java - System.out.println(NOT_AN_EMPTY_STRING);打印出一个空字符串(也就是什么也没有)

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好吧，让我解释一下我的问题：

码：

System.out.println("Message is:" + focusOwner + focusOwner.getPage() == null + focusOwner.getText());

输出：

false

备用代码：

System.out.println("Message is:" + focusOwner);

输出：

Message is:pscript.gui.elements.Answer[,0,98,639x22,invalid,layout=net.miginfocom.swing.MigLayout,alignmentX=0.0,alignmentY=0.0,border=javax.swing.plaf.synth.SynthBorder@677b15f3,flags=9,maximumSize=,minimumSize=,preferredSize=]

一些事实：

focusOwner是扩展Answer的类Answer，它扩展了JPanel。 toString（）方法永远不会被覆盖。证明：

public class Answer extends Element
{
    private static final long serialVersionUID = 1L;
    private JLabel letter;
    private JCheckBox check;
    private Question question;

    //getter and setters
    void setQuestion(Question question)
    {
        this.question = question;
    }

    public Question getQuestion()
    {
        return question;
    }

    JLabel getLetter()
    {
        return letter;
    }

    public boolean isCorrect()
    {
        return check.isSelected();
    }

    public String getLetterText()
    {
        return letter.getText();
    }

    //navigational methods
    @Override
    public Element getPreviousElement()
    {
        Element previousElement = getQuestion().getPreviousAnswer((Answer) getElement());

        if (previousElement != null)
        {
            return previousElement;
        }

        else
        {
            return getQuestion();
        }
    }

    @Override
    public Element getNextElement()
    {
        Element nextElement = getQuestion().getNextAnswer((Answer) getElement());

        if (nextElement != null)
        {
            return nextElement;
        }

        else
        {
            nextElement = getPage().getNextElement(getQuestion());

            if (nextElement != null)
            {
                return nextElement;
            }

            else
            {
                Page nextPage = getChapter().getNextPage(getPage());

                if (nextPage != null)
                {
                    return nextPage.getLastElement().lastElement();
                }

                else
                {
                    Chapter nextChapter = getDocument().getNextChapter(getChapter());

                    if (nextChapter != null)
                    {
                        return nextChapter.getTitle();
                    }

                    else
                    {
                        return getDocument().getChapter(1).getTitle();
                    }
                }
            }
        }
    }

    //other methods
    private Element addNextElement()
    {
        if (getQuestion().getAnswerCount() < 5)
        {
            return addNextAnswer();
        }

        else
        {
            return addNextQuestion();
        }
    }

    private Answer addNextAnswer()
    {
        Answer nextAnswer = new Answer();

        getQuestion().addAnswer(nextAnswer, getQuestion().getAnswerIndex((Answer) getElement())+1);
        getQuestion().rewriteLetters();

        return nextAnswer;
    }

    private Question addNextQuestion()
    {
        Question nextQuestion = new Question();

        getPage().addElement(nextQuestion, getPage().getElementIndex(getQuestion())+1);
        getChapter().rewriteNumbers();

        return nextQuestion;
    }

    private Element removeAnswer()
    {
        Element previousElement = getPreviousElement();

        Question question = getQuestion();

        question.removeAnswer((Answer)getElement());
        question.rewriteLetters();

        return previousElement;
    }

    private void checkLoop()
    {
        Question question = getQuestion();

        if (question != null)
        {
            getDocument().setSaved(false);

            if ((check.isSelected()) && (question.checkFlag == false))
            {
                question.checkFlag = true;
                int answerCount = question.getAnswerCount();

                for (int i = 1; i <= answerCount; i++)
                {
                    question.getAnswer(i).check.setSelected(false);
                }

                check.setSelected(true);
                question.checkFlag = false;
            }
        }
    }

    //actions
    @Override
    void primaryAction()
    {
        addNextElement().takeFocus();
    }

    @Override
    void secondaryAction()
    {
        addNextQuestion().takeFocus();
    }

    @Override
    void deleteAction()
    {
        removeAnswer().takeFocus();
    }

    //constructors
    Answer()
    {
        super();

        setBackground(Color.WHITE);

