How to extends HashMap to allow String, String types

Question

I need to create a custom class that extends java.util.LinkedHashMap and takes String as key and String as value and has a no-arg constructor that pre-populates this map with initial values.

I am stuck at the first task - how to extend the LinkedHashMap in such a way that instead of generic arguments it accepts only?

I tried this, (not working)

import java.util.LinkedHashMap;

@SuppressWarnings("serial")
public class MyMenu<String, String> extends LinkedHashMap<String, String> {


    public MyMenu(){
        this.put("index.jsp", "Home Page");
    }
}

Solution 1

All you need is:

import java.util.LinkedHashMap;

@SuppressWarnings("serial")
public class MyMenu extends LinkedHashMap<String, String> {

    public MyMenu(){
        this.put("index.jsp", "Home Page");
    }
}

Remove <String, String> from your class name and it will work. You are creating a class which is extending LinkedHashMap<String, String> so your class is already a Map which takes String as a key and String as a value. If you want to create your own generic class then you have to do like this:

Create a class for e.g.:

public class MyMenu<K, V> { ... }

and then extend that class:

public class SomeMyMenu extends MyMenu<String, String> { ... }

In that case you will have to specify the key and value for your class in order for SomeMyMenu class use the key and value as String. You can read more about generics here.

But more efficient way to do what you want is to create some final class and declare the map in it like this:

public static final LinkedHashMap<String, String> MAP = new LinkedHashMap<String, String>() {{
    put("index.jsp", "Home Page");
}};

And to get the values from your map use:

SomeClass.MAP.get("Some Key");

Solution 2

If your struggle, as you describe, is just with the declaration, then I would try something like:

class MyCustomizedMap extends LinkedHashMap<String, String> {
    //...
}

Solution 3

You could do something like below:

   public class MyMap extends LinkedHashMap<String,String>{

Solution 4

Composition is a better solution than inheritance in your case:

class MyProperties implements Map<String, String> {

    private Map<String, String> delegatee = new LinkedHashMap<String, String>();

    // ... delegate your methods to delegatee
}

Solution 5

As long as you are using JDK 1.5 or higher, you can use generics and create a LinkedHashMap that stores String keys and String values and initialize it as follows:

LinkedHashMap<String, String> myMap = new LinkedHashMap<String, String>()
{
    {
        put("abc", "xyz");
        ...
    };
}

View more solutions

Comments

[Rohit Jain]

You forgot to complete your last sentence. Anyways, it won't matter, unless you show some code you might have tried.

[Denys Séguret]

Extending LinkedHashMap is almost certainly a bad solution. You can use its parameterized form or use composition instead of inheritance.

[Wil]

I would not have problem with composition but then I would need a getter method to getMap() from my object. I wanted my object to be a HashMap so it would be easier to use from jsp with jstl

[Martin Serrano]

i agree with @dystroy. we use a utility method with varargs to initialize a map with initial values.

[nicholas.hauschild]

You would not necessarily need a getter to get the Map in a composition model. You could implement the Map interface and just delegate to your...delegate...

[Wil]

Can someone maybe post a good link that explains why composition is better than extending HashMap? I don't see a problem with extending it in my particular case - I need to store map of jsp page to the page title so that I can dymanically create navigation menu in JSP

[Louis Wasserman]

Why not just write a static factory method that creates a LinkedHashMap, puts the desired elements into it, and returns a result? No need for a fragile subclass here.

[Wil]

@Louis Wasserman Because I want my class to behave like a bean, so I can use it from jsp page with <jsp:useBean.... it will be easy when my class has no-argument constructor. with wrapper factory class it will probably not be so easy to use it from jsp. I'm just a beginner...

[Wil]

Thanks. This worked. I still would like to get explanation why I don't need <String, String> after MyMenu? I am learning working with generics in Java and any good explanation is very helpful to me.

[durron597]

Composition is almost always better than inheritance. +1

[trashgenerator]

But not "always"