TreeSet的实现：

TreeMap实现：

需要注意：

1. 当利用comparator比较两个元素相等时，插入的时候会失败。而hashset是发现两个元素相等(即：两个元素的hashcode相同，equals方法比较之后也相等)时，插入失败返回false。这说明可能treeset并不是真正的set，因为即使两个元素的hashcode相同，equals方法比较之后也相等，但是如果comparator返回两个对象不相等，也是可以插入的，而hashset插入事则会失败。

2. 排序只在插入时发生，如果后面你修改了元素的数据，元素的位置不会变。

测试类：

ChineseCharCount, 保存一个字符和它出现的次数。本来想测试读取一个文件中的所有中文字符，然后得到出现最多的字符以及出现了多少次的。本想用treeset进行排序，但是发现不行，最后还是直接用的ArrayList，然后用Collections的sort

(Comparator comparator)方法完成的。

package _00_ReadAndCountChineseCharacters;

public class ChineseCharCount {

char ch;

int count;

public ChineseCharCount(char ch, int count) {

// TODO Auto-generated constructor stub

this.ch = ch;

this.count = count;

}

public char getCh() {

return ch;

}

public void setCh(char ch) {

this.ch = ch;

}

public void setCount(int count) {

this.count = count;

}

public String toString() {

return ch + ":appeared " + count + " times";

}

public int getCount() {

return count;

}

@Override

public boolean equals(Object obj) {

// TODO Auto-generated method stub

if (obj == null) {

return false;

}

if (obj == this) {

return true;

}

if (!(obj instanceof ChineseCharCount)) {

return false;

}

ChineseCharCount te = (ChineseCharCount) obj;

if (te.ch == this.ch) {

return true;

}

return false;

}

@Override

public int hashCode() {

// TODO Auto-generated method stub

return (ch + "").hashCode();

}

}

ChineseCharComparator，直接通过比较count来对比大小。

package _00_ReadAndCountChineseCharacters;

import java.util.Comparator;

public class ChineseCharComparator implements Comparator{

public int compare(ChineseCharCount o1, ChineseCharCount o2) {

// TODO Auto-generated method stub

if(o1 == o2)

return 0;

if(o1!=null&&o2==null)

return -1;

else if(o1==null&&o2!=null)

return 1;

if (o1.count < o2.count)

return 1;

else if (o1.count > o2.count)

return -1;

else

return 0;

}

}

ReadAndCountChineseCharacters，测试类。直接读取文件，count，然后排序，输出前三。

package _00_ReadAndCountChineseCharacters;

import java.io.FileReader;

import java.util.ArrayList;

import java.util.Collections;

import java.util.Comparator;

import java.util.HashSet;

import java.util.List;

import java.util.TreeSet;

public class ReadAndCountChineseCharacters {

public static void main(String[] args) throws Exception {

String classPath = ReadAndCountChineseCharacters.class.getResource("").getPath();

System.out.println(classPath);

String filePath = classPath + "ChineseSentence.txt";

// InputStream in = new FileInputStream(filePath);

List chList = new ArrayList();

FileReader fr = new FileReader(filePath);

int i;

while ((i = fr.read()) != -1) {

char next = (char) i;

if(next == '\n' || next == '\t')

continue;

boolean hasCh = false;

for (ChineseCharCount chCount : chList) {

if (next == chCount.ch) {

hasCh = true;

chCount.count = chCount.count + 1;

break;

}

}

if (!hasCh) {

chList.add(new ChineseCharCount(next, 1));

}

}

fr.close();

ChineseCharComparator comparator = new ChineseCharComparator();

Collections.sort(chList, comparator);

for (int j = 0; j < 3; j++) {

System.out.println(chList.get(j));

}

}

}

TestTreeSet 编写排序时，遇到的问题，通过下面的测试类进行了总结：

package _00_ReadAndCountChineseCharacters;

import java.util.HashMap;

import java.util.HashSet;

import java.util.TreeSet;

public class TestTreeSet {

public static void main(String[] args) {

ChineseCharCount ch = new ChineseCharCount('a', 1);

ChineseCharCount ch2 = new ChineseCharCount('a', 2);

ChineseCharCount ch3 = new ChineseCharCount('a', 1);

ChineseCharCount ch4 = new ChineseCharCount('c', 1);

ChineseCharComparator comparator = new ChineseCharComparator();

TreeSet treeSet = new TreeSet<>(comparator);

HashSet hashSet = new HashSet<>();

HashMap haspMap = new HashMap<>();

/*

* Associates the specified value with the specified key in this map.

* If the map previously contained a mapping for the key, the old value is replaced.

*

* the previous value associated with key, or null if there was no mapping for key.

* (A null return can also indicate that the map previously associated null with key.)

*/

haspMap.put("1", "1");

System.out.println("abc123".hashCode());

System.out.println("".hashCode());

System.out.println(new String("abc123").hashCode());

System.out.println(new String("").hashCode());

/*

* Adds the specified element to this set if it is not already present.

* More formally, adds the specified element e to this set if the set

* contains no element e2 such that (e==null ? e2==null : e.equals(e2)).

* If this set already contains the element, the call leaves the set unchanged and returns false.

*/

System.out.println("add into treeset");

System.out.println(treeSet.add(ch));

System.out.println(treeSet.add(ch2));

System.out.println(treeSet.add(ch3));

System.out.println(treeSet.add(ch4));

/*

* Adds the specified element to this set if it is not already present.

* More formally, adds the specified element e to this set if this set

* contains no element e2 such that (e==null ? e2==null : e.equals(e2)).

* If this set already contains the element, the call leaves the set unchanged and returns false.

*/

System.out.println("add into hashSet");

System.out.println(hashSet.add(ch));

System.out.println(hashSet.add(ch2));

System.out.println(hashSet.add(ch3));

System.out.println(hashSet.add(ch4));

System.out.println("From the test result we can see, "

+ "treeset is not really a set. AS it allowed duplicate "

+ "attribute(determinter by hashCode and equals method). "

+ "Instead, it didn't allwed two attributes wihich is "

+ "equals to each other(determined by the comparator).");

/*

* 从测试结果可以看出，如果两个元素利用comparator进行比较是相等的插入会失败。

*/

for(ChineseCharCount chs : treeSet)

System.out.println(chs);

treeSet.first().count = 0;

treeSet.last().count = 2;

System.out.println(treeSet.add(ch3));

/*

* 从测试结果可以看出，treeset只会在存入set的时候进行排序，

* 当做了更改时不会重新排序。

*/

for(ChineseCharCount chs : treeSet)

System.out.println(chs);

}

}