IndexOutOfBoundsException with AWT-EventQueue-0

I'm writing a calculator and for some reason whenever I hit the + sign this error gets thrown

Exception in thread "AWT-EventQueue-0" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
at java.util.ArrayList.RangeCheck(Unknown Source)
at java.util.ArrayList.remove(Unknown Source)
at gui.LongInteger.addition(LongInteger.java:78)
at gui.Calculator$3.actionPerformed(Calculator.java:1 42)

My addition listener is this

Code:

---

//textField is a TextField object
//output is a StringBuilder object
buttonPlus.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent e){
                currentEntry = new LongInteger(output.toString());
                currentValue = currentValue.addition(currentEntry);
                textField.setText(currentValue.toString());
                output.delete(0, output.length());
        }
});

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the code within my addition method that messes up is

Code:

---

num1 = Integer.parseInt(this.arrayIntList.remove(0).toString());

---

and here is the constructor for a LongInteger

Code:

---

LongInteger(String input){
        if((input.charAt(0) == '-') || (input.charAt(0) == '+')){
                if(input.charAt(0) == '-')
                        isPositive = false;
                input = input.substring(1);
        }
        for(int i = 0; i < input.length(); i++){
                char temp = input.charAt(i);
                String tempString = new String(temp+"");
                this.arrayIntList.add(Integer.parseInt(tempString.trim()));
        }
        this.size = arrayIntList.size();       
}

---

can anybody explain why this is happening?