Association, dependency, composition and aggregation

Aggregation.java

public class Aggregation {
    Foo foo;

    public Aggregation(Foo foo) {
        this.foo = foo;
    }
}

Composition.java

public class Composition {
    Foo foo;

    public Composition(int val) {
        foo = new Foo(val);
    }
}

Foo.java

import java.util.HashSet;

public class Foo {
    int val;

    public Foo(int val) {
        this.val = val;
    }

    public void doSomethingUseless() {
        // HashSet is a dependency
        HashSet<String> set = new HashSet<>();
    }

    public void doSomethingUselessAgain(HashSet<String> set) {
        // HashSet also considered dependency
        HashSet<String> aSet = set;
    }
}

Main.java

public class Main {
    public static void main(String[] args) {
        // Aggregation example
        Foo foo1 = new Foo(1);
        Foo foo2 = new Foo(2);
        Aggregation agg = new Aggregation(foo1);
        agg = new Aggregation(foo2);
        // foo1 still exists

        // Composition example
        Composition com = new Composition(10);
        com = new Composition(11);
        System.out.println(com.foo.val); // prints 11. the previous foo can no longer be referenced
    }
}