Java 内存分页、排序

java复制代码/**
 * @Description: 内存分页，字段排序
 * @Author: YanMei.Li
 * @Date: Create in 10:59 2021/11/10
 * @Modified By:
 */
public class MemoryPagination {

    /**
     * 给list的每个属性都指定是升序还是降序
     * list元素的属性可以是数字（byte、short、int、long、float、double等，支持正数、负数、0）、char、String、java.util.Date
     *
     * @param list 数据集合
     * @param pageNum 第几页
     * @param pageSize 每页多少条
     * @param isSort 是否排序
     * @param sortNameArr  参数数组
     * @param sortArr      每个属性对应的升降序数组， true升序，false降序
     */
    public static <E> List<E> sortMemoryPagination(List<E> list, int pageNum, int pageSize, final boolean isSort , final String[] sortNameArr, final boolean[] sortArr) {
        if(isSort){
            if (sortNameArr.length != sortArr.length) {
                throw new RuntimeException("属性数组元素个数和升降序数组元素个数不相等");
            }
            Collections.sort(list, new Comparator<E>() {
                @Override
                public int compare(E a, E b) {
                    int ret = 0;
                    try {
                        for (int i = 0; i < sortNameArr.length; i++) {
                            ret = MemoryPagination.compareObject(sortNameArr[i], sortArr[i], a, b);
                            if (0 != ret) {
                                break;
                            }
                        }
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                    return ret;
                }
            });

        }
        return pagination(list,pageNum,pageSize);
    }

    /**
     * 内存分页
     *
     * @param records  待分页的数据
     * @param pageNum  当前页码
     * @param pageSize 每页显示的条数
     * @return 分页之后的数据
     */
    private static <T> List<T> pagination(List<T> records, int pageNum, int pageSize) {
        if (CollectionUtils.isEmpty(records)) {
            return Collections.emptyList();
        }
        int totalCount = records.size();
        int remainder = totalCount % pageSize;
        int pageCount = (remainder > 0) ? totalCount/pageSize + 1 : totalCount/pageSize;
        if (remainder == 0) {
            return records.stream().skip((pageNum - 1) * pageSize).limit(pageSize).collect(Collectors.toList());
        } else {
            if (pageNum == pageCount) {
                return records.stream().skip((pageNum - 1) * pageSize).limit(totalCount).collect(Collectors.toList());
            } else {
                return records.stream().skip((pageNum - 1) * pageSize).limit(pageSize).collect(Collectors.toList());
            }
        }
    }

    /**
     * 对2个对象按照指定属性名称进行排序
     *
     * @param sortname
     *            属性名称
     * @param isAsc
     *            true升序，false降序
     * @param a
     * @param b
     * @return
     * @throws Exception
     */
    private static <E> int compareObject(final String sortname, final boolean isAsc, E a, E b) throws Exception {
        int ret;
        Object value1 = MemoryPagination.forceGetFieldValue(a, sortname);
        Object value2 = MemoryPagination.forceGetFieldValue(b, sortname);
        String str1 = value1.toString();
        String str2 = value2.toString();
        if (value1 instanceof Number && value2 instanceof Number) {
            int maxlen = Math.max(str1.length(), str2.length());
            str1 = MemoryPagination.addZero2Str((Number) value1, maxlen);
            str2 = MemoryPagination.addZero2Str((Number) value2, maxlen);
        } else if (value1 instanceof java.sql.Date && value2 instanceof java.sql.Date) {
            long time1 = ((java.sql.Date) value1).getTime();
            long time2 = ((Date) value2).getTime();
            int maxlen = Long.toString(Math.max(time1, time2)).length();
            str1 = MemoryPagination.addZero2Str(time1, maxlen);
            str2 = MemoryPagination.addZero2Str(time2, maxlen);
        }
        if (isAsc) {
            ret = str1.compareTo(str2);
        } else {
            ret = str2.compareTo(str1);
        }
        return ret;
    }

    /**
     * 获取指定对象的指定属性值（去除private,protected的限制）
     *
     * @param obj
     *            属性名称所在的对象
     * @param fieldName
     *            属性名称
     * @return
     * @throws Exception
     */
    public static Object forceGetFieldValue(Object obj, String fieldName) throws Exception {
        Field field = obj.getClass().getDeclaredField(fieldName);
        Object object = null;
        boolean accessible = field.isAccessible();
        if (!accessible) {
            // 如果是private,protected修饰的属性，需要修改为可以访问的
            field.setAccessible(true);
            object = field.get(obj);
            // 还原private,protected属性的访问性质
            field.setAccessible(accessible);
            return object;
        }
        object = field.get(obj);
        return object;
    }

    /**
     * 给数字对象按照指定长度在左侧补0.
     *
     * 使用案例: addZero2Str(11,4) 返回 "0011", addZero2Str(-18,6)返回 "-000018"
     *
     * @param numObj
     *            数字对象
     * @param length
     *            指定的长度
     * @return
     */
    public static String addZero2Str(Number numObj, int length) {
        NumberFormat nf = NumberFormat.getInstance();
        // 设置是否使用分组
        nf.setGroupingUsed(false);
        // 设置最大整数位数
        nf.setMaximumIntegerDigits(length);
        // 设置最小整数位数
        nf.setMinimumIntegerDigits(length);
        return nf.format(numObj);
    }

    public static void main(String[] args) {
        List<SysDict> list = new ArrayList<>();
        list.add(new SysDict(1,"test1","1",1,3.32));
        list.add(new SysDict(3,"test3","3",3,4.13));
        list.add(new SysDict(2,"test2","11",2,4.11));
        list.add(new SysDict(4,"test4","4",4,23.23));
        list.add(new SysDict(5,"test5","5",5,0.23));
        list.add(new SysDict(6,"test6","22",6,0.23));
        list.add(new SysDict(7,"test7","3",7,0.23));
        list.add(new SysDict(8,"test8","0.2",8,0.23));


//        String[] sortnameArr = {"ss","sort"};
//        boolean[] typeArr = {true,false};
        String[] sortnameArr = {"name"};
        boolean[] typeArr = {false};
        List<SysDict> ss = sortMemoryPagination(list, 1, 2, true, sortnameArr,typeArr);
        System.out.println(ss.toString());

    }

}


class SysDict {
    private Integer id;
    private String name;
    private String value;
    private Integer sort;
    private Double ss;

    public SysDict(Integer id, String name, String value, Integer sort,Double ss) {
        this.id = id;
        this.name = name;
        this.value = value;
        this.sort = sort;
        this.ss = ss;
    }

    public Double getSs() {
        return ss;
    }

    public void setSs(Double ss) {
        this.ss = ss;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getValue() {
        return value;
    }

    public void setValue(String value) {
        this.value = value;
    }

    public Integer getSort() {
        return sort;
    }

    public void setSort(Integer sort) {
        this.sort = sort;
    }

    @Override
    public String toString() {
        return "SysDict{" +
                "id=" + id +
                ", name='" + name + ''' +
                ", value='" + value + ''' +
                ", sort=" + sort +
                ", ss=" + ss +
                '}';
    }

}

本文转载自: 掘金

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