java8 stream用法收集（后续一直更行）_java stream union-优快云博客

博客介绍了使用Java Stream实现List<Data1>和List<Data2>根据Id进行连接，并将结果输出为List<OutputData>。同时展示了使用Stream进行交集、并集、差集运算，以及合并多个List的操作，给出了具体代码示例。

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// 集合1  
        List<SkillUpgrade> lists = new ArrayList<>();  
        SkillUpgrade s = new SkillUpgrade();  
        s.setLv(1);  
        s.setAppearNum(100);  
        lists.add(s);  
        SkillUpgrade s2 = new SkillUpgrade();  
        s2.setLv(2);  
        s2.setAppearNum(200);  
        lists.add(s2);  
        // 集合1  
        List<SkillUpgrade> listx = new ArrayList<>();  
        SkillUpgrade x = new SkillUpgrade();  
        x.setLv(1);  
        x.setSelectNum(1100);  
        listx.add(x);  
        SkillUpgrade x2 = new SkillUpgrade();  
        x2.setLv(2);  
        x2.setSelectNum(1200);  
        listx.add(x2);  
        // 把list转map,{k=lv,vaule=并为自身}  . SkillUpgrade->SkillUpgrade或Function.identity()  
        Map<Integer, SkillUpgrade> map = listx.stream()  
                .collect(Collectors.toMap(SkillUpgrade::getLv, SkillUpgrade -> SkillUpgrade));  
        System.out.println("map:="+map);  
        // 合并  
        lists.forEach(n -> {  
            // 如果等级一致  
            if (map.containsKey(n.getLv())) {  
                SkillUpgrade obj = map.get(n.getLv());  
                // 把数量复制过去  
                n.setSelectNum(obj.getSelectNum());  
            }  
        });  
        System.out.println("lists:="+lists);  
        // 重复问题  
        Map<Integer, SkillUpgrade> keyRedo = listx.stream()  
                .collect(Collectors.toMap(SkillUpgrade::getLv, Function.identity(), (key1, key2) -> key2));  
        // 方式二：指定实例的map  
        Map<Integer, SkillUpgrade> linkedHashMap = listx.stream().collect(Collectors.toMap(SkillUpgrade::getLv,  
                SkillUpgrade -> SkillUpgrade, (key1, key2) -> key2, LinkedHashMap::new));  
    }

实现List<Data1>和List<Data2>根据Id进行连接，将连接结果输出为一个List<OutputData>：

List<Data2> listOfData2 = new ArrayList<Data2>();

    listOfData2.add(new Data2(10501, "JOE"  , "Type1"));
    listOfData2.add(new Data2(10603, "SAL"  , "Type5"));
    listOfData2.add(new Data2(40514, "PETER", "Type4"));
    listOfData2.add(new Data2(59562, "JIM"  , "Type2"));
    listOfData2.add(new Data2(29415, "BOB"  , "Type1"));
    listOfData2.add(new Data2(61812, "JOE"  , "Type9"));
    listOfData2.add(new Data2(98432, "JOE"  , "Type7"));
    listOfData2.add(new Data2(62556, "JEFF" , "Type1"));
    listOfData2.add(new Data2(10599, "TOM"  , "Type4"));


    List<Data1> listOfData1 = new ArrayList<Data1>();

    listOfData1.add(new Data1(10501, "JOE"    ,3000000));
    listOfData1.add(new Data1(10603, "SAL"    ,6225000));
    listOfData1.add(new Data1(40514, "PETER"  ,2005000));
    listOfData1.add(new Data1(59562, "JIM"    ,3000000));
    listOfData1.add(new Data1(29415, "BOB"    ,3000000));

    List<OutputData> result = listOfData1.stream()
            .flatMap(x -> listOfData2.stream()
                    .filter(y -> x.getId() == y.getId())
                    .map(y -> new OutputData(y.getId(), x.getName(), y.getType(), x.getAmount())))
            .collect(Collectors.toList());
    System.out.println(result);

    /*difference by key*/
    List<Data1> data1IntersectResult = listOfData1.stream().filter(data1 -> listOfData2.stream().map(Data2::getId).collect(Collectors.toList()).contains(data1.getId())).collect(Collectors.toList());
    System.out.println(data1IntersectResult);

Map<Integer, ListContainer> map = Maps.newHashMap();
List<AClass> aClassList1 = Lists.newArrayList();
AClass aClass = new AClass(1, "zhuoli1", "haha1");
aClassList1.add(aClass);
aClassList1.add(new AClass(2, "zhuoli2", "haha2"));
aClassList1.add(new AClass(3, "zhuoli3", "haha3"));

List<AClass> aClassList2 = Lists.newArrayList();
aClassList2.add(aClass);
aClassList2.add(new AClass(5, "zhuoli5", "haha5"));
aClassList2.add(new AClass(6, "zhuoli6", "haha6"));

/*交集*/
/*[AClass(id=1, name=zhuoli1, description=haha1)]*/
List<AClass> intersectResult = aClassList1.stream().filter(aClassList2::contains).collect(Collectors.toList());
System.out.println(intersectResult);

/*并集*/
List<AClass> unionResult = Stream.of(aClassList1, aClassList2).flatMap(Collection::stream).distinct().collect(Collectors.toList());
assertEquals(unionResult.size(), 5);
System.out.println(unionResult);

/*差集*/
/*[AClass(id=2, name=zhuoli2, description=haha2), AClass(id=3, name=zhuoli3, description=haha3)]*/
List<AClass> differenceResult = aClassList1.stream().filter(x -> !aClassList2.contains(x)).collect(Collectors.toList());
System.out.println(differenceResult);

map.put(1, new ListContainer(aClassList1));
map.put(2, new ListContainer(aClassList2));

/*合并多个list*/
List<AClass> aClassListResult = map.values().stream().flatMap(listContainer -> listContainer.getLst().stream()).collect(Collectors.toList());
/*注意跟并集的区别*/
assertEquals(aClassListResult.size(), 6);
System.out.println(aClassListResult);