Don't use static fields, and also encapsulate them.
For ActionListener
i always use inner private classes if the only scope is the outside class or Anoymous Classes
.
Also it seems you have a collection of Button, you may consider grouping into collection.
Example (I comment in code):
private List<JButton> buttons;
private double tempNum1;
private double tempNum2;
private boolean pointOn = false;
private int APC = 1;
//make them private why public and static?
public GUI(){
ActionListener numListener = new GUINumListener();
//initiliatze buttons
int size=10;
buttons= new ArrayList<>(size);
for(int i=0;i<size;i++){
JButton button = new JButton();
button.addActionListener(numListener);
buttons.add(button);
}
}
private class GUINumListener implements ActionListener{
@Override
public void actionPerformed(ActionEvent e){
if (e.getSource() == buttons.get(0)){ // actionCommand returns string you have to use getSource() or setting an actionCommand to the button and compare num0.getActionCommand()
tempNum2 *= 10;
}else if (e.getSource() == buttons.get(1)){
if (!pointOn){ // u were assigning pointOn = false
tempNum2 = (tempNum2 * 10) + 1;
}else{
tempNum2 = (tempNum2 * Math.pow(10, APC) + 1) / Math.pow(10, APC);
APC++;
}
ansField.setText(Double.toString(tempNum2));
}
}