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Chapter 1

Java Fundamentals -

Arrays and References (updated by Dan Fleck)

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Arrays

• Collection of elements with the same data type• Array elements have an order• Support direct and random access• One-dimensional arrays

– Declaration examplefinal int DAYS_PER_WEEK = 7;

double [] maxTemps = new double[DAYS_PER_WEEK];

– Length of an array is accessible using data field length– Use an index or subscript to access an array element

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Arrays

Figure 1-7Figure 1-7One-dimensional array of at most seven elements

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Arrays

• One-dimensional arrays (continued)– Initializer list example

double [] weekDayTemps = {82.0, 71.5, 61.8, 75.0, 88.3};

– You can also declare array of object references

• Multidimensional arrays– Use more than one index

– Declaration examplefinal int DAYS_PER_WEEK = 7;

final int WEEKS_PER_YEAR = 52;

double[][] minTemps = new double[DAYS_PER_WEEK][WEEKS_PER_YEAR];

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Arrays

Figure 1-8Figure 1-8A two-dimensional array

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Arrays

• Passing an array to a method– Declare the method as follows:

public double averageTemp(double[] temps, int n)

– Invoke the method by writing:double avg = averageTemp(maxTemps, 6);

– Location of array is passed to the method• Cannot return a new array through this value

– Method can modify content of the array

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Variables

• Represents a memory location• Contains a value of primitive type or a reference• Its name is a Java identifier• Declared by preceding variable name with data

typedouble radius; // radius of a sphere

String name; // reference to a String object

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Primitive Data Types

Figure 1-5Figure 1-5Primitive data types and corresponding wrapper classes

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References

• Data type used to locate an object

• Java does not allow programmer to perform operations on the reference value

• Location of object in memory can be assigned to a reference variable

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References

int myVar = 23;MyObject myObj = new MyObject(12, “I was here”);

myVar 23

Memory

myObj 0x121AB450

0x121AB450 12

I was here0x121AB470

.

.

.

.

<--primitive

<--reference

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References

myVar 23

Memory

myObj 0x121AB450

0x121AB450 12

I was here0x121AB470

.

.

.

.

<--primitive (literal)

<--reference

method(myVar);

public void method(int x) { x = 45;}

myVar = ?

Really passes in 23

x 23

.

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References

myVar 23

Memory

myObj 0x121AB450

0x121AB450 12

I was here0x121AB470

.

.

.

.

<--primitive (literal)

<--reference

method(myObj);

public void method(MyObject x) { x.myNumber = 45; x.myString = “Was I here?”;}

myObj = ?myObj.myNumber = ?myObj.myString = ?

Really passes in 0x121AB450

x 0x121AB450

.

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References

myVar 23

Memory

myObj 0x121AB450

0x121AB450 12

I was here0x121AB470

.

.

.

.

<--primitive (literal)

<--reference

int otherVar = myVar;otherVar = 67;

myVar = ?

23otherVar

23

67

67

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References

myVar 23

Memory

myObj 0x121AB450

0x121AB450 12

I was here0x121AB470

.

.

.

.

<--primitive (literal)

<--reference

MyObject otherObj = myObj;otherObj.myNumber = 276;

myObj.myNumber = ?

0x121AB450otherObj

276

276

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References

myVar 23

Memory

myObj 0x121AB450

0x121AB450 12

I was here0x121AB470

.

.

.

.

MyObject otherObj = myObj;

0x121AB450otherObj

276

276I was here

myVar otherObj

Balloon analogy:

•Every object is a balloon

•Every reference is a piece of string

•Every variable can hold on to a

piece of String

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References - Questions

How do I copy an object so I end up with two distinct instances?

What do I do if I want a method to modify the value of an int?for example: divideByTwo(int number) ?

What do I do if I want a method to modify an array?For example: sortArray(int [] myArray) ?

Can I modify the values in an object like this:addFortySeven(myObject.myInt);

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Named Constants

• Have values that do not change• Declared as a variable but using the keyword final

• final static int MAX_VALUE=2345;• final static String DAY1 = “Monday”;