尝试使用string作为键和double作为值在java中创建哈希表

Question

In the following program:

import java.util.*;

public class HashTableStringdouble
{
//  private Hashtable<String, double[]> model  = new Hashtable<String, double[]>();;
    private Hashtable<String, double> model  = new Hashtable<String, double>();;                // this does not work

    public static void main(String args[])
    {
    }
}

having double[] works but not double. It gives the following error:

HashTableStringdouble.java:7: error: unexpected type private Hashtable model = new Hashtable();; // this does not work ^ required: reference found: double HashTableStringdouble.java:7: error: unexpected type private Hashtable model = new Hashtable();; // this does not work ^ required: reference found: double 2 errors

I am not sure what am I doing wrong here. Please explain how does a Hashtable works.

Answer 1

You can't use a primitive as a key or value in a Hashtable, you need to use an object. It would work with Double instead of double for example. The reason why it works with double[] is that arrays are objects in Java.

Also, Hashtable is somewhat obsolete and HashMap is preferred in most situations:

private Map<String, Double> model  = new HashMap<String, Double>();
//or if you use Java 7+
private Map<String, Double> model  = new HashMap<>();

Answer 2

1. Don't use Hashtable ; use HashMap . Hashtable is a leftover from Java 1.0, before the times of the Collections Framework.
2. This is not about how maps work in Java, but how Java works in general. You cannot substitute a primitive type for a reference type anywhere.

Answer 3

尝试使用Double类而不是“native”double

Answer 4

1. You can't use primitive types in Collection s. Collection s can contain only descendants of Object type. If you need collections with primitives, you should look at this question: Most efficient Java primitive collections library .

2. Use HashMap , not Hashtable . If you are sure, you need a synchronized collection, have a look at Collections.synchronizedMap() . Eg:

   Map model = Collections.synchronizedMap(new HashMap());

Answer 5

Use wrapper classes. That's one of the reasons why the are invented in the first place.

private Hashtable<String, Double> model  = new Hashtable<String, Double>();

Answer 6

import java.util.*;

public class HashTableStringdouble
{
  private Hashtable<String, Double> model  = new Hashtable<String, Double>();


    public static void main(String args[])
    {
    }
}

Answer 7

use Double instead of double . In Generic primitive data types are not allowed
and don't forget to mark correct answer which you have accepted. welcome to stackoverflow. If you use double[] then it means this is double array object (only objects can be there in generic) and when you use double it means primitive double