We can instantiate a class via Constructor.newInstance() using the Java Reflection API.

Let's consider the following class, which has four constructors:

public class Car {

  private int id;
  private String name;
  private Color color;

  public Car() {}

  public Car(int id, String name) {
    this.id = id;
    this.name = name;
  }

  public Car(int id, Color color) {
    this.id = id;
    this.color = color;
  }

  public Car(int id, String name, Color color) {
    this.id = id;
    this.name = name;
    this.color = color;
  }

  // getters and setters omitted for brevity
}

A Car instance can be created via one of these four constructors. The Constructor class exposes a method that takes the types of the parameters of a constructor and returns a Constructor object that reflects the matched constructor. This method is called getConstructor​(Class<?>... parameterTypes).

Let's call each of the preceding constructors:

Class<Car> clazz = Car.class;

Constructor<Car> emptyCnstr 
  = clazz.getConstructor();

Constructor<Car> idNameCnstr 
  = clazz.getConstructor(int.class, String.class);

Constructor<Car> idColorCnstr 
  = clazz.getConstructor(int.class, Color.class);

Constructor<Car> idNameColorCnstr 
  = clazz.getConstructor(int.class, String.class, Color.class);

Furthermore, Constructor.newInstance​(Object... initargs) can return an instance of Car that corresponds with the invoked constructor:

Car carViaEmptyCnstr = emptyCnstr.newInstance();

Car carViaIdNameCnstr = idNameCnstr.newInstance(1, "Dacia");

Car carViaIdColorCnstr = idColorCnstr
  .newInstance(1, new Color(0, 0, 0));

Car carViaIdNameColorCnstr = idNameColorCnstr
  .newInstance(1, "Dacia", new Color(0, 0, 0));

Now, is time to see how we can instantiate a private constructor via reflection.