Why is the private Modifier makes the program to execute private method in super class although called through sub class object?

Question

I am trying to run a java program to implement the overridding the grow method.

The overridding works perfectly if method grow in class Tree (parent class) is declared with default access modifier and prints "Oak is growing". But when its changed to private, the method from the parent class (Tree) executes.

// Code with  grow() default modifier
public abstract class TreeGrow {
    void grow() {
        System.out.println("Tree is growing");
    }

    public static void main(String args[]) {
        TreeGrow tree = new Oak();
        tree.grow();
    }
}

class Oak extends TreeGrow {
    protected void grow() {
        System.out.println("Oak is Growing");
    } 
}

// Code with private grow
public abstract class TreeGrow {
    private void grow() {
        System.out.println("Tree is growing");
    }

    public static void main(String args[]) {
        TreeGrow tree = new Oak();
        tree.grow();
    }
}

class Oak extends TreeGrow {
    protected void grow() {
        System.out.println("Oak is Growing");
    } 
}

Why is the private method in Tree class executed, when the method is called through Oak object?

Answer 1

The problem is that your object is of type TreeGrow. Your main method therefore calls the method TreeGrow.grow(). The private method is not overridden and therefore the "most-suitable" method is called, which is the private void grow() one.

You need to cast your object to the corresponding type to use its method. Try using ((Oak)tree).grow (or alternatively Oak tree = new Oak();). This should execute your method.

Edit: Sorry for the "vague" explanation. I was in a hurry and wanted to help as fast as possible. After re-visiting this, it really is not well-written. As the linked "duplicate" question (and the respective answers) describe everything in a more elaborate manner, I would recommend reading there.