在java中使用HashMap创建对象

Question

I am trying to create object of a class as a value of HashMap in java. Below is my java code :

public interface Message {
    public String nextMessage(String data);
} 

public class DataMessage implements Message{
    private static String[] randomData;

    public DataMessage(String data) {
        randomData = data.split(",");
    }

    @Override
    public String nextMessage(String data) {
        int index = randomIndex.nextInt(randomData.length-1);
        return randomData[index];
    }
}

When I access that object it is returning null ie throwing null pointer exception.

I am trying to access that object as below :

public class MyMessage {

    public MyMessage() {
    }

    public static void main() {
        HashMap<String,Message> hash = new HashMap<String,Message>();
        if (some condition) {
            hash.put(key,new DataMessage("this is my data"));
        }

        <some code>

        hash.get(key).nextMessage();
        //Throws nullpointer exception
    }
}

Can anyone please help me to understand why this is happening?

I am beginner in java and trying to improve performance of this approach. Please suggest better approaches if any.

Answer 1

I think you can't access to you if(){} so your HashMap is always empty for that you get NullPointException because when you try to use hash.get(key).nextMessage(); the first part hash.get(key) is null so you can't make null.nextMessage();

so to solve your problem make sure that your HashMap is not empty and there are an element with this key.

if (hash .containsKey(key)) {
    //key exists
    hash.get(key).nextMessage();
} else {
    //key does not exists
    System.out.println("No key exist");
}

Note I think you have some error in your function for example this method should return some thing or a void :

public RandomData(String data) {
     randomData = data.split(",");
}

Hope this can help you.