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Given an instance of a class, it is possible to use reflection to set the values of fields in that class. This is typically done only in special circumstances when setting the values in the usual way is not possible. Because such access usually violates the design intentions of the class, it should be used with the utmost discretion.
The
BookField.
import java.lang.reflect.Field;
import java.util.Arrays;
import static java.lang.System.out;
enum Tweedle { DEE, DUM }
public class Book {
public long chapters = 0;
public String[] characters = { "Alice", "White Rabbit" };
public Tweedle twin = Tweedle.DEE;
public static void main(String... args) {
Book book = new Book();
String fmt = "%6S: %-12s = %s%n";
try {
Class<?> c = book.getClass();
Field chap = c.getDeclaredField("chapters");
out.format(fmt, "before", "chapters", book.chapters);
chap.setLong(book, 12);
out.format(fmt, "after", "chapters", chap.getLong(book));
Field chars = c.getDeclaredField("characters");
out.format(fmt, "before", "characters",
Arrays.asList(book.characters));
String[] newChars = { "Queen", "King" };
chars.set(book, newChars);
out.format(fmt, "after", "characters",
Arrays.asList(book.characters));
Field t = c.getDeclaredField("twin");
out.format(fmt, "before", "twin", book.twin);
t.set(book, Tweedle.DUM);
out.format(fmt, "after", "twin", t.get(book));
// production code should handle these exceptions more gracefully
} catch (NoSuchFieldException x) {
x.printStackTrace();
} catch (IllegalAccessException x) {
x.printStackTrace();
}
}
}
This is the corresponding output:
$ java Book BEFORE: chapters = 0 AFTER: chapters = 12 BEFORE: characters = [Alice, White Rabbit] AFTER: characters = [Queen, King] BEFORE: twin = DEE AFTER: twin = DUM
Use of reflection can cause some runtime optimizations to be lost. For example, the following code is highly likely be optimized by a Java virtual machine:
int x = 1; x = 2; x = 3;
Field.set*() may not.
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