How to Use Reflection to Access Fields of a Class in Java?

The following example shows how to get and set fields' value in a class:

Tweedle.java,

package com.xyzws;
public enum Tweedle {
    DEE, DUM
}

Book.java,

package com.xyzws;

public class Book {
    
    private     long chapters = 0;
    protected   String[] characters = { "Alice", "White Rabbit" };
    public      Tweedle twin = Tweedle.DEE;
    
    public long getChapters() {
        return chapters;
    }
    public void setChapters(long chapters) {
        this.chapters = chapters;
    }
    public String[] getCharacters() {
        return characters;
    }
    public void setCharacters(String[] characters) {
        this.characters = characters;
    }
    public Tweedle getTwin() {
        return twin;
    }
    public void setTwin(Tweedle twin) {
        this.twin = twin;
    }

}

and Program.java

package com.xyzws;

import java.lang.reflect.Field;
import java.util.Arrays;
import static java.lang.System.out;

public class Program {

    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");
            chap.setAccessible(true);
            out.format(fmt, "before", "chapters", 
                       chap.getLong(book));
            chap.setLong(book, 12);
            out.format(fmt, "after", "chapters", 
                       chap.getLong(book));

            Field chars = c.getDeclaredField("characters");
            out.format(fmt, "before", "characters",
                  Arrays.asList((String[])chars.get(book)));
            String[] newChars = { "Queen", "King" };
            chars.set(book, newChars);
            out.format(fmt, "after", "characters",
                   Arrays.asList((String[])chars.get(book)));

            Field t = c.getDeclaredField("twin");
            out.format(fmt, "before", "twin", t.get(book));
            t.set(book, Tweedle.DUM);
            out.format(fmt, "after", "twin", t.get(book));

        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    }
}

/*
BEFORE:  chapters     = 0
 AFTER:  chapters     = 12
BEFORE:  characters   = [Alice, White Rabbit]
 AFTER:  characters   = [Queen, King]
BEFORE:  twin         = DEE
 AFTER:  twin         = DUM
*/

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