How do you update/change a instance variable in Java? (Beginner Level)

Question

I am new to java, and I am trying to create a method to update the instance variable age for my objects. I am getting the code to compile, but I am seeing no change in the age value. This code is part of an assignment, so I cannot change the constructors. The method I wrote to update the age (that doesn't work) is shown below. My entire code is shown below that. I am also curious if there's a way to update/change just one of my objects, but before I do that, I need the method to work for both. Any help writing this method properly would be appreciated!

public void setnewAge(int age) {
    dogAge += 1;
    this.dogAge = dogAge;
}

Below is my entire code (including the method I wrote to update age).

public class Dog {
    //Instance Varibles
    private String dogName; 
    private int dogAge;
    private int dogWeight;

    //Two Contructors (One Completely Empty)
    public Dog() {
    }  
    
    public Dog(String name, int age, int weight){
        dogName = name;
        dogAge = age;
        dogWeight = weight;
    }   

    //Getters
    public String getName() { return dogName;}
    public int getAge() { return dogAge;}
    public int getWeight() { return dogWeight;}
    
    //Setters
    public void setName(String theName) { dogName = theName;}
    public void setAge(int theAge) {dogAge = theAge;}
    public void setWeight(int theWeight) {dogWeight = theWeight;}
        
    //to(String) method
    public String toString() {
        return  "The dogs's name is " + getName() + ", the dogs's age is " +
                getAge() + ", " + "\n" + "the dogs's weight is " + getWeight() + ".";
    }
    
    public void setnewAge(int age) {
        dogAge += 1;
        this.dogAge = dogAge;
    }       

    //Main Method
    public static void main(String[] args) {
        Dog poodle = new Dog("Bob", 5, 26);
        System.out.println(poodle);
        Dog lab = new Dog();
        lab.setName("Steve");
        lab.setAge(8);
        lab.setWeight(43);
        System.out.println(lab);
    }
}

How do you know it doesn't work when you don't call it in your code? And why does that method have a parameter, when you don't use that parameter? — Tom
– Tom, Commented Jul 7, 2021 at 21:47
Could you please elaborate or maybe provide an example, I am confused as to what you mean. (thanks for the comment!) — pogu
– pogu, Commented Jul 7, 2021 at 21:52
There's no need to invent new names for parameters to setters (no pun intended). this.age = age; is the way to do it. Also, no need to prepend 'dog' to attributes of Dog - that's obvious. So weight, not dogWeight. — g00se
– g00se, Commented Jul 7, 2021 at 23:45

Basil Bourque · Accepted Answer · 2021-07-07 22:00:57Z

2

As Tom said, you need to actually call the function in your main function otherwise, there will be no change, and also to refine your code for your setnewAge function, try this:

    public void setnewAge() {
           this.dogAge = dogAge + 1;
    }

Then in the main function call setnewAge() and then print your age to see the results.

Dog poodle = new Dog("Bob", 5, 26);
poodle.setnewAge() ;

edited Jul 7, 2021 at 22:00

Basil Bourque

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answered Jul 7, 2021 at 21:52

Avaruto

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1 Comment

Basil Bourque Over a year ago

And perhaps suggest a change in method name away from set…, such as incrementAge.

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