如何通过字段名获取字段的值？[副本]

Question

我试图制作一个方法，获取封闭实例中的任何字段，对于一对2D坐标（其字段名以“X”结尾）的任何“X”组件具有整数值。

    public int getX(String object) throws NullPointerException{
        String objectX = object + "X";
        Component c = null;
        try{
        Integer x = new Integer(object);
        x = c.getX();
        if(object != null){
            return x;
        }
        }catch (NullPointerException e){} // if the referenced object does not exist.
        System.err.println("The object "+object+"does not exist or the name was spelled incorrectly.");
        return (Integer)null;
    }

但我在编译器中得到的只是：

Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: For input     string: "append"
    at java.lang.NumberFormatException.forInputString(Unknown Source)
    at java.lang.Integer.parseInt(Unknown Source)
    at java.lang.Integer.<init>(Unknown Source)
    at PlazmaBurst2.Body.getX(Body.java:118)
    at PlazmaBurst2.Body.append(Body.java:79)
    at PlazmaBurst2.Body.<init>(Body.java:53)
    at PlazmaBurst2.PB2Main$DrawingCanvas.paintComponent(PB2Main.java:40)
    at javax.swing.JComponent.paint(Unknown Source)
    at javax.swing.JComponent.paintChildren(Unknown Source)
    at javax.swing.JComponent.paint(Unknown Source)
    at javax.swing.JLayeredPane.paint(Unknown Source)
    at javax.swing.JComponent.paintChildren(Unknown Source)
    at javax.swing.JComponent.paintToOffscreen(Unknown Source)
    at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(Unknown Source)
    at javax.swing.RepaintManager$PaintManager.paint(Unknown Source)
    at javax.swing.RepaintManager.paint(Unknown Source)
    at javax.swing.JComponent.paint(Unknown Source)
    at java.awt.GraphicsCallback$PaintCallback.run(Unknown Source)
    at sun.awt.SunGraphicsCallback.runOneComponent(Unknown Source)
    at sun.awt.SunGraphicsCallback.runComponents(Unknown Source)
    at java.awt.Container.paint(Unknown Source)
    at java.awt.Window.paint(Unknown Source)
    at javax.swing.RepaintManager$3.run(Unknown Source)
    at javax.swing.RepaintManager$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
    at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
    at javax.swing.RepaintManager.prePaintDirtyRegions(Unknown Source)
    at javax.swing.RepaintManager.access$1000(Unknown Source)
    at javax.swing.RepaintManager$ProcessingRunnable.run(Unknown Source)
    at java.awt.event.InvocationEvent.dispatch(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$200(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)    

方法将这样使用：

Image img = setImage("myImage.png");
int imgX = getX(img);// use of getX(String)

我做错了什么？

Answer 1

似乎您正在将该类中对象的X值的值存储为字段，并且您正试图创建一个查询该对象的X值的方法。

问题在于，您正在将对象的标识符传递给getX（String），而实际上它需要一个字符串（字段的名称？）。如果要获取字段值，可以使用反射按名称获取字段值：

public int getX(String object) {
    try {
        return Plasmaburst2.Body.class.getDeclaredField(object + "X");
    } catch(NoSuchFieldException | SecurityException e) {
        return 0;
    }
}

但是，出于性能原因，可以考虑使用哈希映射而不是反射，或者只为自己的类存储对象的坐标。

Answer 2

也许问题出在物体上！它是一个包含整数的字符串。尝试运行捕获NumberFormatException的代码

  public int getX(String object) throws NullPointerException{
    String objectX = object + "X";
    Component c = null;
    try{
        try{
            Integer x = new Integer(object);
        } catch(NumberFormatException ex){
            System.out.println("You were trying to convert <" + object "> to Integer but it's impossible");
        }

        // The above line c was initiated to null so I delete this line for thest x = c.getX();
        x = (int)(Math.random() * 100) ;
        if(object != null){
            return x;
        }
    }catch (NullPointerException e){} // if the referenced object does not exist.

    System.err.println("The object "+object+"does not exist or the name was spelled incorrectly.");
    return (Integer)null;
}

Answer 3

您似乎认为将对象传递给字符串参数就像参数传递对象的标识符一样。

换句话说，如果你这样做：

public static void printObject(String ostring) {
    System.out.println(ostring);
}

Object object = new Object();
printObject(object);

然后，您认为这将打印“object”，而实际上，您的IDE会给您一个警告，因为它不可编译，或者（可能）静默地更改代码以调用对象的toString方法。

你想要这样做的方式可能是通过一些被黑客攻击的反射来获得任何对象的场”...X"但不要那样做。它被黑了。

相反，如果你真的想做一些像这样的“通用”的事情，你可以使用你的类实现的接口。接口是为这类事情设计的。

// not to be confused with java.awt.Point
public interface Point {
    public Point getPoint();
    public int getX();
    public int getY();
}

任何类都可以实现接口，方法返回的内容取决于该实现。您可以对实现点的任何类调用getX，由该类决定要采取的适当操作。接口的实现类型也是不可知的。

// redundant but shows the point
public static int getXFromPoint(Point pt) {
    return pt.getX();
}

class PointImplementation implements Point {
    private Component c = somewhereElse.getAComponent();

    @Override public int getX() {
        return c.getX();
    }
    @Override public int getY() {
        return c.getY();
    }
    @Override public Point getPoint() {
        return c.getLocation();
    }
}

System.out.println(getXFromPoint(new PointImplementation()));

不确定您实际上想要从映像返回什么（java.awt.Image？）。为什么不直接调用适当的方法呢？

如果您真的希望以您想要的方式（从字符串访问字段？）执行此操作，Java反射教程介绍了以下内容：http://docs.oracle.com/javase/tutorial/reflect/member/field.html