ArrayList question

**steel55677** · February 22nd, 2012, 08:44 PM

Hi,

How do you create ArrayList for individual items like this:

Object str = "A"
ArrayList<Objects> str = new ArrayList<Objects>();

I'm making my own Hashmap but I don't know how to create individual ArrayList with varied names.
eg) ArrayList<String> Keys = new ArrayList<String>();
for(int i = 0; i < n) Keys.add(i); #pseudo

for (int i =0; i <n)
ArrayList<String> Keys.get(i) = new ArrayList<String>(); #1 would have its own arrayList. 2 would have its own arraylist etc.

Thanks

**kenster421** · February 23rd, 2012, 03:36 PM

In order to create your own hashmap, you need a key->value pair.

This would be helpful:

public class HashMapImp
{
	public static void main(String args[])
	{
		MyHashMap mhm = new MyHashMap();
		mhm.addPair("twenty","20");
		System.out.println(mhm.getValue("twenty"));
	}
 
	class MyHashMap
	{
		java.util.ArrayList<KeyValPair> hmap;
 
		public MyHashMap()
		{
			this.hmap = new java.util.ArrayList<KeyValPair>();
		}
 
		public boolean addPair(String key, String value)
		{
			return this.hmap.add(new KeyValPair(key, value);
		}
 
		public String getValue(String key)
		{
			for(int i=0; i<this.hmap.size(); i++)
			{
				if(this.hmap.get(i).getKey().equals(key))
				{
					return this.hmap.get(i).getValue();
				}
			}
			return null;
		}
 
		public boolean setValue(String key, String value)
		{
			for(int i=0; i<this.hmap.size(); i++)
			{
				if(this.hmap.get(i).getKey().equals(key))
				{
					this.hmap.get(i).setValue(value);
					return true;
				}
			}
			return addPair(key,value);
		}
	}
 
	class KeyValPair
	{
		private String key;
		private String value;
 
		public KeyValPair(String key, String value)
		{
			setKey(key);
			setValue(value);
		}
 
		public KeyValPair()
		{
			this("","");
		}
 
		public void setKey(String key)
		{
			this.key = key;
		}
 
		public void setValue(String value)
		{
			this.value = value;
		}
 
		public String getKey()
		{
			return this.key;
		}
 
		public String getValue()
		{
			return this.value;
		}
	}
}

**steel55677** · February 23rd, 2012, 06:42 PM

Thanks, I tried to do it last night. This is my map currently.

public class OwnMap<Key,Value>{
 
		static private Object Key, Value;
 
		public OwnMap(Object Key, Object Value){
 
			this.Key = Key;
			this.Value = Value;
		}	
 
		public static Object getKey(){
              return Key;
		}
 
		public static Object getValues(){
			return Value;
		}
 
 
		public static void main(String args[]){
 
			OwnMap m = new OwnMap("Hello", "123456");
 
			System.out.println(m.getKey());
			System.out.print(m.getValues());	
 
		}
 
	}

I'm just not able to create an ArrayList of an ArrayList which is what a hashmap is.
eg) I have Keys (A,B,C,D,E). They will be added to an Arraylist called Keys. I also want to create an ArrayList of the elements in the Key's ArrayList.
eg) A = new ArrayList();, B = new ArrayList(); and they will contain the values.

I just have trouble with that part =\

**JonLane** · February 23rd, 2012, 08:09 PM

This may be over simplified, but I don't quite understand what you want if not this.

 
ArrayList() keys = new ArrayList();
ArrayList() a = new ArrayList();
ArrayList() b = new ArrayList();
ArrayList() c = new ArrayList();
ArrayList() d = new ArrayList();
ArrayList() e = new ArrayList();
 
keys.add(a);
keys.add(b);
keys.add(c);
keys.add(d);
keys.add(e);

**steel55677** · February 23rd, 2012, 09:15 PM

Yes, that code is exactly what I want. I want an arraylist of arraylist. But, I don't know how to generate generic arraylist so that the names aren't predefined. The name of the arraylist is whatever the elements in the keys arraylist is, or in the code above, put those a,b,c,d,e, into the keys arraylist.

1)
ArrayList() Keys  = new ArrayList();
Keys.add("A")
Keys.add("B")
Keys.add("C")
2)
for (int i =0  to i < Keys.size())
Keys.get(i) = new ArrayList();
 
//A would be its own ArrayList made from the Keys. so it's an Arraylist of Arraylist. 
//I can't have any predefined list because the keys will be entered with random names. I just need to make a method that create ArrayList for every key elements. I don't know how to do that because I've tried to make a generic arraylist name but it doesn't work.

**JonLane** · February 23rd, 2012, 09:41 PM

Well, I can't think of anyway of doing that with out making your own class that adds some addition info to your ArrayList.

public class MyArray {
String myName = "";
ArrayList myList;
 
	//constructor
	public MyArray(String name){
		myName = name;
		//TODO, you will also want to initialize the list here by at least making a new ArrayList object.
	}
}

 
// in your other main class.
Arraylist keys = new ArrayList;
 
//for (int i =0  to i < Keys.size())
//Keys.get(i) = new MyArray();

you will need to write new getter/setter methods to access your MyArray instance variables (myName, myList)
EDIT: if you make them private, and you should.

**steel55677** · February 24th, 2012, 03:15 PM

I'll go ahead and try that. I think I should be able to implement/extend ArrayList class. Thanks

**copeg** · February 24th, 2012, 03:43 PM

I'm making my own Hashmap but I don't know how to create individual ArrayList with varied names.

Not sure why you wish to reinvent the wheel, but a HashMap is a LOT more complicated than creates Lists of Lists...it should utilize a hash function, buckets, and ways to expand the buckets - all together providing constant time look-up performance (as opposed to linear time look-up performance relying on looping over an ArrayList to find an element). My .02

**steel55677** · February 25th, 2012, 11:42 AM

Originally Posted by copeg

Not sure why you wish to reinvent the wheel, but a HashMap is a LOT more complicated than creates Lists of Lists...it should utilize a hash function, buckets, and ways to expand the buckets - all together providing constant time look-up performance (as opposed to linear time look-up performance relying on looping over an ArrayList to find an element). My .02

Because my teacher wants us to code our own code which means we can't use java's api other than arraylist. we had to make our own arraylist class from array. and i get that, i'm just making a simple hashmap to store keys and values and use %m to find the keys or horner's method. then use either quick sort or merge sort to find the keys. appreciate your .02, i wish we didn't have to reinent the wheels

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