将非空键和值放入Hashtable时发生NullPointerException

Question

java.util.Hashtable's null pointer exception is occuring in the following code when neither of the arguments to put() are null :

import java.util.Hashtable;

interface action
{
   void method();
}

class forclass implements action
{
  public void method()
  {
    System.out.println("for(...){");
  }
}

class ifclass implements action
{
  public void method()
  {
    System.out.println("if(..){");
  }
}

public class trial
{
  static Hashtable<String,action> callfunc;
  //a hashtable variable
  public static void init()
  {
    //System.out.println("for"==null); //false
    //System.out.println(new forclass() == null); //false
    callfunc.put("for",new forclass()); //exception occuring here
    callfunc.put("if",new ifclass());
    //putting values into the hashtable
  }
  public static void main(String[] args)
  {
    init(); //to put stuff into hashtable
    action a = callfunc.get("for");
    //getting values for specified key in hashtable
    a.method();
    callfunc.get("if").method();
  }
}

Exception in thread "main" java.lang.NullPointerException -
at trial.init(trial.java:33)
at trial.main(trial.java:38)
why is this exception occuring? how do i fix it?

Answer 1

You have not initialized your Hashtable : -

static Hashtable<String,action> callfunc; // Reference points to null

when neither of the arguments to put() are null

You should rather use HashMap , which allows 1 null key , to avoid getting NPE when using put with null key , method which in case of Hashtable throws NPE , because it does not allow null keys, or value .

So, change your declaration to: -

static Hashtable<String,action> callfunc = new Hashtable<String, action>();

or even better: -

static Map<String, action> callfunc = new HashMap<String, action>();

---

As a side note, you should follow Java Naming Convention in your code. All the class name and interface name, should start with UpperCase letter, and follow CamelCasing thereafter.

Answer 2

callfunc reference is null not the inputs.

Try this:

static Hashtable<String,action> callfunc = new Hashtable<String,action>()

Also this post might be useful as to whether you want Hashtable or HashMap