how to accept an instance of a class as a parameter

[Nyah Bell]

Hi,

The following is apart of an assignment (I changed the code to be brief) and I am not posting it for my work to be done for me - I am just extremely Lost.

abstract class Etc{};

class A extends Etc {

    public String name;
    public Etc source;

    A(String name, Etc source) {
        this.name = name;
        this.source = source;
    }
}

class B extends Etc {

    public int value;

    B(int value) {
        this.value = value;
    }
}

//and then there are several other classes just like those

class C{
int doSomething(Etc e){
//i would then manipulate e here
}
}

class Stuff{
public void main(String[] args){

Etc et = new A("x", new B(2)); 
C cat = new C();
System.out.println(cat.doSomething(et));
}
}

I am just confused as to how to handle this. Do I use something from java.io library (Scanner)? How do I handle a parameter that is just a bunch of instances??

My apologies in advance for any confusion but I am at a loss. Any help is appreciated.

Thanks in Advance

[Mohamed Sanaulla]

Hi Nyah, Welcome to JR

What do you want to do with the instance passed to the method? You can use it just like any other instance.

class Dog
{
    private String name;
    public void setName(String n){ name = n; }
    public String getName() {return name; }
    public Dog(String name){this.name = name; }
    public String toString()
    {
         return "Dog: "+name;
    }
}
public class DogTest
{
    public void handleDog(Dog d)
    {
         d.setName("DogName-2");
    }
    public static void main(String args)
    {
         Dog d = new Dog("DogName-1"); 
         System.out.println(d);
         handleDog(d);
         System.out.println(d);
    }
}

[Nyah Bell]

well - unlike your example - there are only constructors within class A and B, so there are no return values (this is what is confusing me)

I am trying to add code to this:

class C{

int doSomething(Etc e){
//insert code here
}
}

the code that I need to insert is to manipulate the parameter e that is passed

in my original post, inside of main, i created

Etc et = new A("x", new B(2));

I need to get to whats inside of the instances of A and B
(almost like your example of A.methodA but when attempting this ( I am using Netbeans IDE) it is not possible to access the variables that way

[Mohamed Sanaulla]

In the Class A or Class B all the fields are public. So you can access them using <instanceName>.<fieldName>. In the example I provided I had made the fields are private and then provided Getters/Setters to access/manipulate the fields.

[Nyah Bell]

okay, so when it is not given a name but is just

dataType A = new B("string");

and B is just a class with a constructor

B(String s) {
String name;

this.name = s;
}

that instance does not have a name, it is just sitting there on the right side - what would I put in the <instanceName> part? 'A' is being passed, so can I do something like:

A.B.method

??

[Mohamed Sanaulla]

Nyah Bell wrote:
...
that instance does not have a name, it is just sitting there on the right side - what would I put in the <instanceName> part? 'A' is being passed, so can I do something like:

A.B.method

Am not able to understand what you are trying to do with the code you have provided in the above post. Firstly its not understandable and secondly its not syntactically correct.

[Nyah Bell]

okay sorry,

I just did not want to post up my entire actual code but I will post my actual code and go from there.

import java.util.Stack;

class ProjTwo {

    public static void main(String args[]) throws Exception {
//{x=1; {y={x=x+1; x+1}; x+y}}
        Expr e = new Block(new Declaration("x", new Int(1)),
                new Block(new Declaration("y", new Block(new Declaration("x", new BinaryOp("+", new Var("x"), new Int(1))),
                new BinaryOp("+", new Var("x"), new Int(1)))),
                new BinaryOp("+", new Var("x"), new Var("y"))));
        Evaluator ev = new Evaluator();
        
        System.out.println(ev.eval(e));
    }
}

//a dynamic binding from a variable to a value
class Binding {

    public String name;
    public int value;

    Binding(String name, int value) {
        this.name = name;
        this.value = value;
    }
}

//dynamic environment that binds variable to values
class Environment extends Stack<Binding> {
//insert new binding

    public void insert(String name, int value) {
        push(new Binding(name, value));
    }

//find binding of variable//searches stack
    public int lookup(String name) {
        for (int i = size() - 1; i >= 0; i--) {
            if (elementAt(i).name.equals(name)) {
                return elementAt(i).value;
            }
        }
        throw new Error("Variable " + name + "is not bound");
    }
}

abstract class Expr {};

class Var extends Expr {

    public String name;

    Var(String name) {
        this.name = name;
    }
}

class Int extends Expr {

    public int value;

    Int(int value) {
        this.value = value;
    }
}

class BinaryOp extends Expr {

    public String op;
    public Expr left;
    public Expr right;

    BinaryOp(String op, Expr left, Expr right) {
        this.op = op;
        this.left = left;
        this.right = right;
    }
}

class Declaration extends Expr {

    public String name;
    public Expr source;

    Declaration(String name, Expr source) {
        this.name = name;
        this.source = source;
    }
}

class Block extends Expr {

    public Expr left;
    public Expr right;

    Block(Expr left, Expr right) {
        this.left = left;
        this.right = right;
    }
}

class Evaluator{

    Environment env = new Environment();

    int eval(Expr e) throws Exception {
        //this is where I put my code for the assignment

        int ans = 1; //this is just a dummy value
       
        env.insert("y", ans);
        System.out.printf(env.lookup("x") + "\n");//this prints out an error saying "variable not bound"

        return ans; 
    }
}

All of that was given as the teacher's code - he has said not to change it because it should work fine and the only changes we should make is adding our code where he told us to add it (as noted with the comments)

I am expected to use a stack to bind variables, but before I can do so, I need to figure out how to get to those variables. For example, in main, Expr e = new Block(new . . . . . );
e is then passed as a parameter
What can I do within the eval method to get to the variables that were instantiated in e ("x", new Declaration("x"........) etc.?

It is of course perfectly okay if you cannot help because I definitely appreciate you trying to understand me (I can be confusing) and understand what I am trying to do. This is much appreciated.

[Mohamed Sanaulla]

In the eval method-
Firstly you need to cast "e" to Block to be able to use Block class's instance fields. That way you would have to proceed to access other class's instance fields. And before casting try to add an instanceof check. The reason for casting is that- Which fields can be accessed is determined by the Type of the reference.
So if you have the type of reference as Expr and your type of instance is Block, to access the left field in the Block instance you need to have the Type of reference as "Block" and not "Expr". Because the association of the fields with the reference type is done at the compile time.

[Nyah Bell]

Thank you SO much! Now I understand - yes! So by casting I am helping it understand what it is and what it can reach.

Thank you very very much!