java子列表删除

Question

I'm trying to create a list of objects and then a sublist and then delete all the elements in the sublist and then once again display the main list. However when I try to remove elements from a sublist I get error at runtime indexoutofbounds and unknown source. How to fix that so that the app works?

import java.util.*;

class Eval{
    Eval(){         
    }   
}

public class Ch11Ex7 {
    public static void main(String[] args){ 

        Eval e1 = new Eval();
        Eval e2 = new Eval();
        Eval e3 = new Eval();
        Eval e4 = new Eval();
        Eval e5 = new Eval();

        Eval[] eva = {e1, e2, e3, e4, e5};

        //ArrayList<Eval> ev = new ArrayList<Eval>(Arrays.asList(eva));
        List ev = Arrays.asList(eva);
        List<Eval> sub = ev.subList(1, 3);  
        for(int i=0; i< ev.size() ; i++)
              System.out.println(ev.get(i));        
        System.out.println("Sublist");  
        for(int i=0; i< sub.size() ; i++)
              System.out.println(sub.get(i));   
        System.out.println("Remove element");
        sub.remove(2);
    }
}

Answer 1

The second index of subList is exclusive, so if you want the elements between [1..3] you need to use:

List<Eval> sub = ev.subList(1, 4); 

Furthermore, what you are trying to do will not work anyway, because the List implementation returned by subList does not implement the remove operation, so you will get a java.lang.UnsupportedOperationException .

You should create sub as an ArrayList instead:

ArrayList<Eval> sub = new ArrayList<Eval>(ev.subList(1, 4));

Answer 2

Java Lists are zero indexed, so:

List<Eval> sub = ev.subList(1, 3) // = {e2, e3} (subList is not inclusive on second index)

And

sub.remove(2); // Attempts to remove 3rd element from 2 element list

So, reduce your indexes by one.

import java.util.*;

class Eval{
Eval(){         
}   
}

public class Ch11Ex7 {
public static void main(String[] args){ 

    Eval e1 = new Eval();
    Eval e2 = new Eval();
    Eval e3 = new Eval();
    Eval e4 = new Eval();
    Eval e5 = new Eval();

    Eval[] eva = {e1, e2, e3, e4, e5};

    //ArrayList<Eval> ev = new ArrayList<Eval>(Arrays.asList(eva));
    List ev = Arrays.asList(eva);
    List<Eval> sub = ev.subList(0, 2); 
    sub = new ArrayList<Eval>(sub);
    for(int i=0; i< ev.size() ; i++)
          System.out.println(ev.get(i));        
    System.out.println("Sublist");  
    for(int i=0; i< sub.size() ; i++)
          System.out.println(sub.get(i));   
    System.out.println("Remove element");
    sub.remove(1);
}

}

Answer 3

There is no item with index 2 on your sublist sub So the error is self explanatory.

The indexing begins at 0.

In your case

sub =  = {e2, e3}
sub[0] = e2
sub[1] = e3

So, when you try to remove item at index 2, you get the runtime exception. If you are aim is to remove second element from the list, you should call

sub.remove(1)

Answer 4

You want to say sub.remove(1) AND you want to declare the list as an ArrayList - not using Arrays.asList . Your code as is will throw an UnsupportedOperationException because Arrays list is fixed length.

Answer 5

As @Peter said, indexes in Java are 0-based, so you need to use:

sub.remove(1)

Still , with your current program you will get an UnsupportedOperationException, as Arrays.asList() returns a fixed-size list that doesn't support remove (which will be reflected to ev because subList() reflects changes to the original list).

You need to use an ArrayList or similar resizable implementation of the List interface.

Answer 6

List are an ordered type. Therefore (from what I understand) you cannot just remove an item at random within the list. It would change the order.

This here, for example does work :

public class Ch11Ex7 {
    public static void main(String[] args){ 

        Eval e1 = new Eval();
        Eval e2 = new Eval();
        Eval e3 = new Eval();
        Eval e4 = new Eval();
        Eval e5 = new Eval();

        Eval[] eva = {e1, e2, e3, e4, e5};

        ArrayList<Eval> ev = new ArrayList<Eval>(Arrays.asList(eva));
        ArrayList<Eval> sub = new ArrayList<Eval>(ev.subList(1, 3));  
        printList(ev);
        System.out.println("Sublist");  
        printList(sub);   
        System.out.println("Remove element");

        sub.remove(1);

        System.out.println("New List"); 
        printList(ev);
        System.out.println("Sublist"); 
        printList(sub);
    }

    public static void printList(List<Eval> list) {
        for(int i=0; i< list.size() ; i++)
            System.out.println(list.get(i));   
    }
}

I have allowed myself a few changes to code style... but all in all this works (and tested)

Hope this helps!