JDK8集合操作

因为太多了，这里列举几种比较常用的：

1）遍历list

2）遍历list得到map

3）遍历list，根据某个key进行分组

4）遍历多个对象，合并多个对象中的明细行

5）foreach：遍历操作，无返回值

6）distrinct：去重操作

7）filter：过滤，返回filter方法体中为true的对象

首先引入guaua包和fastjson包，方便打印：


               
    
     com.google.guava
                
    
     guava
                
    
     19.0
            
           
               
    
     com.alibaba
                
    
     fastjson
                
    
     1.2.60

1）代码示例：

package com.test;import com.alibaba.fastjson.JSON;import com.google.common.base.Joiner;import com.sun.scenario.effect.Identity;import org.junit.Test;import java.lang.reflect.Array;import java.util.*;import java.util.function.Function;import java.util.stream.Collectors;public class JDK8Test {
       class A{
           private String id;        private String name;        private List lines;        public List getLines() {
               return lines;        }        public void setLines(List lines) {
               this.lines = lines;        }        public void setId(String id) {
               this.id = id;        }        public void setName(String name) {
               this.name = name;        }        public String getId() {
               return id;        }        public String getName() {
               return name;        }    }    /***     * 遍历list     */    @Test    public void testList(){
           List list = new ArrayList<>();        A a = new A();        a.setId("1");        a.setName("张三");        A a2 = new A();        a2.setId("2");        a2.setName("李四");        list.add(a);        list.add(a2);        List
    
      idList = list.stream().map(A::getId).collect(Collectors.toList());        System.out.println(Joiner.on(",").join(idList));        //运行结果：1,2    }    /***     * 遍历list分组得到map     */    @Test    public void testGetMap(){
           List
     list = new ArrayList<>();        A a = new A();        a.setId("1");        a.setName("张三");        A a2 = new A();        a2.setId("2");        a2.setName("李四");        list.add(a);        list.add(a2);        Map
    
      idList = list.stream().collect(Collectors.toMap(A::getId,A::getName));        Map
     
       aList = list.stream().collect(Collectors.toMap(A::getId, Function.identity()));        System.out.println(idList.toString());        System.out.println(JSON.toJSONString(aList));		//运行结果：		//{1=张三, 2=李四}		//{"1":{"id":"1","name":"张三"},"2":{"id":"2","name":"李四"}}    }    /***     * 遍历list，根据id进行分组     */    @Test    public void testGroupMap(){
           List
     
     list = new ArrayList<>();        A a = new A();        a.setId("1");        a.setName("张三");        a.setId("1");        a.setName("王五");        A a2 = new A();        a2.setId("2");        a2.setName("李四");        list.add(a);        list.add(a2);        Map
    > idList = list.stream().collect(Collectors.groupingBy(A::getId));        System.out.println(JSON.toJSONString(idList));        //运行结果：        //{"1":[{"id":"1","name":"王五"}],"2":[{"id":"2","name":"李四"}]}    }    /***     * flatmap合并用法     */    @Test    public void testFlatMap(){
           A a = new A();        a.setName("name1");        a.setId("1");        A a2 = new A();        a2.setName("name2");        a2.setId("2");        List list1 = new ArrayList<>();        list1.add(a);        list1.add(a2);        A a3 = new A();        a3.setName("name3");        a3.setId("3");        A a4 = new A();        a4.setName("name4");        a4.setId("4");        List list2 = new ArrayList<>();        list2.add(a3);        list2.add(a4);        //对象A1        A test1 = new A();        A test2 = new A();        test1.setLines(list1);        test2.setLines(list2);        List list = new ArrayList<>();        list.add(test1);        list.add(test2);        List allList = list.stream().map(A::getLines).flatMap(Collection::stream).collect(Collectors.toList());        System.out.println(JSON.toJSONString(allList));        //运行结果：        //[{"id":"1","name":"name1"},{"id":"2","name":"name2"},{"id":"3","name":"name3"},{"id":"4","name":"name4"}]    }}

当前还有很多其他的用法：

1）foreach：遍历操作，无返回值

2）distrinct：去重操作

3）filter：过滤，返回filter方法体中为true的对象

我们写一段代码来一次性演示3种用法：

@Test    public void testForeach(){
           A a = new A();        a.setId("1");        a.setName("1");        A a2 = new A();        a2.setId("1");        a2.setName("2");        A a3 = new A();        a3.setId("3");        a3.setName("3");        List list = new ArrayList<>();        list.add(a);        list.add(a2);        list.add(a3);        //foreach操作：        list.forEach(item->{
               System.out.println(item.getName());        });        //distinct操作：        List
    
      idList = list.stream().map(A::getId).distinct().collect(Collectors.toList());        System.out.println(Joiner.on(",").join(idList));        //filter操作：        List
     aList = list.stream().filter(item->!"1".equals(item.getId())).collect(Collectors.toList());        System.out.println(JSON.toJSONString(aList));    }

运行结果：