04 Variables

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KNOW YOUR VARIABLES

So far we have discussed variables in two contexts:As object state (instance variables)

As local variables (variables declared within a method)

Now we continue to discuss variables as:Arguments (values sent to a method by the calling code)

Return types (values sent back to the caller of the method)

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Declaring a variableJava cares about type. It won’t let you do something bizarre or dangerous, like stuff a Bicycle reference into a Dog variable – what would happen if someone tries to ask the bicycle to bark()?For the compiler to catch these potential problems, Java requires us to declare the type of the variable. Is it an integer? a Dog? a single Character?Variables come in two flavors:

primitives

object references

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Declaring Variables

You must follow two declaration rules:variables must have a

type1

variables must have a

name2

int count;

type name

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Think of Cups!Think of Java variables as different types of cups. Coffee cups, tea cups, beer cups, massive popcorn cups, Double Gulp cups.

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“I’d like a tall Mocha, no make that an int.”

A variable is just a cup. A container. It holds something. It has a size and a type.

When you go to Starbucks you know they have different sized cups, so you order what you want and tell them the size you would like.

shor

t tall

gra

nde vent

i

In Java, primitives come in different sizes, and those sizes have names. The four containers here are for the four integer primitives in Java.

byte

shor

t

int

lon

g

Just as they write your name on your cup in Starbucks, Java also requires you to give your variable a name.

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Java Primitive Types

Type Bit Depth Value Range Example

boolean & char

boolean (JVM-specific) true/false boolean is fun = true;

char 16 bits 0 to 65535 char c = ‘f’;

numeric (all are signed)

byte 8 bits -128 to 127 byte b = 89;

short 16 bits -32768 to 32767 short s = 67;

int 32 bits -2147483648 to 2147483647 int x;

long 64 bits -huge to huge long big = 3456789;

float 32 bits varies float f = 32.5f;

double 64 bits varies double d = 3456.78;

Note the ‘f’. Java assumes anything with a floating point is a double otherwise.

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Avoid Spillage!Now that you know the limits of each of the primitive Java types it is important not to assign a variable of a bigger type into a smaller one. This would be called spillage. And the compiler will complain!

For example:int x = 24;byte b = x; //won’t

work!You can assign a value to a variable in one of several ways including:

• type a literal value after the equal sign (x = 12, isGood = true)

• assign the value of one variable to another (x = y)• use an expression combining the two (x = y + 43)

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Which statements are legal?

int x = 34.5; boolean boo = x; int g = 17; int y = g; y = y + 10; short s; s = y; byte b = 3; byte v = b; short n = 12; v = n; byte k = 128;

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What Are Good Variable Names?

It must start with a letter, underscore (_), or dollar sign ($). You can’t start a name with a number.

After the first character, you can use numbers as well. Just don’t start it with a number.

It can be anything you like, subject to those two rules, just so long as it isn’t one of Java’s reserved words.

You know you need a name and a type for your variables.You know what the primitive types are.

But what can you use for names?Here are the rules for naming variables

int $_doggy

float $_dog64

long _anything

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Reserved Wordspublic static void

boolean charbyte short intlong float double

Don’t use any of these for you own variable names (memorize these)

…or any of these! (don’t memorize these)

Even if you don’t need to know what they mean, you still need to know you can’t use them yourself.

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Remote Controlling Your Dog!

You know how to declare a primitive variable and assign it a value. But now what about non-primitive variables? In

other words, what about objects? There is actually no such thing as an object variable. There’s only an object reference variable. An object reference variable holds bits that

represent a way to access an object. It doesn’t hold the object itself, but it holds something like a pointer, or mapping to where the object is stored on the heap. We do know that

whatever it is, it represents one and only one object. The JVM knows how to use the reference to get to the object.

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Remote Controlling Your Dog!

You can’t stuff an object into a variable. Objects live in one place and one place only – the garbage collectible heap!Although a primitive variable is full of bits representing the

actual value of the variable, an object reference variable is full of

bits representing a way to get to the object.

byt

e sho

rtin

t

lo ng

You use the dot operator (.) on a reference variable to say, “use the thing before the dot to get me the thing after the dot.” Think of it like pressing a button on the remote control for that object.

BARK

Rufus.bark();

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An Object Reference is Just Another Variable Value

Primitive Variable

byte x = 7;

The bits representing 7 go into the variable. (00000111)

0000

0111

Reference Variable

Dog rufus = new Dog( );

The Dog object itself does not go into the Variable!

int Dog

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3 Steps To Making An Object

1 Declare a reference variable Dog rufus = new

Dog( );

2 Create an object

Dog rufus = new Dog( );

3 Link the object and the reference Dog rufus = new

Dog( );

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What else about Object References?

