Simple HashMap

[Paul Ralph]

Not sure if this goes here or in the Cattle Drive.

I'm trying to develop a HashMap for a Cattle Drive assignment.

import java.util.*; 

class HashMapDemo 
{ 
    public static void main(String args[]) 
    { 
        
        // Create a hash map 
        HashMap NumberTable = new HashMap(); 

        // Put elements to the map 
        NumberTable.put("one", new Integer(1)); 
        NumberTable.put("two", new Integer(2)); 
        NumberTable.put("three", new Integer(3)); 
        NumberTable.put("four", new Integer(4)); 
        NumberTable.put("five", new Integer(5)); 

        Object FirstInt = NumberTable.get(args[0]);
        Object SecondInt = NumberTable.get(args[1]);
                
        System.out.println(FirstInt);
        System.out.println(SecondInt);
                
        //FirstInt = Integer(FirstInt);
        //SecondInt = Integer(SecondInt);
                
        //System.out.println(FirstInt * SecondInt);
    } 
}

Why am I getting Objects instead of Integers? Is there a way to cast the Objects into Integers? Am I even asking the right questions?

Thanks

[Saif Asif]

Hello Paul Ralph ,

What are you trying to achieve ? According to javadoc , the get( String key ) method will always return an Object . But If you want your hashmap to always return an Integer then use parameterized generics like this

         // Create a hash map 
        HashMap<String, Integer> NumberTable = new HashMap<String , Integer>();
        // Specify the generic type for the hashmap. Here key is of type String and the value is of type Integer
                                                                                                                                     
        // Put elements to the map 
        NumberTable.put("one", new Integer(1)); 
        NumberTable.put("two", new Integer(2)); 
        NumberTable.put("three", new Integer(3)); 
        NumberTable.put("four", new Integer(4)); 
        NumberTable.put("five", new Integer(5));
        
        Integer i = NumberTable.get( "three" );        // Now the get( String key ) will always return an object of type Integer . 

However if you look at the back-end , in this example the JVM will do the casting by itself for you , so ultimately casting has to be done since the default return type of get( String key ) is Object.

[Campbell Ritchie]

That is very old‑fashioned code. You should use boxing
myMap.put("Campbell", 123);

[Campbell Ritchie]

Or the valueOf method
myMap.put("Campbell", Integer.valueOf(123));

[Jesper de Jong]

Paul Ralph wrote:Why am I getting Objects instead of Integers? Is there a way to cast the Objects into Integers? Am I even asking the right questions?

Because you are using the raw type HashMap, without using generics. When you use the raw type, the keys and values will be of type Object, and you need to cast them.

Integer firstInt = (Integer)NumberTable.get(args[0]);

The cast would not be necessary if you would use generics:

HashMap<String, Integer> NumberTable = new HashMap<>();

[Paul Ralph]

OK, I got the basic HashMap to work. Now I'm trying to get a method to see the HashMap. NumberTable is null inside convert() and I have no idea why.

(I left out some of the code that's outside of the class)

import java.util.*;

  class Number
    {
        public static HashMap NumberTable ;

        static
        {
            HashMap<String, Integer> NumberTable = new HashMap<>();
                
            // Put elements to the map 
            NumberTable.put("one", 1); 
            System.out.println(NumberTable.get("one"));
            NumberTable.put("two", 2); 
            NumberTable.put("three", 3); 
            NumberTable.put("four", 4); 
            NumberTable.put("six", 6);
            NumberTable.put("seven", 7);
            NumberTable.put("eight", 8);
            NumberTable.put("nine", 9);
            NumberTable.put("ten", 10);
            NumberTable.put("eleven", 11);
            NumberTable.put("twelve", 12);
            NumberTable.put("thirteen", 13);
            NumberTable.put("fourteen", 14);
            NumberTable.put("fifteen", 15);
            NumberTable.put("sixteen", 16);
            NumberTable.put("seventeen", 17);
            NumberTable.put("eighteen", 18);
            NumberTable.put("nineteen", 19);
            NumberTable.put("twenty", 20);
            NumberTable.put("thirty", 30);
            NumberTable.put("forty", 40);
            NumberTable.put("fifty", 50);
            NumberTable.put("sixty", 60);
            NumberTable.put("seventy", 70);
            NumberTable.put("eighty", 80);
            NumberTable.put("ninty", 90);
        }
          
        Integer converted;
        String whole;
        String tens;
        String ones;
        Number()
        {
            //converted = 0;
            //whole = "I'm whole";
            //tens = "I'm tens";
            //ones = "I'm ones";
            System.out.println("Number has been constructed!");
            this.converted = 0;
        }   
        
        public void convert()
        {
                //Integer FirstInt = NumberTable.get(args[0]);
                //Integer SecondInt = NumberTable.get(args[1]);
                System.out.println("Trying to convert");
                System.out.print("This.whole=");
                System.out.println(this.whole);
                //System.out.println(result);
                System.out.println("This.converted="+this.converted);
                System.out.println(this.NumberTable.get("one"));
                this.converted = (Integer)NumberTable.get(this.whole);
                return; //this.converted; NumberToConvert.get(NuberTable.whole);
        }
        //void construct()
        //{
            
    }

Thanks

[Thomas Kasene]

This is essentially what's wrong: you're declaring a local variable with the same name as your class variable (NumberTable). The static block has its own scope just like methods do, so when you assign anything to it, your class variable won't be affected at all.

Good luck!

[Paul Ralph]

OK, got it. Thanks!