Thread: Link objects and calling a variable object's method

Hello,

I've been searching quite a while for an answer but seeing as I could not find it, I decided to come here, hoping someone could help me.

Say I have two classes, Author and Book, and I have 2 author objects and 10 book objects. I would like to know how to do two things:

1) Make some sort of connection that makes clear that author X wrote books A, B and F.

2) Call a method from a book object that is connected to an author.

Seeing as I don't know which books will be connected to an author, is there some way to call a method of an object bases on a variable object name? Something like call the getFirstPage() method for every book that is linked to author X?

Many thanks in advance!

Originally Posted by Norm

1) Have an arraylist in the Author's class that contains the books he wrote
and/or have a field in the Book class that has the address of the Author that wrote it.

2) Not sure what the problem here is. Methods in the Book class should be able to call methods in the Author class. That would be easy if the Book class has a reference to its Author.

--- Update ---

Also posted at: Link objects and calling a variable object's method

Thanks, my second question could be asked in a more simple way I guess: How can I call a method from an object based on a variable object name?

Sorry to ask this before trying but I'm not currently behind my computer:

Say I make a new variable, call it targetBook and give it a value of "bookA"

would targetBook.methodName(argsToMethod) do the same as bookA.methodName(argsToMethod)? because that's what I'm hoping to do.

Originally Posted by Norm

No problem. I'll wait until you can try the code you are asking about with a compiler to see what happens.
Copy any error messages and paste them here with the code.
Be sure to wrap the code in code tags.

Alright, this is my very short example code:

public class TestThing {
 
	public void doSomething(){
		System.out.println("succes");
	}
}

public class Test {
 
	public static void main(String[] args) {
 
		String[] arrObjectNames;
		arrObjectNames = new String[3];	
 
		arrObjectNames[0] = "objFirst";
		arrObjectNames[1] = "objSecond";
		arrObjectNames[2] = "objThird";
 
		TestThing objFirst = new TestThing();
		TestThing objSecond = new TestThing();
		TestThing objThird = new TestThing();
 
		arrObjectNames[2].doSomething;
 
	}
 
}

I'm trying to call the doSomething() method from the objThird object of the TestThing class by using the value stored in arrObjectNames[2]. This does not work as Java can't find the method this way.

Exception in thread "main" java.lang.Error: Unresolved compilation problems: 
	Syntax error, insert "AssignmentOperator Expression" to complete Expression
	doSomething cannot be resolved or is not a field
 
	at Test.main(Test.java:17)

Originally Posted by Norm

The compiler recognizes a method call by the ()s at the end of the symbol.
doSomething is missing the ()s

Whoops, my bad, sorry. It still doesn't work though

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
	The method doSomething() is undefined for the type String
 
	at Test.main(Test.java:17)

Originally Posted by Norm

The compiler can not find the method: doSomething() in the String class.

That method is in the TestThing class. You need to have a reference to an instance of the TestThing class to call the doSomething() method. For example: objFirst is a reference to the TestThing class. Use that.

		arrObjectNames[0] = "objFirst";
		arrObjectNames[1] = "objSecond";
		arrObjectNames[2] = "objThird";
 
		TestThing objFirst = new TestThing();
		TestThing objSecond = new TestThing();
		TestThing objThird = new TestThing();

I don't udnerstand, isn't that what I already did here?

EDIT:

I think I understand now, this works:

public class Test {
 
	public static void main(String[] args) {
 
		TestThing[] arrObjectNames;
		arrObjectNames = new TestThing[3];	
 
		TestThing objFirst = new TestThing();
		TestThing objSecond = new TestThing();
		TestThing objThird = new TestThing();
 
		arrObjectNames[0] = objFirst;
		arrObjectNames[1] = objSecond;
		arrObjectNames[2] = objThird;
 
		arrObjectNames[2].doSomething();
 
	}
 
}

Originally Posted by Norm

Look at the statement the compiler error is on.


What is it that you did there?

If you want to use a String to get a reference to an existing instance of a class, use a Map. Something like: Map<String, TheClass>

I think I was thinking too much in a PHP way, I changed the array from a string type, to a TestThing type and got it working like this:

public class Test {
 
	public static void main(String[] args) {
 
		TestThing[] arrObjectNames;
		arrObjectNames = new TestThing[3];	
 
		TestThing objFirst = new TestThing();
		TestThing objSecond = new TestThing();
		TestThing objThird = new TestThing();
 
		arrObjectNames[0] = objFirst;
		arrObjectNames[1] = objSecond;
		arrObjectNames[2] = objThird;
 
		arrObjectNames[2].doSomething();
 
	}
 
}

By the way, you dont need an array in the first place, you can simply do:

public class Test {
 
	public static void main(String[] args) {
		TestThing objFirst = new TestThing();
		objFirst.doSomething();
	}
 
}

Originally Posted by Cornix

By the way, you dont need an array in the first place, you can simply do:

public class Test {
 
	public static void main(String[] args) {
		TestThing objFirst = new TestThing();
		objFirst.doSomething();
	}
 
}

Well, I was working on something that could call a method of an object without calling a specific object by its name.

I dont think I understand.
Can you give us any useful scenario where you would like to do this?

Originally Posted by Cornix

I dont think I understand.
Can you give us any useful scenario where you would like to do this?

Well, I could have a Team object, inside it is an array with Player objects. There could be 3 players but there just as well could be 11 players. At some point I want to call a method from every player object in this team object.

I would design it as a Person, an Author, and a Book.

This is what I'm thinking:

public class Person  {
   String getName();
   Date getBirthday();
}
 
public class Author  {
   Person getPerson();
   AuthorType getType();
}
 
public enum AuthorType {AUTHOR, CO_AUTHOR, ILLUSTRATOR, ...};
 
public class Book  {
   List<Author> getAuthorList();
   String getTitle();
   String getSubTitle();
}

The reason to separate Person from Author (and for the AuthorType enum) is because they might be an AUTHOR in one book, and a CO_AUTHOR in another.

To go the other way around, you might have a Map where the key is an Author, and the value is a List of all their Books.

Thanks aliteralmind but I don't understand how that would allow me to do something like:

arrObjects[2].doSomething();