Java 从 long 到 int 的可能有损转换？
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possible lossy conversion from long to int?
提问by Daniel Lukish
I have this program that is pretty much a calculator but with a moving JLabel that is supposed to change colors every time you click the label, but i have 3 errors at the very bottom of the code that i have marked with a comment. all three are: error: incompatible types: possible lossy conversion from long to int
我有这个程序，它几乎是一个计算器，但是有一个移动的 JLabel，每次单击标签时它都会改变颜色，但是我在代码的最底部有 3 个错误，我用注释标记了这些错误。所有三个都是：错误：不兼容的类型：从 long 到 int 的可能有损转换
public class TestCalculator {
private ResultPane resultPane;
public static void main(String[] args) {
new TestCalculator();
}
public TestCalculator() {
EventQueue.invokeLater(new Runnable() {
@Override
public void run() {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
resultPane = new ResultPane();
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
}
JFrame frame = new JFrame("Testing");
frame.setGlassPane(resultPane);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setLayout(new BorderLayout());
frame.add(new CalculatorPane(resultPane));
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
frame.setBounds(200,200,500,400);
}
});
}
public class ResultPane extends JPanel {
private JLabel result;
private Timer timer;
private int xDelta = (Math.random() > 0.5) ? 1 : -1;
private int yDelta = (Math.random() > 0.5) ? 1 : -1;
public void setLabelForeground(Color color) {
result.setForeground(color);
}
public ResultPane() {
setOpaque(false);
setLayout(null);
result = new JLabel();
Font font = result.getFont();
font = font.deriveFont(Font.BOLD, 26f);
result.setFont(font);
add(result);
HandlerClass handler = new HandlerClass();
result.addMouseListener(handler);
result.addMouseMotionListener(handler);
timer = new Timer(40, new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
Point point = result.getLocation();
point.x += xDelta;
point.y += yDelta;
if (point.x < 0) {
point.x = 0;
xDelta *= -1;
} else if (point.x + result.getWidth() > getWidth()) {
point.x = getWidth() - result.getWidth();
xDelta *= -1;
}
if (point.y < 0) {
point.y = 0;
yDelta *= -1;
} else if (point.y + result.getHeight() > getHeight()) {
point.y = getHeight() - result.getHeight();
yDelta *= -1;
}
result.setLocation(point);
repaint();
}
});
timer.start();
}
public void setResult(Number number) {
result.setText(NumberFormat.getNumberInstance().format(number));
result.setSize(result.getPreferredSize());
setVisible(true);
}
public String getResultText() {
return result.getText();
}
}
public class CalculatorPane extends JPanel {
private final ResultPane resultPane;
private final JLabel firstNumberLabel = new JLabel("First Number:");
private final JLabel secondNumberLabel = new JLabel("Second Number:");
private final JTextField firstNumberField = new JTextField(5);
private final JTextField secondNumberField = new JTextField(5);
public double result = 0.0;
public CalculatorPane(ResultPane resultPane) {
setLayout(new GridBagLayout());
GridBagConstraints gbc = new GridBagConstraints();
gbc.gridwidth = GridBagConstraints.REMAINDER;
this.resultPane = resultPane;
JPanel fields = new JPanel();
fields.add(firstNumberLabel);
fields.add(firstNumberField);
fields.add(secondNumberLabel);
fields.add(secondNumberField);
add(fields, gbc);
JPanel buttons = new JPanel();
buttons.add(new JButton(new AddAction()));
buttons.add(new JButton(new SubtractAction()));
buttons.add(new JButton(new MultiplyAction()));
buttons.add(new JButton(new DivideAction()));
add(buttons, gbc);
}
public class AddAction extends AbstractAction {
public AddAction() {
putValue(NAME, "+");
}
@Override
public void actionPerformed(ActionEvent e) {
