Section 2.4. Variables

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2.4. Variables

We have used Java to do calculations and to append strings, but we have not stored the results. The results would be in memory but we don't know where they are in memory and we can't get back to them. On a calculator we can store the result of one calculation to memory (Figure 2.4). We can then use that stored value in other calculations. On a calculator you also have access to the result of the last calculation.

Figure 2.4. A calculator with a number in memory.

2.4.1. Declaring Variables

On a computer we can store many calculated values by naming them. We can then access those values by using the variable names. The computer takes care of mapping the name to the memory location (address) that stores the value. We call naming values declaring a variable.

When you declare a variable in Java you specify the type for the variable and a name (type name). You need to specify a type so that Java knows how many bits to reserve in memory and how to interpret the bits. You can also assign a value to a variable using the '=' operator and provide a value or an expression (type name = expression). Don't read '=' as equals but assign the value of the right side to the variable on the left (which makes using '==' for 'is equal to' or 'is equivalent to' make more sense). The bits in the variable will be set to represent the value. We will use the type int for storing integer values (numbers without decimal points) and the type double for storing floating point values (numbers with decimal points).

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2.4.2. Using Variables in Calculations

What if you want to calculate the total bill for a meal including the tip? You would start with the bill value and multiply it by the percentage you want to tip (20%), that would give you the tip amount. You could then add the tip amount to the bill total to get the total amount to leave.

We will use the type double to store the bill amount, tip, and total amount since these can have decimal points. If we also wanted to calculate the cost per person we could divide the total by the number of people. We could use an integer variable to hold the number of people.

> int numPeople = 2; > System.out.println(numPeople); 2 > double bill = 32.45; > System.out.println(bill); 32.45 > double tip = bill * 0.2; > System.out.println(tip); 6.490000000000001 > double total = bill + tip; > System.out.println(total); 38.940000000000005 > double totalPerPerson = total / numPeople; > System.out.println(totalPerPerson); 19.470000000000002

Common Bug: Mistyping

You just saw a whole bunch of Java statements, and some of them are pretty long. What happens if you type one of them wrong? DrJava will complain that it doesn't know what you mean, like this:

> double tip = bil * 0.2; Error: Undefined class 'bil'

It's no big deal. Use the up arrow on the keyboard to bring up the last thing you typed into DrJava and then use the left arrow to get to the place with the error and then fix it. You can use the up arrow to get to any of the commands you have typed in the interactions pane since you started DrJava.

So, each person would need to pay $19.47, which they would probably round up to $19.50.

Making it Work Tip: Variable Names

By convention the first word in a variable name is lowercase. So if the variable name is just one word, then the whole thing is lowercase, such as bill. The first letter of each additional word in a variable name should be uppercase, as shown by the variables named numPeople and totalPerPerson. This is a Java convention (usual way something is done), and it will make your programs easier to read.

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We don't have to print out the value of the variable after we assign a value to it. We are doing that so that you see that the computer does return a value when you use the name of a variable. What about the extra amount for the final answer? The answer should be just $19.47 per person. If we look back at the printing of the tip amount we see where this first occurred. Floating point numbers do not always give exact results.

2.4.3. Memory Maps of Variables

In Java when you declare variables to be of the type int or double you are asking the computer to set aside space for a variable of that type (32 bits for int and 64 for double) and to remember the address of that space. When you assign a value to a variable using the '=' operator you change the value in that space to represent the new value. The code int numPeople reserves 32 bits of space and associates the name "numPeople" with that reserved space (Figure 2.5). The code = 2 sets the value of that space to the integer value 2. The code double bill reserves 64 bits of space and associates the name "bill" with that space. The = 32.45 changes the values in the reserved space to represent the value 32.45.

Figure 2.5. Declaring primitive variables and memory assignment.

When we print out a variable name using System.out.println(bill); the computer looks up the name bill to find the address of that variable in memory and prints the value stored in that space. It knows how many bytes to use and how to interpret the bytes in calculating the value based on the declared type of the variable.

How would you calculate the cost of a shirt that was originally $47.99, but is now 40% off? And, what if you also had a coupon for an additional 20% off the sale price? First you would need to determine the first discount amount by multiplying 40% (0.40) times the original price. Next, calculate the first discount total by subtracting the first discount amount from the original price. Then calculate the second discount amount by multiplying 20% (0.20) times the second discount amount. The second discount total is the first discount total minus the second discount amount. We would need variables to hold the first discount amount, first discount total, second discount amount, and second discount total. What type should those variables be declared to be? Since they have fractional parts they can be declared as double.

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> double originalPrice = 47.99; > double firstDiscountAmount = originalPrice * 0.40; > System.out.println(firstDiscountAmount); 19.196 > double firstDiscountTotal = originalPrice - firstDiscountAmount; > System.out.println(firstDiscountTotal); 28.794 > double secondDiscountAmount = firstDiscountTotal * 0.20; > System.out.println(secondDiscountAmount); 5.758800000000001 > double secondDiscountTotal = firstDiscountTotal - secondDiscountAmount; > System.out.println(secondDiscountTotal); 23.0352

When these statements are executed 64 bits of space is reserved for each variable declared as a double. So how much memory does this calculation take? We have declared five doubles so we have used 5 times 64 bits of space. Each byte has eight bits in it so how many bytes have we used? How much memory does your computer have and how much of it have you used? If your computer has 128 megabytes of memory then that is 128,000,000 bytes of memory and we used 40 bytes then we have only used 0.0000003125% of memory. That isn't very much. We can declare lots of variables and still not use up all of the memory.

Each time we use the variable name above the computer substitutes the value in the memory location associated with that name. What are the values in each of the five declared variables when these statements are finished?

