Use Left Shift Operator To Append to Lists

To append elements to an existing list, use the left shift operator (<<) or call the list's add() function for best performance.

For example, the following code processes a collection of products and adds the value of the Id field from a subset of the products encountered to a new list:

list productIdsToProcess = []
for (prod in products) {
  if (prod.Status == 'RETURNED') {
    // Append the current product id to the list
    // Same as calling productIdsToProcess.add(prod.Id)
    productIdsToProcess << prod.Id 
  }
}

This technique is better than using the plus or plus-equals operator to do the same job because both of those create a new list each time.