Java集合（三）TreeSet

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本文深入探讨了TreeSet集合的特点及使用方法，包括其红黑树结构、排序机制、常用API解析，以及如何通过实现Comparable接口和Comparator接口进行自定义排序。

一.简介

TreeSet是以红黑树的结构存储数据的集合，它能对数据作出顺序排序

二.TreeSet的方法

TreeSet<String> treeSet = new TreeSet<String>();
        treeSet.add("haha");
        treeSet.add("love");
        treeSet.add("me");
        treeSet.add("nihao");
        treeSet.add("yes");
        treeSet.add("you");

        String first = treeSet.first();//第一个元素
        String last = treeSet.last();//最后一个元素
        String lower = treeSet.lower("you");//比"you"小的前一个元素
        String highter = treeSet.higher("you");//比"you"大的后一个元素
        SortedSet<String> subSet = treeSet.subSet("haha", "me");//子集，包括"haha",不包括"me"
        SortedSet<String> headSet = treeSet.headSet("you");//比"you"小的所有元素集合
        SortedSet<String> tailSet = treeSet.tailSet("love");//比"you"大或者等于的所有元素集合

        Log.d(TAG, "testDemo: first = " + first);
        Log.d(TAG, "testDemo: last = " + last);
        Log.d(TAG, "testDemo: lower = " + lower);
        Log.d(TAG, "testDemo: highter = " + highter);
        Log.d(TAG, "testDemo: subSet = " + subSet.toString());
        Log.d(TAG, "testDemo: headSet = " + headSet.toString());
        Log.d(TAG, "testDemo: tailSet = " + tailSet.toString());

输出结果是

2019-08-02 17:01:17.717 24932-24948/com.hezhong.demolauncherservice D/ExampleInstrumentedTest: testDemo: first = haha
2019-08-02 17:01:17.717 24932-24948/com.hezhong.demolauncherservice D/ExampleInstrumentedTest: testDemo: last = you
2019-08-02 17:01:17.717 24932-24948/com.hezhong.demolauncherservice D/ExampleInstrumentedTest: testDemo: lower = yes
2019-08-02 17:01:17.717 24932-24948/com.hezhong.demolauncherservice D/ExampleInstrumentedTest: testDemo: highter = null
2019-08-02 17:01:17.717 24932-24948/com.hezhong.demolauncherservice D/ExampleInstrumentedTest: testDemo: subSet = [haha, love]
2019-08-02 17:01:17.717 24932-24948/com.hezhong.demolauncherservice D/ExampleInstrumentedTest: testDemo: headSet = [haha, love, me, nihao, yes]
2019-08-02 17:01:17.718 24932-24948/com.hezhong.demolauncherservice D/ExampleInstrumentedTest: testDemo: tailSet = [love, me, nihao, yes, you]

三.实现Comparable接口

class Fruit implements Comparable {
        String name;
        int count;


        @Override
        public int compareTo(Object o) {
            Fruit f = (Fruit) o;
            if (this.count == f.count) {
                return 0;
            } else if (this.count > f.count) {
                return 1;
            }
            return -1;
        }

        @Override
        public boolean equals(Object obj) {
            if (this == obj) {
                return true;
            }

            if (obj != null && obj.getClass() == this.getClass()) {
                Fruit f = (Fruit) obj;
                if (this.count == f.count) {
                    return true;
                }
            }
            return false;
        }
    }

四.定制接口，实现Comparator

 TreeSet<AllApp> treeSet = new TreeSet<AllApp>(new Comparator<AllApp>() {
            @Override
            public int compare(AllApp o1, AllApp o2) {

                if (o1.packageName.equals(o2.packageName) && o1.systemApp == o2.systemApp) {
                    return 0;
                }
                if (o1.packageName.hashCode() > o2.packageName.hashCode()) {
                    return 1;
                }
                return -1;
            }
        });