我可以将一个变量用于不同的容器（列表，集合，映射）吗？

Question

It is necessary to change a container type:

import java.util.*;

public class MyContainers {

    public static void main(String[] args) {
        List<String> list = new ArrayList<String>();
        list.add("title1");
        list.add("title2");
        System.out.println(list.indexOf("title1"));

        // change container type
        Set<String> set = new HashSet<String>(list);
    }
}

But it will be better to use only one variable. I wrote such code but there was a restriction (see comment):

import java.util.*;


public class MyContainers {

    public static void main(String[] args) {
        Collection<String> list = new ArrayList<String>();
        list.add("title1");
        list.add("title1");

        System.out.println(list);

        // Can't call indexOf because there are no such method in inteface Collection
        //System.out.println(list.indexOf("title1")); 

        // change container type
        list = new HashSet<String>(list);
        System.out.println(list);   

    }
}

Please, help me with such questions:

1. Is it possible to use one variable for different containers and use full set of containers methods?
2. Is it possible to convert List to Map ? Please, show a code examples.

Answer 1

If some method doesn't exist in one interface , it doesn't exist for a reason. Don't try to have a workaround in order to have it, that'll usually lead to troubles. For example, it doesn't make any sense to have indexOf method for the Set interface .

However, it can be useful sometimes to construct a new object of different type from an existing one, for example, if you have an ArrayList and you don't want to have duplicates, it does make sense to convert it to HashSet .

You should pick the best interface that suits your needs, if you don't find any, you can always implement your own class.

Answer 2

I found the solution, is it good?

import java.util.*;

public class MyContainers {

    static Collection<String> collection = new ArrayList<String>();

    public static void main(String[] args) {
        collection.add("title1");
        collection.add("title1");

        System.out.println(collection); // [title1, title1]
        useIndexOf();
        System.out.println(collection); // [title1, title1]
        deleteDublication();
        System.out.println(collection); // [title1]
    }

    public static void useIndexOf()
    {
        List<String> list = new ArrayList<String>(collection);
        System.out.println(list.indexOf("title1")); // 0
        // Change container type back to universal
        collection = list;
    }

    public static void deleteDublication() {
        Set<String> set = new HashSet<String>(collection);
        collection = set;
    }

}