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[英]Is it a must to pass a parameter to a set method? In this case setting the value is depending on another field in the same object
[英]Setting a field value of an object that is a child of another object
我收到以下异常:
java.lang.IllegalArgumentException:对象不是该类的实例
我对为什么得到它有一个很好的想法-但我一直坚持寻找解决方案。
这是示例代码来解释我要做什么:
public class Car {
private Owner owner;
//Constructor and getter for owner field
}
public class Owner {
private String name;
//Constructor and getter for name field
}
现在,我想做的是:从Car类开始,检索名为“ owner”的字段-然后检索该对象所有者的字段“ name”,该对象所有者是汽车的一部分。 现在,我想读取(然后进行修改,但是我已经无法读取)字段“名称”的值。 问题:我只有car的实例,但没有其“ owner”字段的实例。 为了澄清起见:所描述的过程将通用地工作-所以我将不知道获取器,设置器,字段等的名称。
这是失败的代码(请让我知道是否需要更多代码来澄清问题):
Car car = new Car();
//Set owner and name property of owner for car
...
Field nameField = resolveDatapath(datapathToFeature, car);
nameField.setAccessible(true);
//Here I fail with the afore mentioned IllegalArgumentException, because
//indeed, I am passing car - not its sub-object "owner" (because as I can see it,
//there is no way for me to generically retrieve the owner object - or is there?)
String value = nameField.get(car).toString();
以上内容也适用于任何“深度”(例如,我想要一个字段,并从对象c的对象c操纵它,而对象c的对象又是对象a的字段...)
请让我知道,如果我可以进一步澄清问题-这是检索字段的代码(在上面的示例中为nameField):
private Field resolveDatapath(String path, Object parent) {
String subString = path;
if (!subString.contains("."))
{
//We haven reached the end of the path
return getField(subString, parent.getClass());
}
//We haven't reached the end of the
subString = path.substring(0, path.indexOf("."));
Field field = getField(subString, parent.getClass());
return resolveDatapath(path.substring(path.indexOf(".")+1), field);
}
private Field getField(String name, Class<?> parent) {
Field [] fields = parent.getDeclaredFields();
for(Field f : fields)
{
String current = f.getName();
if(current.equalsIgnoreCase(name))
{
try {
return parent.getDeclaredField(current);
} catch (SecurityException e) {
fail("Not allowed to access field - " + current);
} catch (NoSuchFieldException e) {
fail("No such field exists - " + current);
}
}
}
return null;
}
提前非常感谢您,Ready4Android
该字段只会为您提供给定班级的字段。 它不会导航到类并获取子字段。
我建议您改用像这样的普通获取字段助手
String name = (String) getField(car, datapathToFeature);
好的,感谢Peter为我的大脑拨开了所需的戳戳;)我找到了解决方案,这里是:
我创建一个DatapathObject,其中包含字段和父对象作为对象。 在进行比较之前,我无法获得Owner对象的实例,该对象是Car对象中的一个字段。 当然,就像我现在想的那样,这完全是胡说八道-我的大脑在这里有点慢:P。
这段代码可以正常工作并通过我的单元测试:
public void testDatapathResolution() throws Exception{
String[] path = datapathToFeature.split("\\.");
assertEquals("Owner", path[0]);
assertEquals("Name", path[1]);
DatapathObject result = resolveDatapath(datapathToFeature, car);
result.getField().setAccessible(true);
Object value = result.getField().get(result.getParent());
assertEquals(car.getOwner().getName(), value.toString());
}
这是DatapathObject类:
public class DatapathObject {
private Object parent;
private Field field;
public DatapathObject(Object parent, Field field) {
this.parent = parent;
this.field = field;
}
public Object getParent() {
return parent;
}
public Field getField() {
return field;
}
}
我的resolveDatapath方法的内容如下所示(以适应DatapathObject的引入:
private DatapathObject resolveDatapath(String path, Object parent) throws
IllegalArgumentException, IllegalAccessException
{
String subString = path;
if (!subString.contains("."))
{
//We haven reached the end of the path
Field field = getField(subString, parent.getClass());
return new DatapathObject(parent, field);
}
//We haven't reached the end of the
subString = path.substring(0, path.indexOf("."));
Field field = getField(subString, parent.getClass());
field.setAccessible(true);
return resolveDatapath(path.substring(path.indexOf(".")+1),
field.get(parent));
}
再次感谢Peter-lol,我仍然不明白您的答案,我也不认为这是我想要的,但是它为我的大脑提供了寻找答案的失落之星:)!
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