Can a reference variable be programmed to refer to something else – can a Dog reference variable be programmed to reference a Cat object?Can a Dog reference variable be programmed to reference a different Dog object?

What about objects marked as final?

Can a reference variable ever refer to nothing at all?

Is null a value?

If an object is the only reference to a particular object, and then its set to null (deprogrammed), it means that now nobody can get to that object it once referred to.

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Life On The Garbage-Collectible Heap

HEAP

b

c

Book object

Book object

Book b = new Book();Book c = new Book();

References: 2Objects: 2

2

1

Book

Book

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HEAP

b

c

Book object

Book object

Book d = c;


d

1

2

Book

Book

Book

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Book


HEAP

b

c

Book object

Book object

c = b;


d

1

2

X

Book

Book

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Life and Death on the Heap

HEAP

b

c

Book object

Book object

Book b = new Book();Book c = new Book();


2

1

Book

Book

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HEAP

b

c

Book object

Book object

b= c;

Active References: 2Reachable Objects: 1Abandoned Objects: 1

2

1

This guy is toast.

Book

Book


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HEAP

b

c

Book object

Book object

c = null;

Active References: 1null References: 1Reachable Objects: 1Abandoned Objects: 1

2

1

Still toast.

X

Book

Book


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Array VariablesAn array variable is a remote to an array object.

1 Declare an int array variable.

int[ ] nums;

2 Create a new int array with a length of 6, and assign it to the previously declared int[] variable nums

nums = new int[6];

3 Give each elements in the array an int value.

Remember, elements in an int array are just int variables.

nums[0] = 6;nums[1] = 19;nums[2] = 44;nums[3] = 42;nums[4] = 10;nums[5] = 20;

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Array Variables

int int int int int int

0 1 2 3 4 5

int[ ]

6 19 44 42 10 20

nums

int array object (int[ ])

NOTE: The array is an object, even though it’s an array of primitives.

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Array VariablesEvery element in an array is just a variable. In other words, one of the eight primitive variable types or a reference variable.

Anything you would put in a variable of that type can be

assigned to an array element of that type.

So in an array of type int (int[ ]), each element can hold an int.So in a Dog array, each element can hold a remote control to a Dog.

Arrays are always objects, whether they’re declared to hold primitives or

object references.

The array itself is never a primitive.

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Make an Array of Dogs

Dog[ ]

pets

What’s Missing?

1 Declare an Dog array variable.

Dog[ ] pets;

2 Create a new Dog array with a length of 6, and assign it to the previously declared Dog[ ] variable pets.

pets = new Dog[6];

0 1 2 3 4 5

Dog array object (Dog[ ])

Dog

Dog

Dog

Dog

Dog

Dog

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Make an Array of Dogs

Dog[ ]

pets

3 Create new Dog objects, and assign them to the array elements. Remember, elements in a Dog array are just Dog reference variables. We still need Dogs!!!

pets[0] = new Dog;pets[1] = new Dog;pets[2] = new Dog;

0 1 2 3 4 5Dog array object (Dog[ ])

Dog

Dog

Dog

Dog

Dog

Dog

Dog objects

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Control Your Dog

Dog

name

bark()eat()chaseCat()

Dog rufus = new Dog();rufus.name = “Rufus”;

rufus

Dog

Dog object

String

name

What happens if the Dog is in a Dog array?

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Dog ArrayDog

name


Dog[ ] myDogs = new Dog[3];myDogs[0] = new Dog();myDogs[0].name = “Rufus”;myDogs[0].bark(); Dog Remote

chaseCat

bark

eat

When the Dog is in an array, we don’t have an actual variable name (like rufus). Instead we use array notation and push the remote control button (dot operator) on an object at a particular index (position) in the array.

Eg: myDogs[1].bark();myDogs[0]

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Java Cares About Type

Once you’ve declared an array, you can’t put anything in it except things that are of the declared array type.

You can’t put a Cat into a Dog array, or a double into an int

array (spillage, remember?). You can, however, put a byte into an int array, because a byte will always fit into an int-sized cup. This is known as implicit widening. We’ll get into the details later, but for now just remember that the compiler won’t let you put the wrong thing in an array, based on the array’s declared type.

I don’t’ belong here.

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Dog Example

Do you understand everything in this code?

What does this mean?

Dog

name


Education

04 Variables