如何使用循环从StringBuilder将键/值放入HashMap？ -Java

Question

I'm using several techniques here, so it's hard to find help online. I need to populate a HashMap<String, String> with values I take from part of a StringBuilder , then take the keys and add them into an ArrayList<String> , then print the list. But when I print, I get a list full of nulls . I don't know why, I thought it would print the values I got from the StringBuilder . It should print: Values taken from the hashmap keys: ABCDEFGHI . (The reason I used StringBuilder is because String is immutable, is this correct thinking too?)

Also, I figured using a loop to print the list is okay, since my keys are actually numbers.

I've never created a HashMap before, so maybe I'm missing something. Thanks for your help.

Here is my code:

    // create HashMap from String
    StringBuilder alphaNum = new StringBuilder("1A2B3C4D5E6F7G8H9I");

    Map<String, String> myAlphaNum = new HashMap<String, String>(9);

    // for loop puts key and values in map, taken from String
    for (int i = 0; i < alphaNum.length();)
    {
        myAlphaNum.put(alphaNum.substring(i, ++i), alphaNum.substring(i, ++i));
    }
    if (myAlphaNum.containsKey(1))
        System.out.println("Key is there.");
    else
        System.out.println("Key is null.");

    // create ArrayList, add values to it using map keys
    ArrayList<String> arrayList = new ArrayList<String>();

    // for loop gets the "number" keys from HashMap to get the "letter" values
    for (int j = 1; j <= myAlphaNum.size(); j++)
        arrayList.add(myAlphaNum.get(j));


    System.out.print("Values taken from the hashmap keys: ");
    for (String list : arrayList)
        System.out.print(list);

Console:

Key is null.

Values taken from the hashmap keys: nullnullnullnullnullnullnullnullnull

Answer 1

myAlphaNum has keys of type String, so passing an int to get ( myAlphaNum.get(j) ) will always return null.

There are several ways to iterate over the values (or keys or entries) of the map.

For example (assuming you only care about the values) :

for (String value : myAlphaNum.values())
    arrayList.add(value);

Answer 2

 // create HashMap from String
        StringBuilder alphaNum = new StringBuilder("1A2B3C4D5E6F7G8H9I");

        Map<String, String> myAlphaNum = new HashMap<String, String>(9);

        // for loop puts key and values in map, taken from String
        for (int i = 0; i < alphaNum.length();)
        {
            myAlphaNum.put(alphaNum.substring(i, ++i), alphaNum.substring(i, ++i));
        }

        System.out.println(myAlphaNum);
        if (myAlphaNum.containsKey(1))
            System.out.println("Key is there.");
        else
            System.out.println("Key is null.");

        // create ArrayList, add values to it using map keys
        ArrayList<String> arrayList = new ArrayList<String>();

        // for loop gets the "number" keys from HashMap to get the "letter" values
        for (int j = 1; j <= myAlphaNum.size(); j++)
            arrayList.add(myAlphaNum.get(j+""));


        System.out.print("Values taken from the hashmap keys: ");
        for (String list : arrayList)
            System.out.print(list);

You can try the above code. You have used string key bit while retriving Integer so it wont return anything.

Answer 3

You are using containsKey/get with an Integer as parameter, while your map keys are defined as String. That's why you got null.

I would recommend to use a Map<Integer, String> myAlphaNum = new HashMap<Integer, String>(9); and in your loop myAlphaNum.put(Integer.parseInt(alphaNum.substring(i, ++i)), alphaNum.substring(i, ++i)); . Then you'll get your desired output.

Also you could the ArrayList constructor that takes a Collection as parameter (or just sysout myAlphaNum.values() ) directly.

// create ArrayList, add values to it using map keys
ArrayList<String> arrayList = new ArrayList<String>(myAlphaNum.values());
System.out.println(arrayList); //[A, B, C, D, E, F, G, H, I]