The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
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
class illustrates how to set the values for long, array, and enum field types. Methods for getting and setting other primitive types are described in
Book
Field
.
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
int x = 1; x = 2; x = 3;
Equivalent code using Field.set*()
may not.