Sorting an arraylist of my own type in Java

问题

I have a type in Java called Item which is defined as follows:

private Integer itemNo;
private String itemName;
private String itemDescription;
...

And I would like to be able to sort an arraylist of this type in descending order according to itemName.

From what I read, this can be done via:

Collections.sort(items, Collections.reverseOrder());

Where items is:

ArrayList<Item> items = new ArrayList<Item>();

But I find that the call to Collections.sort gives me a:

Item cannot be cast to java.lang.Comparable

Run time exception.

Can anyone advise on what I need to do?

回答1:

Declare Item to be Comparable, and implement the comapreTo method to compare itemName in reverse order (ie compare "that to this", rather than the normal "this to that").

Like this:

public class Item implements Comparable<Item> {
    private Integer itemNo;
    private String itemName;
    private String itemDescription;

    public int compareTo(Item o) {
        return o.itemName.compareTo(itemName); // Note reverse of normal order
    }

    // rest of class
}

回答2:

You need your custom Item to implement Comparable , or otherwise you can do it using Comparator

回答3:

You should probably make Item implement Comparable, or create a Comparator to your Item, and use Collections.sort(List,Comparator)

code snap:
Comparator:

public static class MyComparator implements Comparator<Item> {
    @Override
    public int compare(Item o1, Item o2) {
        //reverse order: o2 is compared to o1, instead of o1 to o2.
        return o2.getItemName().compareTo(o1.getItemName()); 
    }
}

usage:

    Collections.sort(list,new MyComparator());

(*)note that the static keyword in MyComparator's declaration is because I implemented it as an inner class, if you implement this class as an outer class, you should remove this keyword

回答4:

You need to implement the Comparable interface and define the compareTo(T o) method.

回答5:

You need to make your Item class implement the java.lang.Comparable interface.

public class Item implements Comparable<Item> {

   // your code here, including an implementation of the compare method
}

来源：https://stackoverflow.com/questions/7514451/sorting-an-arraylist-of-my-own-type-in-java

标签

java

collections