Java--Map及泛型

字典（映射）

java.util.Map

• HashMap
• TreeMap
• Map<v,k> - v代表键的类型，k代表值的类型
• 取元素用get取值，里面传键，得到键所对应的值

public class Test01 {

    public static void main(String[] args) {
    
            //Map<Integer, String> map = new HashMap<>();
            Map<Integer, String> map = new TreeMap<>();
            map.put(1, "apple");//添加
            map.put(2, "grape");
            map.put(100, "shit");
            map.put(1, "banana");
            //执行完了后，apple被改成了banana
            System.out.println(map.size());
            //map.remove(100);//删除
            //map.clear();//清空
            for (Integer key : map.keySet()) {
                System.out.println(key + " ---> " + map.get(key));
            }
    }

}

泛型(generic)

• 让类型不再是程序中的硬代码（hard code）
• T代表引用类型，但是不能是基本数据类型

//T类型实现了comparable接口
//此处的extends不是继承而是泛型限定,限定T类型必须是comparable接口的子类型
public static<T extends Comparable<T>> void bubbleSort(T[] array){
        boolean swapped = true;
        for (int i = 1; swapped && i < array.length; i++) {
            swapped = false;
            for (int j = 0; j < array.length - i; j++) {
                if (array[j].compareTo(array[j + 1]) > 0) {
                    T temp = array[j];
                    array[j] = array[j + 1];
                    array[j + 1] = temp;
                    swapped = true;
                }
            }
        }
    }

//T类型没有实现接口
    public static<T> void bubbleSort(T[] array,Comparator<T> comp){
        boolean swapped = true;
        for (int i = 1; swapped && i < array.length; i++) {
            swapped = false;
            for (int j = 0; j < array.length - i; j++) {
                if (comp.compare(array[j],array[j + 1]) > 0) {
                    T temp = array[j];
                    array[j] = array[j + 1];
                    array[j + 1] = temp;
                    swapped = true;
                }
            }
        }
    }

• 测试代码

public static void main(String[] args) {
        Student[] students = {new Student("wangdachui", 18),
                new Student("limochou", 25),
                new Student("zhangsanfeng", 100)};
        //String[] x = {"killer","grape","zoo","blueberry","apple"};
        //Integer[] x = {12,4,45,36,50,67};
        //Double[] y = {2.0,3.2,1.5,10.0};
        //bubbleSort(students);
        bubbleSort(students,(o1,o2)-> {//lambda表达式
            return o1.getName().compareTo(o2.getName());
        });
        
        bubbleSort(students,new Comparator<Student>() {//创造匿名内部类，就地实现

            @Override
            public int compare(Student o1, Student o2) {
                return o1.getAge() - o2.getAge();
            }
        });
        System.out.println(Arrays.toString(students));
    }