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Super Rules

super(...) is Always Called

When we don't write a call to super(), the compiler will try to add one. * The members of the superclass must be initialized before the subclass, and Java uses the constructor to do so.

The class... 

public DataAnalyst(String securityClearance) {
  this.securityClearance = securityClearance;
}
is compiled to become... 
public DataAnalyst(String securityClearance) {
  super();   // Added by the compiler
  this.securityClearance = securityClearance;
}

Whether we have written it or not, every constructor will have a call to super(...) or this(...). * Either we put a call to super(...) or this(...) in our constructors, or the compiler adds super().

super May Be Required

Two points about constructors are very important to remember: * When we code a constructor, the compiler no longer adds the no-arg default constructor. * If we don't see a call to super(...) in a constructor, Java tries to add a call to the no-arg super().

Consider this class hierarchy, which will result in a compiler error.

public class Parent {
  protected String field;

  public Parent(String field) {
    this.field = field;
  }
}

Child initializes the protected field, but will not compile. 

public class Child extends Parent {
  public Child(String field) {
    this.field = field;
  }
}

The problem is that Java has tried to compile the Child constructor to call a no-arg constructor in Parent, which does not exist.

super() does not exist

For Child to compile, we have to add the call to super(field) ourselves (assuming we cannot add a no-arg constructor to Parent).

public class Child extends Parent {
  public Child(String field) {
    super(field);
  }
}

super call to existing constructor

super() Must Be First

super() can only be called from a constructor, and must be the first statement, also like this().

public Employee(String firstName, String lastName, int age,
    String title, double salary) {
  this.title = title;
  this.salary = salary;
  super(firstName, lastName, age);  // COMPILER ERROR
}


Practice Exercise

You can never have this() and super() in the same constructor.

Both this() and super() must each be the first line in a constructor.

If you have both, one is not first, so you can't have both.

Resource: https://docs.oracle.com/javase/tutorial/java/IandI/super.html

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