what is the use of new() and at()

Question

Here is my contract example.

pragma solidity ^0.4.18;
contract LocalEthereum {

    address public owner;
    event Created(bytes32 _tradeHash);
    function createEvent() onlyOwner external {
        Created(0x01);
    } 
}

version one

import {default as contract} from 'truffle-contract';
import Web3 from 'web3';
const contractAbstraction = contract(abi);
contractAbstraction.setProvider(this.web3.currentProvider);
contractAbstraction.deployed().then(function(contractInstance) {

    }).catch(function(e) {
      console.log(e);
    })

version 2

const contractAbstraction = contract(abi);
contractAbstraction.setProvider(this.web3.currentProvider);
contractAbstraction.new().then(instance){}.catch(err){}

}

version 3

const contractAbstraction = web3.eth.contract(abi); 
var contractInstance = contractAbstraction.at(address);

My first question is compared with code 1 and 2, if I am using the truffle framework to deploy the contract, then I just need to contractAbstraction.deployed() for each call, so what is the use of new().

My assumption of new() will make a new instance of the contract, which initialize the contract, the owner address in the contract is the address of those who uses new(). But why do some examples I see don't use new() at all?

Secondly, compared with code 1 and 3, what is the difference with them, I see different examples using the two, I assume they are the same?

Answer 1

The function new() is used to deploy a new instance of a contract. For example you can design your contract to have multiple instances you will need new to create them.

The funciton at() is used to reference an already deployed contract. For example if you want to interact with a third party contract, and you only have the contract ABI and address, in those cases deployed() will not work and you have to use at to interact with it.