12/8/12 Object Arrays

You know what an Object is, and you also know what an Array is, but did you know that you can have an Array of Objects? This means that you could have an array that is a address book; holding objects that represent people in the book. These people will have primitive variables for their address and their phone number. In this post, I will teach you how to use objects in arrays. For this post, I will use the example of people in a phone book.

To create the phone book, you would type:

person[] phoneBook;

This will create an array of people, named phoneBook. Because I did not give phoneBook a size, it is currently pointing to null (nothing). We can fix this problem by saying:

phoneBook = new person[numberOfPeople];

This will give phoneBook the ability to hold numberOfPeople people. What would happen if we then tried to print the elements of phoneBook? Well, we would just print null a bunch of times in a row. To fix this problem, we would need to create a for loop to go through each element of phoneBook and fill them all with the default person. To fill an element of the array with a new person, you would type:

phoneBook[index]= new person();

Now if I wanted to have a value for the person's name, address, and phone number, I would type:

phoneBook[index]= new person(name, address, phoneNumber);

To call a method from a person in phoneBook, I would put phoneBook[index]. and then the method that I want to use. This would allow for me to access the names, addresses, and phoneNumbers of each person in phoneBook.

This is more or less all that you need to know about arrays of objects. Hopefully you have learned a thing or two about java, and I will see you again next week!

12/1/12 How objects really work

A long time ago, I discussed Objects and Instance variables. As you may or may not remember, an Object is not like a primitive data type, but more like a collection of primitive variables that can also be manipulated by the Object. To create an object, you would type:

(Object name) (name) = new (Object name)((parameters));

This code will simply create a (Object name) with the name (name), and starting values (parameters) for the variables in the object that you are creating. Now, what would happen if I tried the following code without a toString method in (Object name)?

System.out.println((name));

Well, you should get something weird like Obj@1cd2e6f. You may then be wondering "what dose this mean?", well it is telling you where in your computers memory you are storing (name). This means that (name) is not actually a (Object name), but a pointer saying where in the memory to look for (name). This may not seem like it makes that much of a difference, but it dose. Let's take the following code:

Object a = new Object(1,"hi");
Object b = new Object(1,"hi");
if(a==b){
 return true;
}
else{
 return false;
}

It seems that this code should return true because the two objects are the exact same, but it dose not. It fails to do so because this code is simply checking if a has the same memory address as b doses. If you want to see if two objects are equal, you will need to create a equals method for that object. This has one other implication, what do you think will be the output of this code:

Person a = new Person("Bob");
Person b = new Person("Joe");
b=a
a.setName("Smith");
System.out.println(b.getName());

It should be Bob, but remember that the name a or b is simply pointing to the object that it is representing, meaning that when you are saying that b = a you mean that b is now pointing to the same place in memory as a is. This means that both person a and b are the same person, and both have the name Smith, so the output would be Smith.

Hopefully you have learned a thing or two about Objects and how they work in Java