Get index of an arraylist using property of an contained object in java [duplicate]

Answer 1

Implement equals (and hashCode) in Object1 class based on idNum field, then you use List.indexOf like this

int i = objList.indexOf(new Object(idNum));

or make a special class for seaching

    final String idNum = "1";
    int i = list.indexOf(new Object() {
        public boolean equals(Object obj) {
            return ((X)obj).idNum.equals(idNum);
        }
    });