Sometimes when we define a new subclass we need to access members or constructors of its superclass. Java provides a special keyword super to do this. This keyword can be used in several cases:

• to access instance fields of the parent class;
• to invoke methods of the parent class;
• to invoke constructors of the parent class (no-arg or parameterized).

Let's consider all of these cases with examples.

Accessing superclass fields and methods

The keyword super can be used to access instance methods or fields of the superclass. In a sense, it is similar to the keyword this, but it refers to the immediate parent class object.

The keyword super is optional if members of a subclass have different names from members of the superclass. Otherwise, using super is the right way to access hidden (with the same name) members of the base class.

Example. There are two classes: SuperClass and SubClass. Each class has a field and a method.

class SuperClass {
    
    protected int field;

    protected int getField() {
        return field;
    }
    
    protected void printBaseValue() {
        System.out.println(field);
    }
}

class SubClass extends SuperClass {
    
    protected int field;

    public SubClass() {
        this.field = 30;  // It initializes the field of SubClass
        field = 30;       // It also initializes the field of SubClass
        super.field = 20; // It initializes the field of SuperClass
    }

    /**     
     * It prints the value of SuperClass and then the value of SubClass
     */
    public void printSubValue() {
        super.printBaseValue(); // It invokes the method of SuperClass, super is optional here
        System.out.println(field);
    }
}

In the constructor of SubClass , the superclass field is initialized using the keyword super. We need to use the keyword here because the subclass field hides the base class field with the same name.

In the body of the method printSubValue , the superclass method printBaseValue is invoked. Here, the keyword super is optional. It is required when a subclass method has the same name as a method in the base class. This case will be considered in the topic concerning overriding.

Invoking superclass constructor

Constructors are not inherited by subclasses, but a superclass constructor can be invoked from a subclass using the keyword super with parentheses. We can also pass some arguments to the superclass constructor.

注意

Two important points:

• invoking super(...) must be the first statement in a subclass constructor, otherwise, the code cannot be compiled;
• the default constructor of a subclass automatically calls the no-argument constructor of the superclass.

Example. Here are two classes Person and Employee. The second class extends the first one. Each class has a constructor to initialize fields.

class Person {

    protected String name;
    protected int yearOfBirth;
    protected String address;

    public Person(String name, int yearOfBirth, String address) {
        this.name = name;
        this.yearOfBirth = yearOfBirth;
        this.address = address;
    }

    // getters and setters
}

class Employee extends Person {

    protected Date startDate;
    protected Long salary;

    public Employee(String name, int yearOfBirth, String address, Date startDate, Long salary) {
        super(name, yearOfBirth, address); // invoking a constructor of the superclass
        
        this.startDate = startDate;
        this.salary = salary;
    }

    // getters and setters
}

In the provided example, the constructor of the class Employee invokes the parent class constructor for assigning values to the passed fields. In a way, it resembles working with multiple constructors using this().