In JDK 9 and other previous versions, you could define a variable of the base class and assign an instance of its derived class to it. The members that you could access using the variable were limited to the ones that were defined in the base class. This is no longer the case with var, since the type of the variable is inferred by using the specific type of the instance that is assigned to it.

Imagine a class  Child extends a class Parent. When you create a local variable and assign it an instance of the Child class, the type of the variable is inferred as Child. This looks simple. The following is an example:

class Parent { 
    void whistle() { 
        System.out.println("Parent-Whistle"); 
    } 
} 
class Child extends Parent { 
    void whistle() { 
        System.out.println("Child-Whistle"); 
    } 
    void stand() { 
        System.out.println("Child-stand"); 
    } 
} 
class Test{ 
    public static void main(String[] args) { 
        var obj = new Child();                             
        obj.whistle(); 
        obj.stand();     // type of obj inferred as Child 
    } 
}

What happens if you assign the value of the obj variable using a method that can return an instance of the Child class or Parent classes? Here's the modified code:

class Parent { 
    void whistle() { 
        System.out.println("Parent-Whistle"); 
    } 
} 
 
class Child extends Parent { 
    void whistle() { 
        System.out.println("Child-Whistle"); 
    } 
    void stand() { 
        System.out.println("Child-stand"); 
    } 
} 
 
class Test{ 
    public static Parent getObject(String type) { 
        if (type.equals("Parent")) 
            return new Parent(); 
        else 
            return new Child(); 
    } 
 
    public static void main(String[] args) { 
        var obj = getObject("Child"); 
        obj.whistle(); 
        obj.stand();              // This line doesn't compile 
    } 
} 

In the preceding code, the type of the instance returned by the getObject() method can't be determined before the code execution. During compilation, the type of the obj variable is inferred as Parent, the return type of the getObject() method. Since the Parent class doesn't define stand(), the main() methods fail to compile.