Java
super
Learn to call the parent with super.
The super keyword
To call the superclass constructor from a subclass constructor, use super(...). The super() call must be the first statement in the subclass constructor. This is how you initialize the superclass's fields.
Example: class Cat extends Animal { Cat(String n) { super(n); } }. Here the Cat constructor calls the Animal constructor with super(n) and initializes name in the superclass. super also accesses superclass methods: super.eat(). This lets you call an overridden method's superclass version.
The position of super()
The super() call must always be the first statement in a subclass constructor. This is mandatory; otherwise you get a compile error. The superclass's fields must be initialized before the subclass's own fields.
Example: Cat(String n) { super(n); int age = 0; }. Here super(n) comes first, then age assignment. If you write super(n) after the age assignment, a compile error occurs. This order ensures the superclass is ready first; the subclass body knows the superclass is prepared.
Exercise Add super to the Cat class
Define a class Animal with a String name field and an Animal(String n) constructor that assigns name = n;. Define Cat extends Animal whose Cat(String n) constructor starts with super(n). Put main inside Cat; create a Cat instance and print its name field.
Show example solution
class Animal {
String name;
Animal(String n) {
name = n;
}
}
class Cat extends Animal {
Cat(String n) {
super(n);
}
public static void main(String[] args) {
Cat c = new Cat("Whiskers");
System.out.println(c.name);
}
}
Test yourself
-
Match the super uses
- super(n)
- Superclass constructor
- super.eat()
- Superclass method
- first line
- Position principle
- Animal
- Superclass
-
Where must the super() call be?
- Inside a method body
- At the first line of the constructor
- At the last line of the constructor
- Outside the class
Show answer
At the first line of the constructor — At the first line of the constructor — super() must be first.
-
Complete the super() call
class Cat extends Animal {
Cat(String n) {
____(n);
}
}
Show answer
class Cat extends Animal {
Cat(String n) {
super(n);
}
}
Next lesson: Method overriding
