interface I {
	void f();
	void g();
}
 
class A implements I {
	public void f() { System.out.println("A: doing f()"); }
	public void g() { System.out.println("A: doing g()"); }
}
 
class B implements I {
	public void f() { System.out.println("B: doing f()"); }
	public void g() { System.out.println("B: doing g()"); }
}
 
// changing the implementing object in run-time (normally done in compile time)
class C implements I {
	I i = null;
	// delegation
	public C(I i){ setI(i); }
	public void f() { i.f(); }
	public void g() { i.g(); }
 
	// normal attributes
	public void setI(I i) { this.i = i; }
}
 
public class Main {
	public static void main(String[] arguments) {
		C c = new C(new A());
		c.f();	// output: A: doing f()
		c.g();	// output: A: doing g()
		c.setI(new B());
		c.f();	// output: B: doing f()
		c.g();	// output: B: doing g()
	}
}

https://en.wikipedia.org/wiki/Helper_object

In software engineering, the delegation pattern is a design pattern in object-oriented programming where an object, instead of performing one of its stated tasks, delegates that task to an associated helper object.

Delegation pattern

java

Java