Simple example of passing by value (primitive) vs reference (arrays and objects)

ReferenceVsValue.java

/**
 * Licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
 * (see http://creativecommons.org/licenses/by-nc-sa/4.0/)
 */
import java.util.Arrays;

/**
 * @author Joseph Moore (joseph_moore@byu.edu)
 */
public class ReferenceVsValue {

    public static void main(String... args) {
        int i = 1;
        int[] array = new int[]{1, 2, 3};
        Foo foo = new Foo();
        foo.bar = "bar";

        int y = array[0];
        array[0] = 2;
        System.out.println(y);

        Foo[] foos = new Foo[] {
                new Foo(),
                new Foo()
        };

        Foo f = foos[0];

        foos[0].bar = "something";

        System.out.println(f);

        f.bar = "changed again";

        System.out.println(foos[0]);

        System.out.println("i before: " + i);
        changeInt(i);
        System.out.println("i outside: " + i);

        System.out.println("array before: " + Arrays.toString(array));
        changeArray(array);
        System.out.println("array outside: " + Arrays.toString(array));

        changeObject(foo);
        System.out.println("outside: " + foo);
    }

    public static void changeInt(int i) {
        i += 1;
        System.out.println("i in method: " + i);
    }

    public static void changeArray(int[] array) {
        array[0] = 42;
        System.out.println("in method: " + Arrays.toString(array));
    }

    public static void changeObject(Foo foo) {
        foo.bar = "ciao";
        System.out.println("in method: " + foo);
    }


    public static final class Foo {
        String bar;

        @Override
        public String toString() {
            return "Foo{" +
                    "bar='" + bar + '\'' +
                    '}';
        }
    }
}