JAVA 中Null值到底是什么

Question

public class test {public static void main(String[] args) throws Exception {Temp temp = new Temp();temp.test("1");temp.test(null);}}public class Temp {public void test(Object a){System.out.println("test Object");}public void test(String a){System.out.println("test string");}}为什么输出结果为test Objecttest Object