📓9.4: super Keyword

Table of Contents

📖 This page is a condensed version of CSAwesome Topic 9.4

The super Keyword

Sometimes you want the subclass to do more than what a superclass’ method is doing. You want to still execute the superclass method, but you also want to override the method to do something additional.

But, since you have overridden the parent method, how can you still call it? You can use super.method() to force the parent’s method to be called.

We’ve used super() before to call the superclass’ constructor. But there are two uses of the keyword super:

  1. super(); or super(arguments); calls just the super constructor if put in as the first line of a subclass constructor.
  2. super.method();calls a superclass’ method (not constructors).

The keyword super is very useful in allowing us to first execute the superclass method, and then add on to what it does in the subclass.

In the example below, the Student class overrides the getFood method of the Person class, and it uses super.getFood() to call the Person getFood method before adding on to it. Here, a Person is associated with the food “Hamburger” and a Student is associated with “Hamburger” and “Taco”.

public class Person {
   private String name;

   public Person(String theName) {
           name = theName;
   }

   public String getFood() {
      return "Hamburger";
   }

   public static void main(String[] args) {
      Person p = new Student("Javier");
      System.out.println(p.getFood());
   }
}

public class Student extends Person {
   private int id;
   private static int nextId = 0;

   public Student(String theName) {
      super(theName);
      id = nextId;
      nextId++;
   }

   public String getFood() {
      String output = super.getFood();
      return output + " and Pizza";
   }

   public int getId() {
      return this.id;
   }

   public void setId(int theId) {
      this.id = theId;
   }
}

💻 TRY IT OUT: Add another subclass called Vegan that inherits from the Student class. Add a constructor that takes a name as an argument and passes it to the super() constructor. Override the getFood() method in Vegan to call the superclass getFood(), but add a “No “ in front of it and then say “but “ and add a vegan food. Change Javier to a Vegan object in main() and try it out!

How does this work? Remember that an object always keeps a reference to the class that created it and always looks for a method during execution starting in the class that created it.

  • If it finds the method in the child class that created it, it will execute that method.
  • If it doesn’t find it in the class that created it, it will look at the parent of that class.
    • It will keep looking up the ancestor chain until it finds the method, all the way up to the Object class. The method has to be there, or else the code would not compiled!

⛓️ When the student getFood() method is executed it will start executing the getFood method in Student. When it gets to super.getFood() it will execute the getFood method in Person. This method will return the string "Hamburger". Then execution will continue in the getFood method of Student and return the string "Hamburger and Taco".

toString Override

The toString method is commonly overridden. A subclass can override toString, but in its new toString method, it can also call super.toString() to get a starting String to which it can add on its own instance variables:

// overridden toString() in subclass
public String toString() {
   return (super.toString() + "\n" + subclassInstanceVariables);
}

💻 In-Class Activity: Customer Info

  1. Go to
  2. Make sure you SIGN IN!
  3. Complete the Programming Challenge: Customer Info activity in pairs.

⭐️ Summary

  • The keyword super can be used to call a superclass’s constructors and methods.
    • The superclass method can be called in a subclass by using the keyword super with the method name and passing appropriate parameters.

Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.