Java Swing：JComboBox和ActionListener是否有问题？

Question

我在创建转换器的项目上遇到了困难。问题是无法从中获得任何输出。我认为动作监听器有问题吗？我尝试调试代码，但发现问题不关乎输入或输出字段（我将这两个链接在一起，然后进行了尝试。我得到了正确的答案）。其次，我尝试将打印内容添加到代码的不同部分。这就是我发现该软件永远不会进入

的原因

if(from.getSelectedItem()=="meter") { 
    meterConvertor();..."                  

我将不胜感激。

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.awt.*;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
import java.applet.*;

public class Converter extends JFrame implements ActionListener, ItemListener {
    JTextField input1_field;
    JTextField answer;
    double input1 = 0;
    double result = 0;
    String[] pituudet = new String[] { "Choose to", "meter", "kilometer", "feet" };
    String[] pituudet1 = new String[] { "Choose from", "meter", "kilometer", "feet" };
    JButton calculate;
    JComboBox<String> from;
    JComboBox<String> to;

    Converter() {
        JFrame frame = new JFrame("");
        frame.setSize(1000, 200);
        frame.setLayout(new BorderLayout());
        // Body
        JPanel body = new JPanel();
        body.setLayout(new GridLayout(1, 1, 50, 50));
        body.setPreferredSize(new Dimension(300, 400));
        body.setBackground(Color.lightGray);
        // from JComboBox1
        JComboBox<String> from = new JComboBox<>(pituudet);
        from.setSize(200, 2000);
        from.setFont(new Font("Serif", Font.PLAIN, 35));
        add(from);
        from.addActionListener(from);
        // to JComboBox2
        JComboBox<String> to = new JComboBox<>(pituudet1);
        to.setFont(new Font("Serif", Font.PLAIN, 35));
        to.addActionListener(to);
        // input1 field
        input1_field = new JTextField(15);
        input1_field.setFont(new Font("Serif", Font.PLAIN, 35));
        input1_field.setBounds(650, 300, 400, 100);
        add(input1_field);
        input1_field.addActionListener(from);
        // output field
        answer = new JTextField("");
        answer.setFont(new Font("Serif", Font.PLAIN, 35));
        answer.setBounds(650, 300, 400, 100);
        add(answer);
        answer.setEditable(false);
        // Calculate button
        JButton calculate = new JButton("Calculate");
        calculate.setFont(new Font("Serif", Font.PLAIN, 20));
        calculate.addActionListener(this);
        // add staff to body panel
        body.add(from);
        body.add(to);
        body.add(input1_field);
        body.add(calculate);
        body.add(answer);
        // add to frame
        frame.add(body);
        frame.setVisible(true);
        frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    public void meterConvertor() {
        if (to.getSelectedItem() == "meter") {
            result = (input1);
            answer.setText("" + result);
        } else if (to.getSelectedItem() == "kilometer") {
            result = (input1 * 0.001);
            answer.setText("" + result);
        } else if (to.getSelectedItem() == "feet") {
            result = (input1 * 0.3048);
            answer.setText("" + result);
        } else {
            result = (input1 * 29.7);
            answer.setText("" + result);
        }
    }

    public void kilometerConvertor() {
        if (to.getSelectedItem() == "meter") {
            result = (input1 * 1000);
            answer.setText("" + result);
        } else if (to.getSelectedItem() == "kilometer") {
            result = (input1);
            answer.setText("" + result);
        } else if (to.getSelectedItem() == "feet") {
            result = (input1 * 3280.8399);
            answer.setText("" + result);
        } else {
            result = (input1 * 0.681);
            answer.setText("" + result);
        }
    }

    public void feetConvertor() {
        if (to.getSelectedItem() == "meter") {
            result = (input1 * 0.3048);
            answer.setText("" + result);
        } else if (to.getSelectedItem() == "kilometer") {
            result = (input1 * 0.0003048);
            answer.setText("" + result);
        } else if (to.getSelectedItem() == "feet") {
            result = (input1);
            answer.setText("" + result);
        } else {
            result = (input1 * 0.388);
            answer.setText("" + result);
        }
    }

    public void actionPerformed(ActionEvent e) {
        input1 = Double.parseDouble(input1_field.getText());
        String a = input1_field.getText();
        if (e.getSource() == calculate)
            if (from.getSelectedItem() == "meter") {
                meterConvertor();
            } else if (from.getSelectedItem() == "kilometer") {
                kilometerConvertor();
            } else if (from.getSelectedItem() == "feet") {
                feetConvertor();
            } else {
                kilometerConvertor();
            }
    }

    public void itemStateChanged(ItemEvent e) {
    }

    public static void main(String args[]) {
        Converter window = new Converter();
        window.setVisible(true);
    }
}