Write to a Field

Similarly, you can write to a field using the .set method on a Field object. This will also throw IllegalAccessException if its not something you are allowed to do.

import java.lang.reflect.Field;

class Main {
    void main() throws IllegalAccessException {
        Class<Drink> drinkClass = Drink.class;

        Field caffeinatedField;
        try {
            caffeinatedField = drinkClass.getField("caffeinated");
        } catch (NoSuchFieldException e) {
            throw new RuntimeException(e);
        }

        var water = new Drink("Water", false);

        // You can put drugs in anything you set your mind to, kids
        caffeinatedField.set(water, true);

        System.out.println(caffeinatedField.get(water));
    }
}

class Drink {
    public String name;
    public boolean caffeinated;

    Drink(String name, boolean caffeinated) {
        this.name = name;
        this.caffeinated = caffeinated;
    }
}

If you try to set a field to the wrong type of value - like setting a boolean field to have a String value - you will get an IllegalArgumentException. Unlike NoSuchFieldException and IllegalAccessException this is an unchecked exception, so you do not need to explicitly account for it.

import java.lang.reflect.Field;

class Main {
    void main() throws IllegalAccessException {
        Class<Drink> drinkClass = Drink.class;

        Field caffeinatedField;
        try {
            caffeinatedField = drinkClass.getField("caffeinated");
        } catch (NoSuchFieldException e) {
            throw new RuntimeException(e);
        }

        var soda = new Drink("Soda", true);

        caffeinatedField.set(soda, "yes, very much so");

        System.out.println(caffeinatedField.get(soda));
    }
}

class Drink {
    public String name;
    public boolean caffeinated;

    Drink(String name, boolean caffeinated) {
        this.name = name;
        this.caffeinated = caffeinated;
    }
}

The same will happen if you try to set a final field.¹

import java.lang.reflect.Field;

class Main {
    void main() throws IllegalAccessException {
        Class<Drink> drinkClass = Drink.class;

        Field nameField;
        try {
            nameField = drinkClass.getField("name");
        } catch (NoSuchFieldException e) {
            throw new RuntimeException(e); // Should have this field
        }

        var water = new Drink("Water", false);

        // You can put drugs in anything you set your mind to, kids
        nameField.set(water, true);

        System.out.println(nameField.get(water));
    }
}

class Drink {
    public String name;
    public final boolean caffeinated;

    Drink(String name, boolean caffeinated) {
        this.name = name;
        this.caffeinated = caffeinated;
    }
}