在arrayList Java中搜索对象

Question

package generics;

import java.util.ArrayList;
import java.util.List;


public class Generics {
    private static List <Box> newlist = new ArrayList<>();

    public static void main(String[] args) {

        newlist.add(new Box("charlie",30));
        newlist.add(new Box("max",29));
        newlist.add(new Box("john",22));

        // Testing method find -- Start
        find ("max",29);


        //Testing method find2 -- Start
        Box <String,Integer> search = new Box("max",29);

        find2(search);

    }

    public static void find (String parameter, Integer parameter1){

        for (Box e : newlist){
            if(e.getName() != null && e.getMoney() !=null 
                                   && e.getName().equals(parameter)
                                   && e.getMoney().equals(parameter1)){
                System.out.println("found on position " + newlist.indexOf(e));
                break;
            }

        }

    }

        public static void find2 (Box e){
        for (Box a : newlist){
            if (a.equals(e)){
                System.out.println("Found");
            }else {
                System.out.println("Not found");
            }
        }
    }
}


public class Box<T , D>{


    private T name;
    private D money;

    public Box(T name, D money) {
        this.name = name;
        this.money = money;
    }

    public T getName() {
        return name;
    }

    public D getMoney() {
        return money;
    }

    @Override
    public String toString() {
        return name + " " + money;
    }

}

Can someone show me how to search for an object in ArrayList.

Method find() it works perfect but in my opinion is wrong and the reason why I am thinking like that, because I am passing as parameter a string and an integer but should be an box object or maybe I wrong?

In my second method find2() I am trying to pass as parameter an object of Box and when I am trying to search for it I got a false result =(

I am noobie I am trying to understand and to learn.

Answer 1

You should override Object.equals() on the Box class. Try to handle null correctly too. Because 2 Box with null names and/or null money are in fact equal.

(you DON'T need to override Object.hashCode() for this, but it's a good practice to do so, just in case it is used in a hashmap or hashset or such).

Answer 2

Stop using raw types!

Box is generic, so if you are not targeting older Java versions, always add generic parameters !.

The declaration of find2 should be like this:

public static void find2 (Box<String, Integer> e)

And you should check whether two boxes are equal in exactly the way you did in find . equals will not work because you did not define an equals method in Box . So:

for (Box<String, Integer> a : newlist){
    if (a.getName().equals(e.getName()) &&
        a.getMoney().equals(e.getMoney())){
        System.out.println("Found");
    }else {
        System.out.println("Not found");
    }
}

Answer 3

The easiest way to search and find something in an arraylist is to use the .equals method combined with a for loop to iterate through your lists.

for(int i = 0; i < newList; ++i)
{
if(newlist.equals(Stringname))
    {
     //it matches so do something in here
    }
}

what it is doing here is moving through the list 1 by 1 until it finds something that matches what you entered -> stringName