        //TODO
        check = new JCheckBox();
        check.setBackground(Color.WHITE);

        check.addItemListener(new ItemListener()
        {
            @Override
            public void itemStateChanged(ItemEvent arg0)
            {
                checkLoop();
            }
        });

        add(check, "top, gapleft " + LayoutConstants.CHECK_SPACE_LEFT + ", gaptop "+ LayoutConstants.CHECK_ABOVE + ", w " + LayoutConstants.CHECK_WIDTH);

        letter = new JLabel("a)", SwingConstants.LEFT);
        letter.setFont(Main.otherFont);
        letter.setBackground(Color.WHITE);
        add(letter, "top, gaptop " + LayoutConstants.LETTER_ABOVE + ", w " + LayoutConstants.LETTER_WIDTH);

        getTextBox().setMargin(LayoutConstants.ANSWER_INSETS);
        add(getTextBox(), "span, grow, push, w " + LayoutConstants.ANSWERBOX_WIDTH);
    }

    public Answer(String text, boolean correct)
    {
        this();
        getTextBox().setText(text);
        check.setSelected(correct);
    }
}

元件：

public class Element extends JPanel
{
    private static final long serialVersionUID = 1L;
    private JTextArea textBox = new JTextArea();
    private Page page = null;
    private InputMap inputM = getTextBox().getInputMap();
    private ActionMap actionM = getTextBox().getActionMap();

    //getter and setters
    Element getElement()
    {
        return this;
    }

    void setPage(Page page)
    {
        this.page = page;
    }

    public Page getPage()
    {
        return page;
    }

    Chapter getChapter()
    {
        return getPage().getChapter();
    }

    Document getDocument()
    {
        return getChapter().getDocument();
    }

    public JTextArea getTextBox()
    {
        return textBox;
    }

    public String getText()
    {
        return getTextBox().getText();
    }

    public void setText(String text)
    {
        getTextBox().setText(text);
    }

    public boolean isEmpty()
    {
        return getTextBox().getText().equals("");
    }

    //other methods
    public void takeFocus()
    {
        getDocument().setFocusOwner(getElement());
        scrollRectToVisible(getTextBox().getBounds());
        getTextBox().grabFocus();
    }

    void addListener()
    {
        AbstractDocument document = (AbstractDocument) getTextBox().getDocument();
        document.addDocumentListener(new DocumentListener()
        {
            @Override
            public void changedUpdate(DocumentEvent e)
            {
                getDocument().setSaved(false);
            }

            @Override
            public void insertUpdate(DocumentEvent e)
            {
                getDocument().setSaved(false);
            }

            @Override
            public void removeUpdate(DocumentEvent e)
            {
                getDocument().setSaved(false);
            }
        });
    }

    Element lastElement()
    {
        if (getElement() instanceof Question)
        {
            if (((Question) getElement()).getAnswerCount() != 0)
            {
                return ((Question) getElement()).getLastAnswer();
            }
        }

        return getElement();
    }

    //navigational methods
    public Element getPreviousElement()
    {
        return null;
    }

    public Element getNextElement()
    {
        return null;
    }

    //actions
    void primaryAction() {}

    void secondaryAction() {}

    private void tertiaryAction()
    {
        Chapter chapter = getChapter();
        int chapterIndex = getDocument().getChapterIndex(chapter);

        new CreateChapterDialog(getDocument(), chapterIndex+1); //TOD quaternary maybe??

        getDocument().setKeyPressed(false);
    }

    void deleteAction() {}

    private void upAction()
    {
        getPreviousElement().takeFocus();
    }

    private void downAction()
    {
        getNextElement().takeFocus();
    }

    //constructors
    Element()
    {
        setLayout(new MigLayout("insets 0, gap 0!"));

        getTextBox().setLineWrap(true);
        getTextBox().setWrapStyleWord(true);

        getTextBox().addKeyListener(new KeyAdapter()
        {
            @Override
            public void keyReleased(KeyEvent e)
            {
                getDocument().setKeyPressed(false);
            }
        });

        CustomDocumentFilter filter;

        if (this instanceof Title)
        {
            getTextBox().setFont(Main.titleFont);
            filter = new CustomDocumentFilter(250);
        }

        else if (this instanceof Question)
        {
            getTextBox().setFont(Main.questionFont);
            filter = new CustomDocumentFilter(500);
        }

        else
        {
            getTextBox().setFont(Main.otherFont);
            filter = new CustomDocumentFilter(500);
        }

