Learning about Objects

Hi All,

I am trying to learn about the OO areas of Java and I believe that I have managed to create some objects but now I am trying to work out how to access information from these objects and then use System.out.print(); to display information of these on screen.
However, I have become stuck and if I can get any advise then it will be great :@:


I am using the following,

Code:

---

public class RunProgram {

        public static Object[] myObjects = new Object[10];
       
        public static void main(String[] args) {
               
                myObjects[0] = new ObjTest("Object 1", "An Object", 2, 2.2);
                myObjects[1] = new ObjTest("Object  2", "An Object", 0, 1.3);
                myObjects[2] = new ObjTest("Object  3", "An Object", 5, 5.57);
                myObjects[3] = new ObjTest("Object  4", "An Object", 3, 3.31);
                myObjects[4] = new ObjTest("Object  5", "An Object", 3, 0.34);
                myObjects[5] = new ObjTest("Object  6", "An Object", 0, 5.20);
               
                // Print list of toys to screen using Object Accessor's
               
               
        }
}

---

And this is supported by,

Code:

---

public class ObjTest {

        String objID, description;
        int aNumber;
        double aDouble;

        public ObjTest(String objID, String description, int aNumber, double aDouble) {
                this.objID = objID;
                this.description = description;
                this.aNumber = aNumber;
                this.aDouble = aDouble;
        }
}

---

Everything compiles okay but I am stuck trying to access these objects in the array again to print data from them :(

You already wrote the solution in line #14: define the necessary accessor methods in the ObjTest class; here's one of them:

Code:

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public String getObjID() { return objID; }

---

You have to implement the others and you can use them if you have an instantiation of yourr ObjTest class.

kind regards,

Jos

Excellent, thank you.
I have created the following and it has given what I want :)

Code:

---

public String getObj() {
                return objID + description + aNumber;
        }

---

and,

Code:

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System.out.println(((ObjTest) myObjects[0]).getObj());

---

Thank you so much, you have saved me a lot of time.
I had already spent a few hours researching this and on the bright side learnt alot about Java that I did not know.

Now to format the output to look better, this should be the easy part :)

Here is a handy technique for formatting strings. You can read more about it in the Java tutorials on the Oracle site.

Code:

---

return String.format("ObjectID(%s): %s %d", objID, description, aNumber);

---

In the project I work on, we use this a lot for logging purposes. There are some implications to doing so related to performance because these end up in a lot of mainline paths and therefore can impact performance. If you care to understand more, I can elaborate, possibly in a different thread related to Java logging. Heck, maybe it's time for my first blog!

Thank you,
For now I have used the following at it has worked how I want it to work, but once completed the project I will review everything to ensure I am happy.

Code:

---

return String.format("%-8s %-20s %d", objID, description, aNumber);

---

Here's something else that comes in handy: if a PrintStream or a PrintWriter needs to output an object of any type it calls its toString() method and prints that. Every type has such a method, either implemented or ultimately inherited from the Object class. So if you define something like this in your ObjTest class:

Code:

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public String toString() { return String.format("ObjectID(%s): %s %d", objID, description, aNumber); }

---

(I copied that code from the previous reply ;-) and somewhere in your code you do this:

Code:

---

System.out.println(yourTestObj);

---

the toString() method is called and you'll see the resulting String on stdout.

kind regards,

Jos

If you know that your myObjects array is only going to contain ObjTest objects, then declare it as an ObjTest[].
That'll save you having to do that casting when you;re reading things in the array.