如何将 ArrayList 类型的类元素传递给方法

Question

如何将 ArrayList 类型的元素传递给方法？我的天真尝试如下。

public double example( ArrayList<CMyType> list, String type ){

    double out = list.get(0).type // type should be a placeholder for one of the variables which I defined in CMyType
    return out;

}

public class CMyType {
    public double var1;
    public double var2;
}

Answer 1

你在这里尝试做的事情：

double out = list.get(0).type // type should be a placeholder for one of 

不使用反射是不可能的，像这样：

public double example( ArrayList<CMyType> list, String type ) {
  CMyClass obj = list.get(0);
  Field field = obj.getClass().getDeclaredField(type);
  Object objOut = field.get(obj);
  // you could check for null just in case here
  double out = (Double) objOut;
  return out;
}

您还可以考虑将您的 CMyType 类修改为如下所示：

class CMyType {
  private double var1;
  private double var2;

  public double get(String type) {
    if ( type.equals("var1") ) {
      return var1;
    } 
    if ( type.equals("var2") ) {
      return var2;
    }

    throw new IllegalArgumentException();
}

然后像这样从您的代码中调用它：

public double example( ArrayList<CMyType> list, String type ) {
  CMyClass myobj = list.get(0);
  return myobj.get(type);
}

更好的解决方案是像这样在CMyType 中使用Map<String, Double>：

class CMyType {
  private Map<String, Double> vars = new HashMap();

  public CMyType() {
    vars.put("var1", 0.0);
    vars.put("var2", 0.0);
  }

  public double get(String type) {
    Double res = vars.get(type);
    if ( res == null ) throw new IllegalArgumentException();
    return res;
}

Answer 2

为什么不直接

public double example( ArrayList<CMyType> list, String type ){
    double out = list.get(0).type // type should be a placeholder for one of the variables which I defined in CMyType
    return out;
}

public class CMyType {
    public double var1;
    public double var2;
}

public invocationTest() {
   ArrayList<CMyType> arrayList = new ArrayList<CMyType>(); //or populate it somehow
   return myExample(arrayList.get(0));
}

public double myExample( CMyType member, String type ){
    double out = member.type;
    return out;
}