Java中的多态方法

public class Main {
    public void test(Object o) {
        System.out.println("Object");
    }
    public void test(String s) {
        System.out.println("String");
    }
    public static void main(String[] args) {
        Main that = new Main();
        that.test(null);
    }
}

这段代码运行的结果是

J:\test>java Main
String

J:\test>

因为Java处理null时会优先处理更确切的对象

如果此时还有一个多态方法是ArrayList，程序会报错（编译阶段），因为不知道去适配String还是ArrayList

J:\test>javac Main.java
Main.java:14: error: reference to test is ambiguous
                that.test(null);
                    ^
  both method test(String) in Main and method test(ArrayList) in Main match
1 error

如果是基础数据类型和对象类型，则会适配对象类型

J:\test>type Main.java
import java.util.ArrayList;
public class Main {
        public void test(Object o) {
                System.out.println("Object");
        }
        public void test(String s) {
                System.out.println("String");
        }
//      public void test(ArrayList a){
//              System.out.println("ArrayList");
//      }
        public void test(int i){
                System.out.println("int");
        }
        public static void main(String[] args) {
                Main that = new Main();
                that.test(null);
        }
}
J:\test>javac Main.java

J:\test>java Main
String

J:\test>

如果只有基础数据类型和Object，因为Object是对象，则调用Object

J:\test>type Main.java
import java.util.ArrayList;
public class Main {
        public void test(Object o) {
                System.out.println("Object");
        }
//      public void test(String s) {
//              System.out.println("String");
//      }
//      public void test(ArrayList a){
//              System.out.println("ArrayList");
//      }
        public void test(int i){
                System.out.println("int");
        }
        public static void main(String[] args) {
                Main that = new Main();
                that.test(null);
        }
}
J:\test>javac Main.java

J:\test>java Main
Object

J:\test>

如果只有基础数据类型，则编译报错，因为java不支持int与null转换

J:\test>javac Main.java
Main.java:17: error: incompatible types: <null> cannot be converted to int
                that.test(null);
                          ^
Note: Some messages have been simplified; recompile with -Xdiags:verbose to get full output
1 error

J:\test>