学步园

public class GetSpecifiedElementFromAList {
    
    public static void main(String[] args) {
        List<POJO> list = new ArrayList<POJO>();
        POJO a = new POJO();
        a.setKey("aaa");
        a.setContent("aaa111111");
        POJO b = new POJO();
        b.setKey("bbb");
        b.setContent("bbb222222");
        list.add(a);
        list.add(b);
        // So, we're going to find out where it is. I mean the element which key is 'bbb'.        
        // If you do it with the original object, everything seems OK.
        int idx = list.indexOf(b);
        System.out.println(idx); // You can get a '1' here.
        // But if you do it with a brand new object
        // containing the same value with the original one...
        POJO c = new POJO();
        // like this...
        c.setKey("bbb");
        c.setContent("bbb222222");
        idx = list.indexOf(c);
        System.out.println(idx); // you'll get nothing but '-1'.
        // which means it is not supposed to be used like this at all.
    }

}