java - JAVA 中 ActionPerformed 方法中的 IF 语句

Question

我正在尝试构建一个基本游戏，测试用户对州首府的了解。在这个阶段，我只是尝试为按钮设置一些基本功能。

我正在尝试获取它，以便当用户输入与正确答案相对应的字符串时，程序会显示“正确!”。我试过使用 IF ELSE 语句，但即使我输入正确的答案，它也会输出“错误!”。这可能很简单，但我无法发现它。如果有人能指出我正确的方向，将不胜感激。我是一个初学者，所以也许这完全是错误的方法，但这是我到目前为止所拥有的:

public class Main {

    public static void main(String[] args){

        Gui a = new Gui();

    }

}

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class Gui extends JFrame{
    private static final long serialVersionUID = 1L;

        JPanel row1 = new JPanel();
        JLabel instructions = new JLabel("What is the capital of Alabama?");

        JPanel row2 = new JPanel();
        JLabel aLabel = new JLabel("Answer: ");
        JTextField aField = new JTextField(14);

        JPanel row3 = new JPanel();
        JButton submit = new JButton("Submit");
        JButton reset = new JButton("Reset");

    public Gui() {

        super("State Capitals Game");
        setLookAndFeel();
        setSize(400, 200);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);
        setResizable(false);

        setLayout(new GridBagLayout());
        GridBagConstraints gc = new GridBagConstraints();

        gc.insets = new Insets(5,5,5,5);

        gc.gridx = 0;
        gc.gridy = 0;
        add(instructions, gc);

        gc.gridx = 0;
        gc.gridy = 1;
        add(aLabel, gc);

        gc.gridx = 0;
        gc.gridy = 2;
        add(aField, gc);

        gc.gridwidth = 3;
        gc.fill = GridBagConstraints.HORIZONTAL;
        gc.gridx = 0;
        gc.gridy = 4;
        add(submit, gc);

        ///this is where I'm stuck////

        submit.addActionListener(new ActionListener() {

            public void actionPerformed(ActionEvent e) {
                String userIP = aField.getText();

                String mg = "Montgomery";
                if(userIP == mg){
                    System.out.println("correct!");
                }else{
                    System.out.println("WRONG!");
                }

            }
        });

        gc.gridwidth = 3;
        gc.fill = GridBagConstraints.HORIZONTAL;
        gc.gridx = 0;
        gc.gridy = 5;
        add(reset, gc);

        reset.addActionListener(new ActionListener() {

            public void actionPerformed(ActionEvent e) {
                reset.getInputMap().put(KeyStroke.getKeyStroke("ENTER"), "pressed");
                aField.setText("");             
            }
        });
    }

    private void setLookAndFeel() {

        try {
            UIManager.setLookAndFeel(
                "com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel"
            );
        } catch (Exception exc) {
            // ignore error
        }

    }

}

Answer 1

当比较字符串时，你应该使用 equals() 方法:

String mg = "Montgomery";
                if(userIP.equalsIgnoreCase(mg)){ // like this
                    System.out.println("correct!");
                }else{
                    System.out.println("WRONG!");
                }

否则你不会比较字符串的内容，而是比较不是你想要的引用

编辑: 实际上，您可能想要的是使用 equalsIgnoreCase()，否则如果用户输入“montgomery”，它将抛出一个 false，因为 equals 区分大小写。你可以找到 api here