Accessing and modifying properties of a super class from sub class?

Question

while trying to get a grasp of polymorphism and inheritance, I made a small program to demonstrate these topics. The program consists of a superclass 'Tree' and three subclasses 'Birch', 'Maple', and 'Oak'. Tree's constructor makes it so that all trees start off with a height of 20 and 200 leaves. In Tree I have an abstract method called grow().

Here's the code for Tree:

public abstract class Tree { private int height; private int numberOfLeaves; public Tree() { height = 20; numberOfLeaves = 200; } public Tree(int aheight, int anum) { height = aheight; numberOfLeaves = anum; } public int getHeight() { return height; } public int getNumberOfLeaves() { return numberOfLeaves; } public void setNumberOfLeaves(int anum) { numberOfLeaves = anum; } public abstract String getType(); public void setHeight(int aheight) { height = aheight; } public abstract void grow();

}

Here's the code in Birch for grow().

public void grow() { int height = super.getHeight(); super.setHeight(height++); int num = super.getNumberOfLeaves(); super.setNumberOfLeaves(num+=30); System.out.println("The Birch is Growing..."); }

However, when I call code to make an array of trees grow, none of their heights or number of leaves change. Here's the code I used to populate the array of trees (I did it manually):

ArrayList<Tree> treeArray = new ArrayList<Tree>(); treeArray.add( new Oak()); treeArray.add(new Birch()); treeArray.add(new Maple());

And Here's the code I used to call grow:

for (Tree tree : treeArray) { tree.grow(); System.out.println("The " + tree.getType() + "'s height is " + tree.getHeight() + " and it's number of leaves is "+ tree.getNumberOfLeaves() +"."); }

Clearly, the values in the superclass aren't being modified. Any help will be appreciated! Thanks!

although it still depends on how tree class being written, try super.setHeight(++height); — Rudy
– Rudy, Commented Nov 27, 2013 at 5:31
your Tree code seems OK. Show us how you populate the treeArray. — Rudy
– Rudy, Commented Nov 27, 2013 at 5:45
@Rudy I populated the treeArray manually as a matter of fact. I simply did treeArray.add(new Birch()) — Sidd Menon
– Sidd Menon, Commented Nov 27, 2013 at 5:48

Rudy · Accepted Answer · 2013-11-27 06:05:21Z

Change your code to :

int height = super.getHeight(); super.setHeight(++height);

note that you don't need to call super.method(). as long as the method is protected (public even better) you can just simplify it to :

int height = getHeight(); setHeight(++height);

You only call super. if you implement the method again in your child class and want to specifically call the parent class, which usually can be seen in constructor calling parent constructor.

One more thing : your accessor need to be changed a bit just for pre-caution case. see code below. Usually your IDE should support auto generation of accessor.

 public int getHeight() { return height; } public void setHeight(int height) { this.height = height; }

Even after changing the operator on height to pre-increment, it still isn't changing the value; the main function is still leaving height at 20
I just put the code for Tree. Oak is written almost exactly as Birch and maple with the exception of the name of the constructor and how many leaves it increases by in grow().
there is no weird thing in your Tree class. show how you populate treeArray

musical_coder · Accepted Answer · 2013-11-27 05:31:46Z

This code:

int height = super.getHeight(); super.setHeight(height++);

isn't going to change anything, because the increment of height will occur after the call to super.setHeight(). So you're just setting the height to its current value.

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