问题描述

.Java 中多个 class 的使用问题

背景：这是一个教科书上的计算器代码，因为代码中只有第一个是public class，所以我放到同一个.java文件中。

过程：用sublime将代码抄到电脑里，在Eclipse中打开却发现是 class 调用错误。检查了一边发现代码和书中相同，找不到错误的地方，随后我为各个class建了各自的.java文档，想这次如果有错误就是书中代码错了，但……显示正常，运行ok。

坑爹，我把代码粘贴回原来的.java文档，复制文档＋换文件夹，拷贝的文件没有显示错误。但是class的名字是黄色未被调用（这不是废话么？不能调用你还运行个什么）。我再点击修复，得到 private static final long serialVersionUID = 1L;

秉着打破砂锅问到底的精神，所以为什么第一次写上代码时是显示错误，复制粘贴之后又 ok 了呢？
private static final long serialVersionUID = 1L; 这个量的意义何在？

而且：只有 DigitButton 调用是 ok 的，剩下的AddButton、SubtractButton、MultiplyButton、ClearButton…ect 全是提示错误。OperatorButton 类被继承也是不可以的。

/** This program implements a simple four-function calculator */public class come extends Program{    /* Initializes the user interface */    public void init(){        setLayout(new TableLayout(54));        display = new CalculatorDisplay();        add(displaygridwidth=4 height="" + BUTTON_SIZE);        addButtons();        addActionListeners();    }    public void actionPerformed(ActionEvent e) {        Object source = e.getSource();        if(source instanceof CalculatorButton){            ((CalculatorButton) source).action(display);        }    }    private void addButtons(){        String constraint = ""width="" + BUTTON_SIZE + "" height="" + BUTTON_SIZE;        add(new DigitButton(7) constraint);        add(new DigitButton(8) constraint);        add(new DigitButton(9) constraint);        add(new AddButton() constraint);        add(new DigitButton(4) constraint);        add(new DigitButton(5) constraint);        add(new DigitButton(6) constraint);        add(new SubtractButton() constraint);        add(new DigitButton(1) constraint);        add(new DigitButton(2) constraint);        add(new DigitButton(3) constraint);        add(new MultiplyButton() constraint);        add(new ClearButton() constraint);        add(new DigitButton(0) constraint);        add(new EqualsButton() constraint);        add(new DivideButton() constraint);    }    private static final int BUTTON_SIZE = 40;    private CalculatorDisplay display;}class CalculatorDisplay extends IntField{    /* Creates a new calculator display that is not directly editable by the user */    public CalculatorDisplay(){        setEditable(false);        setFont(new Font(""SansSerif"" Font.PLAIN 24));        setValue(0);        startNewValue = false;        op = null;    }    /* Adds a digit to the display clearing the old value if startNewValue is set */    public void addDigit(int digit){        int value = (startNewValue) ? 0 : getValue();        setValue(10 * value + digit);        startNewValue = false;    }    /* Sets a new operator applying the previous one if one exists */    public void setOperator(OperatorButton button){        if (op == null){            memory = getValue();        } else {            memory = op.apply(memory getValue());            setValue(memory);        }        op = button;        startNewValue = true;    }    /* private instance variables */    private OperatorButton op;       /* The last operator button pressed */    private int memory;              /* The value to which the operator is applied */    private boolean startNewValue;   /* Set after an operator to start a new value */}/*   This abstract class is the superclass for every calculator button. Every button must define an action method which is called whenever the button is cliked.  */abstract class CalculatorButton extends JButton {    /* Creates a new CalculatorButton with the specified name */    public CalculatorButton(String name) {        super(name);        setFont(new Font(""SansSerif"" Font.PLAIN 24));    }    /* Called when the button is clicked (every subclass must implement this        method)     */    public abstract void action(CalculatorDisplay display);}/*   This class is used for each of the digit buttons. The action consists of    adding the digit used as a label on the button which is returned by getText.*/class DigitButton extends CalculatorButton {    /* Creates a new DigitButton for the digit n */    public DigitButton(int n) {        super("""" + n);    }    /* Adds this digit to the display */    public void action(CalculatorDisplay display) {        display.addDigit(Integer.parseInt(getText()));    }}/*   This abstract class is the superclass of the various operator buttons.   Each concrete subclass must override the apply method.*/abstract class OperatorButton extends CalculatorButton{    /* Creates a new OperatorButton with the specified name */    public OperatorButton(String name) {        super(name);    }    /* Informs the display that this operator button has been clicked */    public void action(CalculatorDisplay display) {        display.setOperator(this);    }    /* Applies this operator (every subclass must implement this method) */    public abstract int apply(int lhs int rhs);}/*   The classes AddButton SubtractButton MultipButton and DicideButton   are the same except for their label and the implementation of apply.   */class AddButton extends OperatorButton {    public AddButton() { super(""+""); }    public int apply(int lhs int rhs) { return lhs + rhs; }}class SubtractButton extends OperatorButton {    public SubtractButton() { super(""-""); }    public int apply(int lhs int rhs) { return lhs - rhs; }}class MultiplyButton extends OperatorButton {    public MultiplyButton() { super(""x""); }    public int apply(int lhs int rhs) { return lhs * rhs; }}class DivideButton extends OperatorButton {    public DivideButton() { super(""/""); }    public int apply(int lhs int rhs) { return lhs / rhs; }}/*    The EqualsButton class displays the current value. As it happens this    Operation can be implemented simply by setting the operator to null.*/class EqualsButton extends CalculatorButton {    public EqualsButton() {        super(""C"");    }    public void action(CalculatorDisplay display) {        display.setOperator(null);        display.setValue(0);    }}public class ClearButton extends CalculatorButton {    public ClearButton(){        super(""C"");    }    public void action(CalculatorDisplay display) {        display.setOperator(null);        display.setValue(0);    }}

解决方案

public class ClearButton extends CalculatorButton
这里不是还有一个public类