java8 转对象，Java8转Map，Java8转Llist

1.准备数据

public static List<Persion> getData(){List<Persion> arrayList = new ArrayList<>();arrayList.add(new Persion("李四","20","男"));arrayList.add(new Persion("王麻子","30","男"));arrayList.add(new Persion("wangwu","10","女"));arrayList.add(new Persion("wangwu","10","女"));arrayList.add(null);return arrayList;}

2.List对象转Map

    public static void main(String[] args) {List<Persion> persions = getData();//list对象转 Map<String,String> name 是key,age是value。Map<String, String> stringMap = persions.stream().collect(Collectors.toMap(Persion::getName, Persion::getSex));System.out.println("stringMap:"+stringMap);// 结果：stringMap:{李四=男, 王麻子=男, wangwu=女}}

3.List对象转Map对象并去重

    public static void main(String[] args) {List<Persion> persions = getData();Map<String, Persion> stringMap = persions.stream().collect(Collectors.toMap(Persion::getName, Function.identity(), (key1, key2) -> key2));System.out.println("stringMap:"+stringMap);}
//输出：stringMap:{李四=Persion{name='李四', age='20', sex='男'}, 王麻子=Persion{name='王麻子', age='30', sex='男'}, wangwu=Persion{name='wangwu', age='10', sex='女'}}

//收集对象本身并转换Map<String, Persion> persionMap2 = persions.stream().collect(Collectors.toMap(Persion::getName, v -> new Persion(v.getName(),"","")));

4.List对象转List转Set转List 都类似

    public static void main(String[] args) {List<Persion> persions = getData();List<Persion> persions1 = persions.stream().collect(Collectors.toList());
//        list对象转list对象的属性Set<String> collect2 = persions.stream().map(Persion::getName).collect(Collectors.toSet());System.out.println("stringMap:"+collect2);// 结果：stringMap:stringMap:[李四, 王麻子, wangwu]}

5.多个字段排序 第一个字段降序,第二个字段升序

    public static void main(String[] args) {List<Persion> persions = getData();List<Persion> persions1 = persions.stream().sorted(Comparator.comparing(Persion::getAge).reversed().thenComparing(Persion::getAge)).collect(Collectors.toList());System.out.println("stringMap:"+persions1);}// 结果：stringMap:[Persion{name='王麻子', age='30', sex='男'}, Persion{name='李四', age='20', sex='男'}, Persion{name='wangwu', age='10', sex='女'}, Persion{name='wangwu', age='10', sex='女'}]

6.根据某个字段分组

  public static void main(String[] args) {List<Persion> persions = getData();Map<String, List<Persion>> collect3 = persions.stream().collect(Collectors.groupingBy(Persion::getAge));System.out.println("stringMap:"+collect3);}
// 结果：stringMap:{30=[Persion{name='王麻子', age='30', sex='男'}], 20=[Persion{name='李四', age='20', sex='男'}], 10=[Persion{name='wangwu', age='10', sex='女'}, Persion{name='wangwu', age='10', sex='女'}]}

7.去除空元素

    public static void main(String[] args) {List<Persion> persions = getData();boolean removeAll = persions.removeAll(Collections.singleton(null));System.out.println("stringMap:"+removeAll);System.out.println("persions:"+persions);}
//结果：stringMap:true
//persions:[Persion{name='李四', age='20', sex='男'}, Persion{name='王麻子', age='30', sex='男'}, Persion{name='wangwu', age='10', sex='女'}, Persion{name='wangwu', age='10', sex='女'}]