336.TreeMap集合

自定义对象作为键存储到TreeMap集合

@Test
    public void demo1() {
        TreeMap<Student, String> treeMap = new TreeMap<>();
        treeMap.put(new Student("张三", 23), "北京");
        treeMap.put(new Student("李四", 13), "上海");
        treeMap.put(new Student("王五", 33), "广州");
        treeMap.put(new Student("赵六", 43), "深圳");
        /**
         * 如果Student未实现Comparable,将报如下错误:
         *
         * java.lang.ClassCastException: cn.giteasy.common.Student cannot be cast to java.lang.Comparable
         * 异常:TreeMap的KEY对象需要实象java.lang.Comparable接口,如果不实现,不知道如何比较,如何报错
         *
         * 此处我们Student的父类Person类实现了Comparable接口,可以正常打印
         *    打印结果:
         * {Person [name=李四, age=13]=上海, Person [name=张三, age=23]=北京, Person [name=王五, age=33]=广州, Person [name=赵六, age=43]=深圳}
         */
        System.out.println(treeMap);
    }

创建TreeMap的时候,传入一个比较器

@Test
    public void demo2(){
        TreeMap<Student, String> treeMap = new TreeMap<>(new Comparator<Student>() {

            @Override
            public int compare(Student s1, Student s2) {

                return s1.getAge() - s2.getAge();//按照年龄比较
            }
        });
        treeMap.put(new Student("张三", 23), "北京");
        treeMap.put(new Student("李四", 13), "上海");
        treeMap.put(new Student("赵六", 43), "深圳");
        treeMap.put(new Student("王五", 33), "广州");

        System.out.println(treeMap);
        //{Person [name=李四, age=13]=上海, Person [name=张三, age=23]=北京, Person [name=王五, age=33]=广州, Person [name=赵六, age=43]=深圳}
    }