Java 1.8新特性使用记录：Filter、数据容器的转换、排序Sorted

一、过滤Filter
注意：符合条件的才返回。List的对象如果为null会报空指针，安全起见要加上判断

public class MyCar {
   
    private Integer id;
    private String name;
    private String number;
    private Integer age;
}

public static void myFilter() {
   
        List<MyCar> carList = new ArrayList<>();
        carList.add(new MyCar("c1",1));
        carList.add(new MyCar("c2",2));
        carList.add(new MyCar("c3",3));
        carList.add(new MyCar("c4",4));
        carList.add(new MyCar("c5",5));
        carList.add(null);
        //符合条件的才会返回
        carList = carList.stream().filter(e->e != null && e.getAge() >2).collect(Collectors.toList());

        carList.forEach(e->System.out.println(e));

    }

    public static void main(String[] args) {
   
        FilterDemo.myFilter();
    }

二、数据容器的转换
2.1、将对象数组一个属性抽出来变成List

//List的生成看上面
List<String> nameList = carList.stream().map(e->e.getName()).collect(Collectors.toList());
        nameList.forEach(e->System.out.println(e));

2.2、将对象List指定属性内容以逗号隔开，例如：name

//生产环境注意判断空
String nameStr = carList.stream().map(e->e.getName()).collect(Collectors.joining(","));
System.out.println(nameStr);
//c3,c4,c5

//上面也可以分成两步操作
nameList = carList.stream().map(e->e.getName()).collect(Collectors.toList());
nameStr = nameList.stream().collect(Collectors.joining(","));
System.out.println(nameStr);
//c3,c4,c5

三、List 排序

3.1.List排序，Integer,BigDecimal,Date可以一样使用
注意：判空是很重要的一步。null一般放在后面。下面都有介绍

public static void mySortedInteger() {
   
        //这里其中有一个为空的
        List<Integer> numList = Arrays.asList( 1,2,3,4,5,6,7,8,8,9,null,10);
        System.out.println(numList.stream().map(e->e+"").collect(Collectors.joining(",")));
        //默认升序，判空，null放在后面
        numList = numList.stream().sorted(Comparator.nullsLast(Integer::compareTo)).collect(Collectors.toList());
        //numList = numList.stream().sorted(Comparator.comparing(e->e,Comparator.nullsLast(Integer::compareTo))).collect(Collectors.toList());
        System.out.println(numList.stream().map(e->e+"").collect(Collectors.joining(",")));
        //默认升序，判空，null放在前面
        numList = numList.stream().sorted(Comparator.nullsFirst(Integer::compareTo)).collect(Collectors.toList());
        //numList = numList.stream().sorted(Comparator.comparing(e->e,Comparator.nullsFirst(Integer::compareTo))).collect(Collectors.toList());
        System.out.println(numList.stream().map(e->e+"").collect(Collectors.joining(",")));
        //降序,判空，null放在后面
        numList = numList.stream().sorted(Comparator.nullsLast(Comparator.reverseOrder())).collect(Collectors.toList());
        //numList = numList.stream().sorted(Comparator.comparing(e->e,Comparator.nullsLast(Comparator.reverseOrder()))).collect(Collectors.toList());
        System.out.println(numList.stream().map(e->e+"").collect(Collectors.joining(",")));
        //降序,判空，null放在前面
        numList = numList.stream().sorted(Comparator.nullsLast(Integer::compareTo).reversed()).collect(Collectors.toList());
        System.out.println(numList.stream().map(e->e+"").collect(Collectors.joining(",")));
        //当然可以加filter,但是这样长度就会减少
        //numList = numList.stream().filter(e->e != null).sorted(Comparator.comparing(e->e,Comparator.reverseOrder())).collect(Collectors.toList());
        numList = numList.stream().filter(e->e != null).sorted(Comparator.reverseOrder()).collect(Collectors.toList());
        System.out.println(numList.stream().map(e->e+"").collect(Collectors.joining(",")));
    }

3.2、对象数组
对象是使用率最高的，也需要注意判空！
跟上面的用法基本一致。

public static void mySortedTest() {
   
        List<MyCar> carList = new ArrayList<>();
        //初始化对象
        Calendar cal = Calendar.getInstance();
        carList.add(new MyCar("c1",1,cal.getTime(),new BigDecimal(1)));
        cal.add(Calendar.DAY_OF_MONTH, -1);
        carList.add(new MyCar("c2",2,cal.getTime(),new BigDecimal(2)));
        carList.add(new MyCar("c3",3,null,new BigDecimal(3)));
        cal.add(Calendar.DAY_OF_MONTH, 5);
        carList.add(new MyCar("c4",4,cal.getTime(),new BigDecimal(4)));
        cal.add(Calendar.DAY_OF_MONTH, 2);
        carList.add(new MyCar("c5",5,cal.getTime(),new BigDecimal(5)));
        carList.add(new MyCar("c6",null,cal.getTime(),new BigDecimal(5)));
        carList.add(new MyCar("c7",null,cal.getTime(),new BigDecimal(5)));
        System.out.println("原始顺序 :");
        //这里 Integer,BigDecimal,Date的排序使用大同小异，以Inter为例
        carList.forEach(e->System.out.println(e));
        System.out.println("Sorted by Age 升序,null放后面 :");
        carList = carList.stream().sorted(Comparator.comparing(MyCar::getAge,Comparator.nullsLast(Integer::compareTo))).collect(Collectors.toList());
        carList.forEach(e->System.out.println(e));
        System.out.println("Sorted by Age 升序,null放前面 :");
        carList = carList.stream().sorted(Comparator.comparing(MyCar::getAge,Comparator.nullsFirst(Integer::compareTo))).collect(Collectors.toList());
        carList.forEach(e->System.out.println(e));
        System.out.println("Sorted by Age 降序,null放在后面:");
        carList = carList.stream().sorted(Comparator.comparing(MyCar::getAge,Comparator.nullsFirst(Integer::compareTo)).reversed()).collect(Collectors.toList());
        carList.forEach(e->System.out.println(e));
        System.out.println("Sorted by Age 降序,null放在前面:");
        carList = carList.stream().sorted(Comparator.comparing(MyCar::getAge,Comparator.nullsLast(Integer::compareTo)).reversed()).collect(Collectors.toList());
        carList.forEach(e->System.out.println(e));
        //其它与Integer基本一样
        System.out.println("Sorted by Date 升序,null放在后面:");
        carList = carList.stream().sorted(Comparator.comparing(MyCar::getCreateDate,Comparator.nullsLast(Date::compareTo))).collect(Collectors.toList());
        carList.forEach(e->System.out.println(e));
    }