Question

public class EqualsTest {
  public static <T> boolean equalTest(T o1, T o2) {
      return o1.equals(o2);
  }
  public static void main(String[] args) {
      EqualsTest et1 = new EqualsTest();
      EqualsTest et2 = new EqualsTest();
      System.out.println(et1.equals(et2));
      System.out.println(equalTest(et1, et2));
  }
  public boolean equals(Object o) {
      if (o instanceof EqualsTest) {
          System.out.println("equals(Object)");
          return true;
      }
      return false;
  }
  public boolean equals(EqualsTest et) {
      System.out.println("equals(EqualsTest)");
      return this.equals((Object)et);
  }
}

Answer 1

There is none, it is implicit. Since it does not explicitly extends any class, it will implicitly extend Object. And since Object has a no-arg constructor, it will implicitly call up to that one.

Really, the class could contain a constructor written as this, and it would be equivalent:

public class EqualsTest {
    public EqualsTest() {
        super();
    }

    //the methods...
}

You cannot use implicit constructors if the class you extend does not also have a ‘no-arg’ constructor (whether implicit or explicit). If you provide any explicit constructor, there will be no implicit no-arg constructor created for you.