Java - Checking for duplicates in arrayList and stop user from adding an order to the array

A GUI program that displays a label at the top with "Order not added -duplicate" when a duplicate order is attemped to be added by the user.

for the order in orderList to be considered duplicate all inputs in textfields (when the button is clicked) have to be the same.

I suppose that I have to use a for loop to check if the order is already in the arrayList but I'm not sure how, I'm pretty new to java.

Instead of an array you should be using a Set which is a collection specifically designed to prevent duplicates:

> A Set is a Collection that cannot contain duplicate elements. It models the mathematical set abstraction. The Set interface contains only methods inherited from Collection and adds the restriction that duplicate elements are prohibited. Set also adds a stronger contract on the behavior of the equals and hashCode operations, allowing Set instances to be compared meaningfully even if their implementation types differ. Two Set instances are equal if they contain the same elements.

Since you are new to Java language take a step back and first learn about basic collections provided by the standard library: List, Set and Map. Once you understand this abstractions and their implementation variants you will be able to solve a lot of common day-to-day problems.

While a Set would be good in a real world scenario, this appears to be a homework assignment with arbitrary limitations so let's see how it can be done using an ArrayList.

First thing I would recommend doing is overriding your .equals() method on your Order object to properly compare field values. Something like...

@Override
public boolean equals(Object obj) {
  //Same reference so same object in every sense of the word
  if(obj == this)
    return true;
  //Not instanced nor the same class so clearly not equal
  if(obj == null || obj.getClass() != getClass())
    return false;

  //Cast it to the same object type
  Order o = (Order) obj;
  //Start comparing every variable, will return false if any of them don't match
  //Be sure to use the equals operation for any variable objects such as String
  return this.name.equals(o.name) && this.value == o.value && this.someVar == o.someVar; //ect...
}

Then you just need to take your fields and turn it into a object like normal...

Order order = new Order(field1.getText(), field2.getText(), field3.getText()); //ect...

And run it in a compare against the existing values. I recommend making a separate method for this...

public static boolean orderExists(Order order) {
  for(Order otherOrder : orderArrayList) {
    if(order.equals(otherOrder))
      return true;
  }
  return false;
}

From there, the orderExists() method will tell you if it's already in there and you can implement your code for displaying the error. Hope this helps you get set up enough to handle the rest on your own alright!