Why deep copy with putAll() is invalid in this example?

Question

import java.util.HashMap;

public class test {
    public static void main(String[] args) {
        HashMap<String, Object> map1 = new HashMap<>();
        HashMap<String, Object> map2 = new HashMap<>();
        HashMap<String, Object> map3 = new HashMap<>();
        map3.put("isEnd", 2);
        map2.put("x", map3);
        map1.put("y", map2);
        String sentence = "xyxy";
        sensitiveWordFilter(sentence, map1);
    }

    public static String sensitiveWordFilter(String sentence, HashMap<String, Object> map){
        System.out.println("original map: " + map);
        HashMap<String, Object> futureMap = new HashMap<>();
        for(int i = 0; i < sentence.length(); i++){
            char char_ = sentence.charAt(i);
            for (String key : map.keySet()){
                if (String.valueOf(char_).equals(key)){
                    HashMap<String, Object> tempMap = new HashMap<>();
                    tempMap.putAll((HashMap<String, Object>) map.get(key));
                    HashMap<String, Object> newMap = new HashMap<>();
                    newMap.put(key, tempMap);
                    futureMap = newMap;
                }
            }

            if (futureMap.get(String.valueOf(char_)) != null){
                futureMap = (HashMap<String, Object>) futureMap.get(String.valueOf(char_));
                if (futureMap.get("isEnd") != null){
                    System.out.println("---------------" + map + "--------------------");
                    futureMap.remove("isEnd");
                    System.out.println("---------------" + map + "--------------------");
                }
            }
        }
        return sentence;
    }
}

This is the code that can be executed directly. You can clearly see that the value of map changes with the change of futureMap. It can be said that the deep copy is invalid. What's wrong?

score -1 · Answer 1 · answered Mar 15 '23 at 06:06

Try This:

public static String sensitiveWordFilter(String sentence, HashMap<String, Object> map) throws Exception {
    System.out.println("original map: " + map);
    HashMap<String, Object> futureMap = new HashMap<>();
    for(int i = 0; i < sentence.length(); i++){
        char char_ = sentence.charAt(i);
        for (String key : map.keySet()){
            if (String.valueOf(char_).equals(key)){
                HashMap<String, Object> tempMap = (HashMap<String, Object>)map.get(key);
                ByteArrayOutputStream bos = new ByteArrayOutputStream();
                ObjectOutputStream oos = new ObjectOutputStream(bos);
                oos.writeObject(tempMap);
                oos.flush();
                oos.close();
                bos.close();
                byte[] byteData = bos.toByteArray();
                ByteArrayInputStream bais = new ByteArrayInputStream(byteData);
                HashMap<String, Object> clonedMap = (HashMap<String, Object>) new ObjectInputStream(bais).readObject();
                Map<String, Object> newMap = new HashMap<>();
                newMap.put(key, clonedMap);
                futureMap = (HashMap<String, Object>)newMap;
            }
        }

        if (futureMap.get(String.valueOf(char_)) != null){
            futureMap = (HashMap<String, Object>) futureMap.get(String.valueOf(char_));
            if (futureMap.get("isEnd") != null){
                System.out.println("---------------" + map + "--------------------");
                futureMap.remove("isEnd");
                System.out.println("---------------" + map + "--------------------");
             }
        }
    }
    return sentence;
}

Thank you very much！！！ You have solved the problem that has troubled me for a long time. — speed milo, Mar 15 '23 at 06:29
No problem and sorry for not explaining, the issue in your code is with the line futureMap = newMap; inside the if block. This line assigns a new reference to futureMap, which causes it to no longer point to the original map. Instead of creating a new map and assigning it to futureMap, you should modify the existing futureMap object,So To achieve a deep copy, you can use the clone() method or create a new instance of the HashMap. — Nick, Mar 15 '23 at 06:56
Your answer could be improved with additional supporting information. Please [edit] to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers [in the help center](/help/how-to-answer). — Community, Mar 20 '23 at 14:06

Why deep copy with putAll() is invalid in this example?

1 Answers1