try {
double num1 = Double.parseDouble(firstNumberField.getText());
double num2 = Double.parseDouble(secondNumberField.getText());
double result = num1 + num2;
resultPane.setResult(result);
String resultText = resultPane.getResultText();
firstNumberField.setText(resultText);
} catch (NumberFormatException exp) {
}
}
}
public class SubtractAction extends AbstractAction {
public SubtractAction() {
putValue(NAME, "-");
}
@Override
public void actionPerformed(ActionEvent e) {
try {
double num1 = Double.parseDouble(firstNumberField.getText());
double num2 = Double.parseDouble(secondNumberField.getText());
double result = num1 - num2;
resultPane.setResult(result);
String resultText = resultPane.getResultText();
firstNumberField.setText(resultText);
} catch (NumberFormatException exp) {
}
}
}
public class MultiplyAction extends AbstractAction {
public MultiplyAction() {
putValue(NAME, "x");
}
@Override
public void actionPerformed(ActionEvent e) {
try {
double num1 = Double.parseDouble(firstNumberField.getText());
double num2 = Double.parseDouble(secondNumberField.getText());
double result = num1 * num2;
resultPane.setResult(result);
String resultText = resultPane.getResultText();
firstNumberField.setText(resultText);
} catch (NumberFormatException exp) {
}
}
}
public class DivideAction extends AbstractAction {
public DivideAction() {
putValue(NAME, "/");
}
@Override
public void actionPerformed(ActionEvent e) {
try {
double num1 = Double.parseDouble(firstNumberField.getText());
double num2 = Double.parseDouble(secondNumberField.getText());
double result = num1 / num2;
resultPane.setResult(result);
String resultText = resultPane.getResultText();
firstNumberField.setText(resultText);
} catch (NumberFormatException exp) {
}
}
}
}
private class HandlerClass implements MouseListener, MouseMotionListener {
public void mouseClicked(MouseEvent event) {
int r = Math.round(Math.random() * 255);//error here
int g = Math.round(Math.random() * 255);//here
int b = Math.round(Math.random() * 255);//and here
Color col = new Color(r, g, b);
resultPane.setLabelForeground(col);
}
public void mouseReleased(MouseEvent event) {
}
public void mousePressed(MouseEvent event) {
}
public void mouseExited(MouseEvent event) {
}
public void mouseEntered(MouseEvent event) {
}
public void mouseMoved(MouseEvent event) {
}
public void mouseDragged(MouseEvent event){
}
}
}
回答by rgettman
You are trying to have the compiler provide an implicit primitive narrowing conversion, by assigning a long(the result of Math.round) to an int. It cannot be done implicitly; do it explicitly with a cast.
您试图让编译器通过将 a long（的结果Math.round）分配给int. 它不能隐式地完成；用演员表明确地做到这一点。
int r = (int) Math.round(Math.random() * 255);
回答by awksp
That's because a longis 64 bits and an intis 32 bits, not to mention you're going from floating-point to integer. To go from longto int, you're going to have to discard some information, and the compiler can't/won't do that automatically. You're going to have to explicitly say so through a cast:
那是因为 along是 64 位而 anint是 32 位，更不用说您从浮点数到整数了。要从long到int，您将不得不丢弃一些信息，并且编译器不能/不会自动执行此操作。您将不得不通过演员表明确表示：
int g = (int) Math.round(Math.random() * 255);
^^^^^
Alternatively, you can use java.util.Random:
或者，您可以使用java.util.Random：
Random r = new Random();
int g = r.nextInt(256);
回答by Eric J.
Math.random() gives you a long, which can far exceed the range of values that an integer can represent. You then assign that value to an integer.
Math.random() 为您提供一个 long，它可能远远超出整数可以表示的值范围。然后将该值赋给一个整数。
That is what the compiler is warning you about.
这就是编译器警告你的。
http://docs.oracle.com/javase/7/docs/api/java/lang/Math.html
http://docs.oracle.com/javase/7/docs/api/java/lang/Math.html
