
        private static void test2(List<Long> list) {  
            int size = 16;  
            List<List<Long>> subSets = Lists.partition(list, size);    
            System.out.println(subSets.toString());  
        }  
    }

### 补充：guava分割其他collections[iew plain][] ###

@Test    
    public void givenCollection_whenParitioningIntoNSublists_thenCorrect() {    
        Collection<Integer> intCollection = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);    
         
        Iterable<List<Integer>> subSets = Iterables.partition(intCollection, 3);    
         
        List<Integer> firstPartition = subSets.iterator().next();    
        List<Integer> expectedLastPartition = Lists.<Integer> newArrayList(1, 2, 3);    
        assertThat(firstPartition, equalTo(expectedLastPartition));    
    }

以上需要注意的是，partition返回的是原list的subview.视图，也即，原list改变后，partition之后的结果也会随着改变。

@Test    
    public void givenListPartitioned_whenOriginalListIsModified_thenPartitionsChangeAsWell() {    
        // Given    
        List<Integer> intList = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);    
        List<List<Integer>> subSets = Lists.partition(intList, 3);    
         
        // When    
        intList.add(9);    
         
        // Then    
        List<Integer> lastPartition = subSets.get(2);    
        List<Integer> expectedLastPartition = Lists.<Integer> newArrayList(7, 8, 9);    
        assertThat(lastPartition, equalTo(expectedLastPartition));    
    }

**3、使用apache commons collection：**

@Test    
    public void givenList_whenParitioningIntoNSublists_thenCorrect() {    
        List<Integer> intList = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);    
        List<List<Integer>> subSets = ListUtils.partition(intList, 3);    
         
        List<Integer> lastPartition = subSets.get(2);    
        List<Integer> expectedLastPartition = Lists.<Integer> newArrayList(7, 8);    
        assertThat(subSets.size(), equalTo(3));    
        assertThat(lastPartition, equalTo(expectedLastPartition));    
    }

1.没有对应的Iterable.partions方法，类似guava那样

2.partition后的结果同样是原集合的视图。

**4、Java8方法**

1）通过grouping by：

@Test    
    public final void givenList_whenParitioningIntoNSublistsUsingGroupingBy_thenCorrect() {    
        List<Integer> intList = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);    
         
        Map<Integer, List<Integer>> groups =     
          intList.stream().collect(Collectors.groupingBy(s -> (s - 1) / 3));    
        List<List<Integer>> subSets = new ArrayList<List<Integer>>(groups.values());    
         
        List<Integer> lastPartition = subSets.get(2);    
        List<Integer> expectedLastPartition = Lists.<Integer> newArrayList(7, 8);    
        assertThat(subSets.size(), equalTo(3));    
        assertThat(lastPartition, equalTo(expectedLastPartition));    
    }

按年龄分组：

Map<Integer, List<Person>> personGroups = Stream.generate(new PersonSupplier()).    
     limit(100).    
     collect(Collectors.groupingBy(Person::getAge));    
    Iterator it = personGroups.entrySet().iterator();    
    while (it.hasNext()) {    
     Map.Entry<Integer, List<Person>> persons = (Map.Entry) it.next();    
     System.out.println("Age " + persons.getKey() + " = " + persons.getValue().size());    
    }

2）通过partition by：

@Test  
    public void givenList_whenParitioningIntoSublistsUsingPartitionBy_thenCorrect() {  
        List<Integer> intList = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);  
       
        Map<Boolean, List<Integer>> groups =   
          intList.stream().collect(Collectors.partitioningBy(s -> s > 6));  
        List<List<Integer>> subSets = new ArrayList<List<Integer>>(groups.values());  
       
        List<Integer> lastPartition = subSets.get(1);  
        List<Integer> expectedLastPartition = Lists.<Integer> newArrayList(7, 8);  
        assertThat(subSets.size(), equalTo(2));  
        assertThat(lastPartition, equalTo(expectedLastPartition));  
    }

按照成年未成年人分组：

Map<Boolean, List<Person>> children = Stream.generate(new PersonSupplier()).  
     limit(100).  
     collect(Collectors.partitioningBy(p -> p.getAge() < 18));  
    System.out.println("Children number: " + children.get(true).size());  
    System.out.println("Adult number: " + children.get(false).size());

注意：  
1.Java8方式，分组后的list不再是原list的视图。所以，原list的改变不会影响分组后的结果。  
2partitioningBy 其实是一种特殊的 groupingBy，它依照条件测试的是否两种结果来构造返回的数据结构，get(true) 和 get(false) 能即为全部的元素对象。

[iew plain]: http://blog.csdn.net/pandawang1989/article/details/72801630#