java - 如何使用相同的递归函数迭代 Map 和 Map> ？

Question

例如，为了使用相同的函数迭代 List< String> 和 List< List< String>> ，我可以编写如下内容:

import java.util.*;
public class Test{
    public static void print(Object obj) {
        if(obj instanceof List){
            List list=(List)obj;
            System.out.print("[");
            for(Object obj2 : list){
                print(obj2);
            }
            System.out.print("]");
        }else{
            System.out.print(obj+",");
        }
    }

    public static void main(String[] args){
        String l0="a";
        System.out.println(l0);

        List<String> l1=Arrays.asList("a","b");
        print(l1);

        System.out.println("");

        List<List<String> > l2=Arrays.asList(Arrays.asList("a","b"),Arrays.asList("c","d"));
        print(l2);
    }
}

输出:

a
[a b ]
[[a b ][c d ]]

现在我想类似地迭代 Map< String,String> 和 Map< String,Map< String,String>> ，我尝试过:

import java.util.*;
public class Test{
    public static void print(Object obj) {
        if(obj instanceof Map){
            System.out.print("{");
            Map map=(Map)obj;
            for(Map.Entry<Object,Object> entry : map.entrySet()){
                print(entry.getKey());
                System.out.print(":");
                print(entry.getValue());
                System.out.print(",");
            }
            System.out.print("}");
        }else{
            System.out.print(obj);
        }
    }
    public static void main(String[] args){
        String m0="a";
        print(m0);

        System.out.println("");

        Map<String,String> m1=new HashMap<String,String>();
        m1.put("surname","Tom");
        m1.put("lastname","Bob");
        print(m1);

        System.out.println("");

        Map<String,HashMap<String,String>> m2=new HashMap<String,HashMap<String,String>>();
        HashMap<String,String> mm1=new HashMap<String,String>();
        mm1.put("surname","Tom");
        mm1.put("lastname","Bob");
        mm1.put("nickname","Penguin");
        m2.put("owner",mm1);
        HashMap<String,String> mm2=new HashMap<String,String>();
        mm2.put("name","Lucky");
        mm2.put("type","cat");
        m2.put("pet",mm2);
        print(m2);
    }
}

预期输出类似于:

a
{surname:Tom,lastname:Bob,}
{owner:{surname:Tom,nickname:Penguin,lastname:Bob,},pet:{name:Lucky,type:cat,},}

但无法编译:

Test.java:20: error: incompatible types: Object cannot be converted to Entry<Object,Object>
        for(Map.Entry<Object,Object> entry : map.entrySet()){

这是什么原因呢？可以修复吗？如果没有，如何像在开头所示的递归迭代列表一样递归地迭代嵌套映射？

Answer 1

一个Map与 Map<Object,Object> 不同。

它会转换为 Map<Object,Object>或使用Entry没有泛型。