如何逐个将项添加到ArrayList中而不是互相替换？

Question

I want to create a shopping list application. I'm not sure how to add items one by one so the list expands instead of replacing an existing item. This is my code at the moment. I was unsuccessfully trying to use for loop.

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         

    List<ShoppingItem> list = new ArrayList<>();

    String name = ShoppingListUI.jTextField1.getText();    
    double price = Double.parseDouble(jTextField2.getText());
    int quantity = Integer.parseInt(jTextField3.getText());

    ShoppingItem item = new ShoppingItem(name, price, quantity);
    list.add(item);

    for (ShoppingItem temp : list) {
        System.out.println("Item: " + temp.getName() + ", Price: " + temp.getPrice() + ", Quantity: " + temp.getQuantity());
    }

    jTextField1.setText("");
    jTextField2.setText("");
    jTextField3.setText("");    
}

Answer 1

You are creating a new List within your method :

List<ShoppingItem> list = new ArrayList<>();

so each time jButton1ActionPerformed is called, a new List is created, which contains just one element.

You should change that list to be an instance variable of the class that contains this method.

public class YourClass {

    List<ShoppingItem> list = new ArrayList<>();

    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
        String name = ShoppingListUI.jTextField1.getText();    
        double price = Double.parseDouble(jTextField2.getText());
        int quantity = Integer.parseInt(jTextField3.getText());

        ShoppingItem item = new ShoppingItem(name, price, quantity);
        list.add(item);

        for (ShoppingItem temp : list) {
            System.out.println("Item: " + temp.getName() + ", Price: " + temp.getPrice() + ", Quantity: " + temp.getQuantity());
        }

        jTextField1.setText("");
        jTextField2.setText("");
        jTextField3.setText("");    
    }
}

Answer 2

I'm not sure how to add items one by one so the list expands instead of replacing an existing item.

That's because you are creating a new list on each action. You should move your list declaration to top level. So that you list stays constant and it keeps adding items in it.

List<ShoppingItem> list = new ArrayList<>();

......// other codes 

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         

String name = ShoppingListUI.jTextField1.getText();    
double price = Double.parseDouble(jTextField2.getText());
int quantity = Integer.parseInt(jTextField3.getText());

ShoppingItem item = new ShoppingItem(name, price, quantity);
list.add(item);

for (ShoppingItem temp : list) {
    System.out.println("Item: " + temp.getName() + ", Price: " + temp.getPrice() + ", Quantity: " + temp.getQuantity());
}

jTextField1.setText("");
jTextField2.setText("");
jTextField3.setText("");    

}

---

And looking your codes further

String name = ShoppingListUI.jTextField1.getText();    
double price = Double.parseDouble(jTextField2.getText());
int quantity = Integer.parseInt(jTextField3.getText());

You are parsing the user inputted text right way. You might want to check for parsing errors and ask user to reenter.

Answer 3

You always create a new list, when the Action is performed. The "old" list istn't referenced anymore after that. You add the new Item to the new List. So this is the only item you see. You need to instantiate the List outside of your Action actionPerformed method.

private List<ShoppingItem> list = new ArrayList<>();

private void jButton1ActionPerformed(java.awt.event.ActionEvent evt){                                          

    String name = ShoppingListUI.jTextField1.getText();    
    double price = Double.parseDouble(jTextField2.getText());
    int quantity = Integer.parseInt(jTextField3.getText());

    ShoppingItem item = new ShoppingItem(name, price, quantity);
    list.add(item);

    for (ShoppingItem temp : list) {
        System.out.println("Item: " + temp.getName() + ", Price: " + temp.getPrice() + ", Quantity: " + temp.getQuantity());
    }

    jTextField1.setText("");
    jTextField2.setText("");
    jTextField3.setText("");    
}

Answer 4

Currently list in created every call to your method, You need to move it outside the function.

You need for make it Instance variables, Notice in case you create new class instance every time when calling the function - define list in class as static field and use it.

public static List<ShoppingItem> list = new ArrayList<>();