在java中将属性从一个类传递到另一个+ instaiation

Question

I am tying to wrap my head around using a method of an object in one class (in this case the Category class) and having it pass attributes/fields into an object of another class (in this case subCategory class). You can see in my main method below the actual results I am getting from the compiler versus the results I am trying to obtain.

I think the biggest challenge for me is trying the following:

1. Passing a class as an argument in a method(I could not find online references that talk about it.)
2. Instantiating an object of an external class inside a method of a local class.

Thanks for the help.

Category class

public class Category {

private String name;
private SubCategory subCategory = new SubCategory();

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public void createSubcategory(SubCategory subCategory, String name){
    SubCategory m = new SubCategory();
    m.setParent(Category.this);
    m.setName(name);

}

SubCategory class

public class SubCategory {


private String name;
private Category parent;

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public Category getParent() {
    return parent;
}

public void setParent(Category parent) {
    this.parent = parent;
}    }

Main (method)

public static void main(String[] args) {


        Category mainCat = new Category();
        SubCategory m = new SubCategory(); // I want to instantiate this inside the method of class Category called createSubCategory()

        mainCat.setName("TITLE");
        System.out.println(mainCat.getName()); // Returns --> "TITLE" THIS IS OK!


        mainCat.createSubcategory(m, "SUBTITLE1");
        System.out.println(m.getName()); // Returns "null", I'm trying to make it return --> "SUBTITLE1"
        System.out.println(m.getParent());// Returns "null", I'm trying to make it return --> "mainCat (object)"

    }

Answer 1

right now your createSubCategory method receives a SubCategory as a parameter, but it does nothing with it (creates a new instance inside). and on the other hand- it creates an instance of SubCategory, but does nothing with it as the method returns void.

Two ways you can go here:

1. make createSubCategory method use the provided subCategory, and work on it (in this case, I'd call it setSubCategory , as it doesn't create it):

    public void setSubcategory(SubCategory subCategory, String name){ subCategory.setParent(Category.this); subCategory.setName(name); } 

and in main:

    mainCat.setSubcategory(m, "SUBTITLE1");
    System.out.println(m.getName()); //you will get 'SUBTITLE1' here
    System.out.println(m.getParent()); //you will get the parent category

2. make createSubCategory create it, and in this case you don't need to pass it to it, but instead return it:

    public SubCategory createSubcategory(String name){ SubCategory m = new SubCategory(); m.setParent(Category.this); m.setName(name); return m; } 

and in main:

public static void main(String[] args) {
    Category mainCat = new Category();
    mainCat.setName("TITLE");
    System.out.println(mainCat.getName()); 

    Subcategory m = mainCat.createSubcategory("SUBTITLE1");
    System.out.println(m.getName());
    System.out.println(m.getParent());
}

Answer 2

I would even go little bit further and modify example 2 given by @Nir Levy:

public SubCategory createSubcategory(String name){
    return new SubCategory(this, name);
}

If both attributes are required they should be injected via constructor ( Constructor or Setter )