Updated:

As per ur request, modified already provided function.

private Userlist getExistingUser(){

    Userlist existingUser = null;

    //#1. Iterate through list
    for(Userlist userlist: UserListArray){

        //#2. Check if user id is matching
        if(userlist.mId == Users.getId()){

            //#3. Matches, user id exists, get the record and out from loop
            existingUser = userlist;
            break;
        }
    }

    return existingUser;
}

Where getting this function, pls check with null or not.

 Userlist user = getExistingUser();

  If (user!=null){ //means user is present in the list array
         //so, Get the quantity here..
  }

It might solve your problem.

Old:

Try with following function, it will return boolean as user id exists or not

 private boolean checkUserIdExist(){

    boolean isUserIdExist = false;

    //#1. Iterate through list
    for(Userlist userlist: UserListArray){

        //#2. Check if user id is matching
        if(userlist.mId == Users.getId()){

            //#3. Matches, user id exists, out from loop
            isUserIdExist = true;
            break;
        }
    }

    return isUserIdExist;
}

public boolean checkUserExist(User user){

    for(i=0; i<UserListArray.size(); i++){

       if(UserListArray.get(i).getmId == user.getmId){
        return true;
       }
    }

    return false
}

You can use anyMatch of Stream API

if(UserListArray.stream().anyMatch(u -> u.getmId().equals(Users.getId()))){

}

And to get user

UserListArray.stream().filter(u -> u.getmId().equals(Users.getId())).findFirst().get()

This Is My Data Handler Class

public class Userlist {
    private String mId;
    private String mQuantity;

    public String getmId() {
        return mId;
    }

    public void setmId(String mId) {
        this.mId = mId;
    }

    public String getmQuantity() {
        return mQuantity;
    }

    public void setmQuantity(String mQuantity) {
        this.mQuantity = mQuantity;
    }

    public Cartlist(String mId, String mQuantity) {
        this.mId = mId;
        this.mQuantity = mQuantity;
    }
}

And This Is My ArrayList Code

    public static ArrayList<Userlist> UserListArray = new ArrayList<>();

And Here Is I Add Users

    Home.UserListArray.add(new Userlist (  "1" , "595" ));
    Home.UserListArray.add(new Userlist (  "2" , "5558" ));
    Home.UserListArray.add(new Userlist (  "55" , "5154" ));
    Home.UserListArray.add(new Userlist (  "97" , "513" ));
    Home.UserListArray.add(new Userlist (  "16" , "565" ));
    Home.UserListArray.add(new Userlist (  "21" , "598" ));

Now I Want To Check If There Is Userid Exits In Arraylist Or Not

if (UserListArray.contains( Users.getId() ) == true){

        Toast.makeText(context,"List Conatin User",Toast.LENGTH_SHORT).show();

    }else{

        Toast.makeText(context,"List Not Conatin User",Toast.LENGTH_SHORT).show();
    }

ANYONE CAN HELP

REGARDS

AHSAN JAVED