how can I implement the constructor of a hashtable in java

Question

Suppose I need to implement a hashtable on my own,but I have some problems about implementing my constructor. For example,if I need to initialize List[] buckets,but when I write like the following codes,the computer just gave a wrong signal of"buckets[i]=new List()" , can someone tell me how to finish the constructor in this case? import java.util.List;

import java.util.LinkedList;

public class GeneralHashMap extends MyHashMap {

  public List< String>[] buckets;

  public GeneralHashMap() {
    for(int i=0;i<120;i++)
    {
        buckets[i]=new List<String>(); 
    }

    // TODO: IMPLEMENT CONSTRUCTOR

  }


  public GeneralHashMap(int newsize)
  {
    for(int i=0;i<newsize;i++)
    {
        buckets[i]=new List<String>();
    }
  }

  @Override
  protected int hash(String token) {

    // TODO: IMPLEMENT HASHING FUNCTION FOR GENERAL HASHMAP
    return -1;

  }

  @Override
  public void add(String token) {

    // TODO: IMPLEMENT ADD METHOD USING BUCKETS

  }

  @Override
  public void display() {

    // TODO: IMPLEMENT DISPLAY METHOD TO SHOW CONTENTS OF ALL BUCKETS

  }

}

I want to set up a list of list so that I can store strings in every bucket,but I have some problems to implement the construct — python3, May 02 '15 at 16:57

score 2 · Accepted Answer · answered May 02 '15 at 16:59

2

In Java List is just an interface which cannot be instantiated. What you need is a class implementing such interface, for example, ArrayList. Try

buckets = new List[120];
for(int i=0;i<120;i++)
{
    buckets[i]=new ArrayList<String>(); 
}

answered May 02 '15 at 16:59

Tagir Valeev

97,161
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would also be worth mentioning the initialization of the `new List[120]` rather than just including it. The second constructor also needs one. – CupawnTae May 02 '15 at 17:12

Manuel Jain · Answer 2 · 2015-05-02T17:00:56.247

0

initialize your public List< String>[] buckets; properly before filling it:

public List< String>[] buckets = new ArrayList< String>[120];

edited May 02 '15 at 17:00

answered May 02 '15 at 16:57

Manuel Jain

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That won't work for the second constructor if a size > 120 is passed - you should initialize in the constructor instead – CupawnTae May 02 '15 at 16:59
oops, you're right, didn't saw the other constructor – Manuel Jain May 02 '15 at 17:00
Oh and `List` isn't a class, so you need a concrete implementation – CupawnTae May 02 '15 at 17:01
And also `new ArrayList< String>[120];` is not valid... See http://stackoverflow.com/questions/3903196/error-generic-array-creation – CupawnTae May 02 '15 at 17:08

how can I implement the constructor of a hashtable in java

2 Answers2