2.4.4. Object Variables

Variables that are declared to be of any of the primitive types: byte, short, int, long, float, double, boolean or char reserve space and associate the variable name with the starting location of that space. Variables that are declared to be of any other type are object variables. This is because all other types inherit from the class Object.

You can think of inheritance as saying that one class "is a kind of" another class, like saying that a dog is a kind of mammal (Figure 2.6). If you need a mammal you can use a dog, but if you need a dog another mammal (like a cat) won't do. Because a dog is a kind of mammal we know that it has the same characteristics that a mammal does such as breathing oxygen, bearing live young, having hair, etc. We say that it inherits characteristics from mammal. The String class is a child of the Object class so it is a kind of object (Figure 2.6). All of the classes that you define will inherit from the Object class either directly or indirectly.

Figure 2.6. Showing the parent and child relationship between mammal and dog (left) and `Object` and `String` (right). (This item is displayed on page 29 in the print version)

When you declare a variable you specify the type and a name type name; or type name = expression;. What if you want to declare a variable that will refer to a string? What type can you use? Well it can't be int or double because those represent numbers. It can't be char because that represents a single character.

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Java has a class String that is used to represent character strings. The String class inherits from the Object class. So to declare a variable that represents a string of characters use: String name;.

Object variables reserve space for something which is a reference to an object. A reference isn't the address of the object in memory. It is more like a Dewey Decimal System number. Once you know the Dewey Decimal System number for a book, you can find the book on the library shelves. An object reference gives the computer a way to find an object in memory.

Object variables do not reserve space for the object. If the object variable doesn't reference an object yet it has the value null.

> String test; > System.out.println(test); null > test = "Hi"; > System.out.println(test); Hi > test = new String("Bye"); > System.out.println(test); Bye

When the variable test was declared as type String space was reserved for an object reference and the value of the test variable was set to null (Figure 2.7). The default value for an object variable is null which means it isn't referring to any object yet. The compiler will create a String object when it sees characters enclosed in double quotes so the "Hi" creates an object of the String class and sets the characters in that String object to be the characters "Hi". The code test = "Hi" changes the value of the space reserved for the object reference from null to a reference to the String object with the characters "Hi".

Figure 2.7. Declaring object variables and memory assignment. (This item is displayed on page 30 in the print version)

What happens to the String object with the characters "Hi" in it when you changed the variable test to refer to the new String object with the characters "Bye"? Java keeps track of used space and if there are no valid references to the used space it will put it back into available space. This is called garbage collection.

The fact that Java automatically handles freeing used memory when it is no longer needed is one of the advantages to Java over languages like C++ which required the programmer to free memory when it was no longer needed. Programmers aren't very good at keeping track of when memory is no longer needed, so many programs never free memory when it is no longer needed. This is called a memory leak, and it is why some programs use more and more memory while they are running. Sometimes programmers free memory when it is still being used, which can cause major problems, such as incorrect results, and even cause your computer to crash.

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Making it Work Tip: Variables versus Literals

Notice that we have changed the value of the variable test in Figure 2.7 several times. We call items like test variables because the values inside of them can change. This is different from literals such as the string literal "Hi" in that the value of that won't change. You can set the value of a variable to a literal but you can't set the value of a literal to a variable.

2.4.5. Reusing Variables

Once we have declared variables we can reuse them by assigning new values to them.

> String myName = "Mark"; > System.out.println(myName); Mark > myName = "Barb"; > System.out.println(myName); Barb

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This actually means to first set the variable myName to refer to the String object with the characters "Mark" in it. Then it changes the variable myName to refer to another String object with the characters "Barb" in it. The first String object with the characters "Mark" in it still exists and can be garbage collected (reused as available space) (Figure 2.8).

Figure 2.8. Shows creation and reuse of an object variable.

You can't declare the same variable name twice. Declare the name one time (by specifying the type and name) and then you can use it many times.

> String myName = "Mark"; > System.out.println(myName); Mark > String myName = "Sue"; Error: Redefinition of 'myName'

The binding between the name and the data only exists (a) until the name gets assigned to something else or (b) you quit DrJava or (c) you reset the interactions pane (by clicking on the RESET button). The relationship between names and data in the interactions pane only exists during a session of DrJava.

Common Bug: Redefinition Error

You can't declare a variable with the same name more than once in the interactions pane. If you do you will get a "Redefinition Error''. If you want to "start over'' click the RESET button in DrJava to let it know that you want to get rid of all currently defined variables. Or, just remove the types and you won't be redeclaring the variables, just changing their values (reusing them).

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2.4.6. Multiple References to an Object

You can have several variables that reference an object. You can use any of the references to access the object.

> String name1 = "Suzanne Clark"; > System.out.println(name1); Suzanne Clark > String name2 = name1; > System.out.println(name2); Suzanne Clark

When the compiler encounters the characters inside the pair of double quotes it creates a String object. The code String name1 creates a variable name1 that will refer to this string object. Print out name1 to see what it refers to using System.out.println(name1);. Next the code String name2 = name1; creates another variable name2 and sets the value of it to refer to the same string. Printing the new variable name2 will result in the same string being printed (Figure 2.9).

Figure 2.9. An object with multiple references to it.

An object can only be garbage collected when there are no current references to it. To allow the String object with the characters "Suzanne Clark" in it to be garbage-collected set the variables that refer to it to null.

> name1 = null; > System.out.println(name1); null > System.out.println(name2); Suzanne Clark > name2 = null; > System.out.println(name2); null

Now all references to the String object are set to null so the object can be garbage-collected (Figure 2.10).