Question

我实际上能够仅根据键和值对我的Map进行排序，我什至尝试按以下方式对其进行排序：

我根据国家/地区对学生进行了排序，如果两个学生碰巧具有相同的states，则仅在匹配的国家/地区中按StudentID进行排序。

到目前为止我尝试过的事情：

final Map<Integer, String> studentMaster = new HashMap<>() {{
        put(146, "Sweden");
        put(148, "Sweden");
        put(110, "Orebro");
        put(6, "Malmo");
        put(14, "Orebro");
        put(26, "Malmo");
    }   
    };
    studentMaster.entrySet().stream()
    .sorted((i1,i2)->i1.getValue().compareTo(i2.getValue()))
    .sorted((j1,j2)->j1.getKey().compareTo(j2.getKey()))
    .forEach(System.out::println);

我得到的结果**（实际输出）**

14=Orebro
26=Malmo
110=Orebro
146=Sweden
148=Sweden

预期输出：

  26=Malmo
  14=Orebro
  110=Orebro
  146=Sweden
  148=Sweden

Answer 1

注意：您的预期和实际输出与您添加到Map的键不匹配。

代码不起作用的原因是，您两次调用Stream#sorted并使用两个单独的Comparator，因此在您的情况下，第一次调用Stream#sorted是没有用的（因为它被第二个调用覆盖了。

通过将自定义Comparator传递给Stream#sorted，我能够实现您的预期输出：

Map.Entry.<Integer, String>comparingByValue()
    .thenComparing(Map.Entry.comparingByKey())

输出：

6=Malmo
26=Malmo
14=Orebro
110=Orebro
146=Sweden
148=Sweden

Answer 2

有时我会回答How to sort the name along with age in java，除了用于存储的数据结构外，还与您的问题有很多相似之处。要遍历每个键并对其进行排序，然后再对值进行排序，然后再对它进行排序，这将非常繁琐，并且会使您感到非常困惑。只需记住当您不习惯使用 Stream 时是如何遍历Map的即可：

for (Map.Entry<String,String> entry : somemap.entrySet()){..Some Statements..};

studentMaster.entrySet().stream()
    .sorted(Comparator.comparing((Map.Entry<Integer, String> m) -> m.getValue())
              .thenComparing(Map.Entry::getKey)).forEach(System.out::println);

输出

6=Malmo
26=Malmo
14=Orebro
110=Orebro
146=Sweden
148=Sweden

Answer 3

Comparator应该看起来像这样：

Comparator<Entry<Integer, String>> comparator = (o1, o2) -> {
    int i = o1.getValue().compareTo(o2.getValue());
    if (i == 0) {
        return o1.getKey().compareTo(o2.getKey());
    } else {
        return i;
    }
};

然后将其传递给Stream#sorted方法：studentMaster.entrySet().stream().sorted(comparator).forEach(System.out::println);

输出：

6=Malmo
26=Malmo
14=Orebro
110=Orebro
146=Sweden
148=Sweden

Answer 4

2种方式：

将TreeSet与Comparable pojo一起使用。
将TreeSet与自定义的Comparator一起使用。

代码

Tmp.java

（将TreeSet与Comparable pojo一起使用。）

import java.util.*;

public class Tmp {
    static class StudentMaster implements Comparable<StudentMaster> {
        private Integer id;
        private String master;

        public StudentMaster(Integer id, String master) {
            this.id = id;
            this.master = master;
        }

        @Override
        public int compareTo(StudentMaster other) {
            int masterFlag = master.compareTo(other.master);
            return (masterFlag == 0) ? id.compareTo(other.id) : masterFlag;
        }

        @Override
        public boolean equals(Object o) {
            StudentMaster osm = (StudentMaster) o;
            return id == osm.id && master.equals(osm.master);
        }

        @Override
        public int hashCode() {
            return Objects.hash(id, master);
        }

        public String toString() {
            StringBuilder sb = new StringBuilder();
            Formatter fm = new Formatter(sb);

            fm.format("id = %d, master = %s\n", id, master);

            fm.close();
            return sb.toString();
        }
    }

    public static void test() {
        final Set<StudentMaster> smSet = new TreeSet<>();

        smSet.add(new StudentMaster(146, "Sweden"));
        smSet.add(new StudentMaster(148, "Sweden"));
        smSet.add(new StudentMaster(110, "Orebro"));
        smSet.add(new StudentMaster(6, "Malmo"));
        smSet.add(new StudentMaster(14, "Orebro"));
        smSet.add(new StudentMaster(26, "Malmo"));

        for (StudentMaster sm : smSet) {
            System.out.print(sm);
        }
    }

    public static void main(String[] args) {
        test();
    }
}

TmpComparator.java

（将TreeSet与自定义的Comparator一起使用。）

import java.util.*;

public class TmpComparator {
    static Comparator<StudentMaster> smc = new Comparator() {
        @Override
        public int compare(Object o1, Object o2) {
            StudentMaster sm1 = (StudentMaster) o1, sm2 = (StudentMaster) o2;

            int masterFlag = sm1.master.compareTo(sm2.master);
            return (masterFlag == 0) ? sm1.id.compareTo(sm2.id) : masterFlag;
        }
    };

    static class StudentMaster {
        private Integer id;
        private String master;

        public StudentMaster(Integer id, String master) {
            this.id = id;
            this.master = master;
        }

        @Override
        public boolean equals(Object o) {
            StudentMaster osm = (StudentMaster) o;
            return id == osm.id && master.equals(osm.master);
        }

        @Override
        public int hashCode() {
            return Objects.hash(id, master);
        }

        public String toString() {
            StringBuilder sb = new StringBuilder();
            Formatter fm = new Formatter(sb);

            fm.format("id = %d, master = %s\n", id, master);

            fm.close();
            return sb.toString();
        }
    }

    public static void test() {
        final Set<StudentMaster> smSet = new TreeSet<>(smc);

        smSet.add(new StudentMaster(146, "Sweden"));
        smSet.add(new StudentMaster(148, "Sweden"));
        smSet.add(new StudentMaster(110, "Orebro"));
        smSet.add(new StudentMaster(6, "Malmo"));
        smSet.add(new StudentMaster(14, "Orebro"));
        smSet.add(new StudentMaster(26, "Malmo"));

        for (StudentMaster sm : smSet) {
            System.out.print(sm);
        }
    }

    public static void main(String[] args) {
        test();
    }
}

只需运行main()方法。

两者的输出相同：

id = 6, master = Malmo
id = 26, master = Malmo
id = 14, master = Orebro
id = 110, master = Orebro
id = 146, master = Sweden
id = 148, master = Sweden

提示

在生产代码中，equals()需要改进，这是仅用于测试的简化版本。

首先按值然后按键对地图排序

4 个答案:

代码

提示