Type inference with interfaces

Let's extend the content of the preceding section to the use of interfaces. Imagine that the Child class implements a MarathonRunner interface, as follows:

interface MarathonRunner{ 
    default void run() { 
        System.out.println("I'm a marathon runner"); 
    } 
} 
 
class Child implements MarathonRunner { 
    void whistle() { 
        System.out.println("Child-Whistle"); 
    } 
    void stand() { 
        System.out.println("Child-stand"); 
    } 
}  

Let's define an obj local variable, assigning it an instance of the Child class:

class Test{ 
    public static void main(String[] args) { 
        var obj = new Child();              // inferred type of var obj 
                                            // is Child 
        obj.whistle(); 
        obj.stand();                     
        obj.run(); 
    } 
}  

If the same variable is initialized using a method whose return type is MarathonRunner, its inferred type is MarathonRunner (irrespective of the type of the instance returned by it):

class Test{ 
    public static MarathonRunner getObject() { 
        return new Child(); 
    } 
    public static void main(String[] args) { 
        var obj = getObject();     // inferred type of var obj is 
                                   // MarathonRunner 
        obj.whistle(); 
        obj.stand(); 
        obj.run(); 
    } 
}