有没有一种方法可以将String对象分配为自定义类实例？

Question

Tried to extend from String class but it is final, hence looking for a way so that I can assign a String object as my custom class instance?

Example:

public class MyClass {
    public CustomString cString;
}

MyClass myClass = new MyClass();
Field field = myClass.getField("cString");
field.set(myClass, "TestValue");

Answer 1

If you must do this with reflection, how about using methods instead of fields. Specifically, try using a setter convention like so:

class MyClass {
    private CustomString customString = new CustomString();

    public CustomString getCustomString() {
        return customString;
    }

    public String getCustomStringValue() {
        return customString.getValue();
    }

    public void setCustomString(String customString) {
        this.customString.setValue(customString);
    }
}

class CustomString {
    private String value;

    public void setValue(String value) {
        this.value = value;
    }

    public String getValue() {
        return value;
    }
}

public static void main(String[] args) throws InvocationTargetException, IllegalAccessException {
    String fieldName = "customstring";
    MyClass myClass = new MyClass();
    Method[] methods = myClass.getClass().getMethods();
    for(Method method : methods) {
        String name = method.getName();
        if (name.equalsIgnoreCase("set" + fieldName) {
            method.invoke(myClass, "Hello");
        }
    }
    System.out.println(myClass.getCustomStringValue());
}

This way you can still use the field names but allow the field to be set by a method which can be arbitrarily interesting.

Answer 2

You can set the field with a little encapsulation. If the field's type is CustomString , we transform the value to CustomString object. Here is the complete codde:

import java.lang.reflect.Field;

public class Q47027440 {

  public static void main(String[] args) throws Exception {
    MyClass myClass = new MyClass();
    Field field = MyClass.class.getField("str");
    set(field, myClass, "TestValue");
    System.out.println(myClass.str.actual);
  }

  public static void set(Field field, Object obj, Object value)
      throws InstantiationException, IllegalAccessException {
    if (field.getType() == CustomString.class && value instanceof String) {
      CustomString cs = CustomString.class.newInstance();
      cs.actual = (String) value;
      value = cs;
    }
    field.set(obj, value);
  }

  public static class CustomString {
    public String actual;
  }

  public static class MyClass {
    public CustomString str;
  }
}