Get index of an arraylist using property of an con

Question

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Searching in a ArrayList with custom objects for certain strings (9 answers)

Closed 6 years ago.

I'm having an list of Object type. In that I have one String property idNum. Now I want to get the index of the object in the list by passing the idNum.

List<Object1> objList=new ArrayList<Object1>();

I don't know how to give objList.indexOf(// Don't know how to give here);

Is it possible to do this without iterating the list. I want to use indexOf() method only.

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);
        }
    });

Answer 2

You cannot do this with indexOf. Instead all of the objects in the list should inherit from a common interface - for example

interface HasIdNum {
    String getIdNum();
}

Now you list can be List<HasIdNum> and you can loop through it to find the object by id using:

for (HasIdNum hid: objList) {
   if (hid.getIdNum().equals(idNumToFind) {
       return hid;
   }
}
return null;

To get the index rather than the object do:

for (int i=0;i<objList.size();i++) {
   HasIdNum hid = objList.get(i);
   if (hid.getIdNum().equals(idNumToFind) {
       return i;
   }
}
return -1;

Alternatively you can use reflection to query the methods of the object, but that will be much slower and much less safe as you lose all the compile time type checking.

Answer 3

Write a small helper method.

 private int getIndexByProperty(String yourString) {
        for (int i = 0; i < objList.size(); i++) {
            if (object1 !=null && object1.getIdNum().equals(yourString)) {
                return i;
            }
        }
        return -1;// not there is list
    }

Do not forget to return -1 if not found.