前言

前段时间刷到一个牛客题目，问输出的结果。虽苦思冥想蒙对了，但查看解析终不解其意，遂有自己查找资料测试之意。

牛客题目如下，问输出是什么？

Java
public class Demo {
    public static void main(String[] args) {
        Collection<?>[] collections
                = {new HashSet<String>(), new ArrayList<String>(), new HashMap<String, String>().values()};
        Super subToSuper = new Sub();
        for (Collection<?> collection : collections) {
            System.out.println(subToSuper.getType(collection));
        }
    }

    abstract static class Super {
        public static String getType(Collection<?> collection) {
                    return "Super: collection";
        }
        public static String getType(List<?> list) {
                    return "Super: list";
        }
        public String getType(ArrayList<?> list) {
                    return "Super: arrayList";
        }
        public static String getType(Set<?> set) {
                    return "Super: set";
        }
        public String getType(HashSet<?> set) {
                    return "Super: hashSet";
        }
    }
    
    static class Sub extends Super {
        public static String getType(Collection<?> collection) {
            return "Sub";
        }
    }
}

答案是 Super:collection Super:collection Super:collection，所以是为什么呢？

静态绑定和动态绑定

测试

static 方法的静态绑定

Java
import java.util.ArrayList;
import java.util.List;

public class Main {

    public static void main(String[] args) {
        Test test = new SubTest();
        List<Integer> list = new ArrayList<>();
        test.print(list);
    }

    abstract static class Test {

        public static void print(List<Integer> list) {
            System.out.println("static list");
        }

        public void print(ArrayList<Integer> list) {
            System.out.println("non-static ArrayList");
        }
    }

    static class SubTest extends Test {

        public static void print(List<Integer> list) {
            System.out.println("sub static list");
        }

        public void print(ArrayList<Integer> list) {
            System.out.println("sub non-static ArrayList");
        }
    }
}