I have a class called Food.java that has two constructors. The driver code I have been given calls its copy constructor inside its constructor with its own object as the parameter. The copy constructor has the parameters food and flavor. This is an object is then printed out using a toString() method but the toString() method is giving me the food variable and the flavor variable are both null. Would anyone be able to tell me why? Thank you. Driver code:

public static void main(String args[]) {
    System.out.println(new Food(new Food("Ice-cream", "Vanilla")));
}

My code :

public class Food {

    private String food;
    private String flavor;
    private Food f;

    public Food(String food, String flavor) {
        this.food = food;
        this.flavor = flavor;
    }


    public Food(Food f) {
        this.f = f;
    }

    public String toString() {
        return food   " "   flavor;
    }

}

Output:

null null

CodePudding user response：

When you call System.out.println (new Food (new Food ("Ice-cream", "Vanilla"))); you are creating to Food instances. When the inner instance is created this constructor is called:

public Food(String food, String flavor) {
    this.food = food;
    this.flavor = flavor;
}

Here food and flavor are set. Then the outer Food instance is created with the other constructor. Here only f is set, not food and flavor. And the outer instance is thrown into the println, so this is the instance toString is called on. What you want is a constructor doing this:

public Food(Food other) {
    this.food = other.food;
    this.flavor = other.flavor;
}

And then you can delete the field f.

CodePudding user response：

What's Happening:

The first object creation correctly stores the food and flavour as "Ice-cream" and "Vanilla" but leaves the variable f uninitialized.

The second object creation takes the Food object you just created as input and then stores it in its f variable. food and flavour are left uninitialized.

Therefore, when the printline is called it gets the uninitialized values in food and flavour.

How to Fix it:

Firstly, you can probably remove the f variable as it seems to be redundant - the object itself will already store the information you require. Generally speaking an object will never be a property of itself.

Then, in the second constructor that uses a Food as input you just change the code to use the food and flavour instead.

public Food(Food f) {
    this.food = f.food;
    this.flavor = f.flavor;
}

CodePudding user response：

public class Food {

    private final String food;
    private final String flavor;
    // you should not add Food f; you have 2 copies of the same object

    // copy constructor should check for null and set all parameters
    public Food(Food f) {
        food = f == null ? null : f.food;
        flavor = f == null ? null : f.flavor;
    }

    public Food(String food, String flavor) {
        this.food = food;
        this.flavor = flavor;
    }

    @Override
    public String toString() {
        return food   ' '   flavor;
    }

}

CodePudding user response：

You need the .toString() to be called from somewhere. Your driver code should probably look something like this.

    public static void main(String[] args){
        System.out.println((new Food ("Ice-cream", "Vanilla")).foodToString();
    }

foodToString() would be your toString method in your food class unless you override the generic to string method.