Question
Answer and Explanation
You might encounter issues when trying to set a java.lang.Integer field using reflection in Java due to the fact that Integer objects are immutable. Here's why, along with explanations and potential workarounds:
1. Immutability of Integer:
- java.lang.Integer is an immutable class. Once an Integer object is created, its value cannot be changed. The internal int value wrapped by the Integer cannot be directly modified.
2. Example of Attempting to Set the Integer Field:
- If you have code like this:
class MyClass {
private Integer myInteger = 5;
public Integer getMyInteger() {
return myInteger;
}
}
public class Main {
public static void main(String[] args) throws Exception {
MyClass obj = new MyClass();
Field field = MyClass.class.getDeclaredField("myInteger");
field.setAccessible(true);
field.set(obj, 10); // This might not behave as expected
System.out.println(obj.getMyInteger());
}
}
- The line field.set(obj, 10); will not directly modify the original Integer object. Instead, it will either replace the reference with a new Integer object (if the field is not final) or throw an exception if the field is final.
3. Why it Appears to Work (Sometimes):
- If the field is not final, the reflection will replace the reference to the old Integer object with a reference to a new Integer object with the value 10. The original Integer object remains unchanged, but the field now points to a different Integer object.
4. When it Won't Work:
- If the field is final, attempting to set it via reflection will typically result in an IllegalAccessException or a similar exception, depending on the Java version and security settings, because final fields are meant to be unchangeable after initialization.
5. Workarounds and Proper Approaches:
- Create a New Object: The correct way to "change" the value is to replace the Integer object with a new one. This ensures immutability is maintained.
- Mutable Wrapper (Not Recommended): You could create a mutable wrapper class that holds an Integer (or int). However, this is generally not recommended because it defeats the purpose of using Integer in the first place and can lead to confusing code.
6. Example of Correct Approach:
class MyClass {
private Integer myInteger = 5;
public Integer getMyInteger() {
return myInteger;
}
public void setMyInteger(Integer value) {
this.myInteger = value;
}
}
public class Main {
public static void main(String[] args) throws Exception {
MyClass obj = new MyClass();
Field field = MyClass.class.getDeclaredField("myInteger");
field.setAccessible(true);
field.set(obj, new Integer(10)); // Replace with a new Integer object
System.out.println(obj.getMyInteger());
}
}
- In this example, we are replacing the reference to the Integer object with a new Integer object, which is the correct way to handle immutability.
In summary, you cannot directly modify the value of an existing java.lang.Integer object using reflection because Integer is immutable. The only safe approach is to replace the Integer reference with a new Integer object.