7.1 Arrays

7.1.1 Introduction to arrays • Any array is a collection of objects or

primitives• Useful when the number of reference variables

is large or can’t be determined until Runtime

7.1.2 Variables in arrays • An array is a collection of variables• The variables are either a primitive or a

reference to an object• There is also a reference to the array itself

7.1.3 Index

• Arrays are a built-in object• All members of the array are of the same type and size• Accessing array members is quick and efficient

7.1.4 Arrays of primitives

• This example shows two arrays of primitives

7.1.5 Arrays of objects

• Arrays of objects are a collection of references

• The array variable references the array object

• Each member of the array is itself a reference to another object

7.2.1 Creating arrays

• Step 1: Declare the array reference variableint[] arrayOfInts;

int arrayOfInts[];

• Step 2: Create the array using the ‘new’ operatorarrayOfInts = new int[5];

• The array is then initialized to default values

7.2.2 Use subscripts to access elements of an array

• Access the members of the array using an index

• Index is an int• The index for the first member is 0• The following code creates an array of ints,

then assigns a value to the 3rd element

Int[] myArray;

myArray = new int[5];

myArray[2] = 57;

7.3.1 Array of primitives

• Arrays of primitives can be assigned initial values when they are declared

• The array members can still be changed if necessary

7.3.2 Array of objects

• Initializing arrays of objects requires that the objects be instantiated first, ie:

Account[] accountList;

accountList = new Account[2];

accountList[0] = new Account(203.50);

accountList[1] = new Account(4500.00);

7.4.1 Accessing array elements

• The index value must resolve to an int

• Can be an explicit value of the result of a calculationloopValue = arrayOfInts[index – 1];

• System.arraycopy() can be used to copy the contents of one array to anotherSystem.arraycopy(arrayOfInts, 0, anotherArrayOfInts, 0,

arrayOfInts.length);

7.4.2 Using arrays

• An array can be a way of passing a large number of arguments to a method

• Parallel arrays can be used to store related informationString names = new String[20];

String addresses = new String[20];

String phone = new String[20];

Char grades = new char[20];

• Array members can be searched and sorted

7.5.1 Multidimensional arrays

• Multidimensional arrays can be used instead of parallel arrays

• Declared with multiple sets of ‘[]’int[][] multiArray = new int[20][30];

• Use a separate index for each ‘dimension’loopValue = multiArray[5][20];

7.5.2 Traversing a multidimensional array

• Nested ‘for’ loops can be used to index all the elements of an array

public int[][] myArray = new int[4][5];

for(x = 0; x < 4; x++) {

for(y = 0; y < 5; y++) {

myArray[x][y] = x * y;

}

}