        AbstractDocument document = (AbstractDocument) getTextBox().getDocument();
        document.setDocumentFilter(filter);

        //primary action
        inputM.put(KeyStroke.getKeyStroke("ENTER"), "enter");
        actionM.put("enter", new AbstractAction()
        {
            private static final long serialVersionUID = 1L;

            @Override
            public void actionPerformed(ActionEvent arg0)
            {
                if (!getDocument().isKeyPressed())
                {
                    getDocument().setKeyPressed(true);
                    getDocument().setSaved(false);
                    primaryAction();
                }
            }
        });

        //secondary action
        inputM.put(KeyStroke.getKeyStroke("shift ENTER"), "pressedShiftEnter");
        actionM.put("pressedShiftEnter", new AbstractAction()
        {
            private static final long serialVersionUID = 1L;

            @Override
            public void actionPerformed(ActionEvent e)
            {
                if (!getDocument().isKeyPressed())
                {
                    getDocument().setKeyPressed(true);
                    getDocument().setSaved(false);
                    secondaryAction();
                }
            }
        });

        //tertiary action
        inputM.put(KeyStroke.getKeyStroke("control ENTER"), "pressedCtrlEnter");
        actionM.put("pressedCtrlEnter", new AbstractAction()
        {
            private static final long serialVersionUID = 1L;

            @Override
            public void actionPerformed(ActionEvent e)
            {
                if (!getDocument().isKeyPressed())
                {
                    getDocument().setKeyPressed(true);
                    getDocument().setSaved(false);
                    tertiaryAction();
                }
            }
        });

        //up action
        inputM.put(KeyStroke.getKeyStroke("shift TAB"), "shiftTab");
        actionM.put("shiftTab", new AbstractAction()
        {
            private static final long serialVersionUID = 1L;

            @Override
            public void actionPerformed(ActionEvent e)
            {
                upAction();
            }
        });

        //down action
        inputM.put(KeyStroke.getKeyStroke("TAB"), "Tab");
        actionM.put("Tab", new AbstractAction()
        {
            private static final long serialVersionUID = 1L;

            @Override
            public void actionPerformed(ActionEvent e)
            {
                downAction();
            }
        });

        //delete action
        inputM.put(KeyStroke.getKeyStroke("shift DELETE"), "shiftDelete");
        actionM.put("shiftDelete", new AbstractAction()
        {
            private static final long serialVersionUID = 1L;

            @Override
            public void actionPerformed(ActionEvent e)
            {
                if (!getDocument().isKeyPressed())
                {
                    getDocument().setKeyPressed(true);
                    getDocument().setSaved(false);
                    deleteAction();
                }
            }
        });

        getTextBox().addFocusListener(new FocusAdapter()
        {
            @Override
            public void focusGained(FocusEvent e)
            {
                getDocument().setFocusOwner(getElement());
            }
        });
    }
}

focusOwner.getPage（）返回其私有变量之一，该私有变量为null或不为null（在这种情况下不是）。

focusOwner.getText（）调用类中包含的JTextArea的getText（）方法并返回其返回值（在这种情况下，它实际上是一个空字符串）。

那里。这是我可以给您的最接近SSCCE的东西。

编辑：

您如何解释这一点：

System.out.println("Message is:" + focusOwner + focusOwner.getPage() == null + focusOwner.getText());
System.out.println("Message is:" + focusOwner);

返回此：

false
Message is:pscript.gui.elements.Answer[,0,98,639x22,invalid,layout=net.miginfocom.swing.MigLayout,alignmentX=0.0,alignmentY=0.0,border=javax.swing.plaf.synth.SynthBorder@1b2a8c70,flags=9,maximumSize=,minimumSize=,preferredSize=]

我们正在谈论的是在两个连续的代码行中打印出SAME变量的值，并且它没有相同的值。

最佳答案

假设问题是为什么

System.out.println("Message is:" + focusOwner + focusOwner.getPage() == null + focusOwner.getText());

打印false而不是在另一种情况下得到的长字符串：

+运算符的优先级高于==运算符。因此，您要打印的是：

("Message is:" + focusOwner + focusOwner.getPage()) == (null + focusOwner.getText())

或两个String对象之间相等性比较的结果。显然，双方并不平等。

你想要的是

System.out.println("Message is:" + focusOwner + (focusOwner.getPage() == null) + focusOwner.getText());