Array Basics Suppose you need to process daily temperatures for a 12-month period in a science project, would you use 365 variables? You can, but would you? An array is a collection of data values of the same data type. If your program needs to deal with 100 integers, 365 real numbers, etc., you will use an array.

Arrays Variables are Reference Variables Primitive variables contain values Reference variables point at objects byte short int double long float boolean String Applet MessageBox HiLo InputBox etc. char primitive reference Data Type

Primitive Arrays A primitive array stores multiple values of the same primitive data type. rainfall The index of the first position in an array is 0.

Arrays of Primitive Data Types Array Declaration [ ] //variation 1 [ ] //variation 2 Array Creation = new [ ] Example double[] rainfall; rainfall = new double[12]; Variation 1 double rainfall [ ]; rainfall = new double[12]; Variation 2 An array is like an object!

Accessing Individual Elements Individual elements in an array accessed with the indexed expression. double[] rainfall = new double[12]; rainfall rainfall[2] This indexed expression refers to the element at position #2

Example Programs Zadoot … out to reality … ArrayElements1.java Zadoot … out to reality … ArrayElements2.java

Array Lengths double[] rainfall = new double[12]; double annualAverage; double sum = 0.0; int index; for (index = 0; index < rainfall.length; index++) { rainfall[index] = keyboard.nextDouble(); sum += rainfall[index]; } annualAverage = sum / rainfall.length; The public constant length returns the capacity of an array. Phrrud … out to reality … ArrayRain.java Phrrud … out to reality … ArrayAverages.java

Array Bounds Errors Trying to access an array element that does not exist causes a runtime error Negative indices Indices beyond the size Falop … out to reality … ArrayBoundsError.java

Array Initialization Like other data types, arrays can be declared and initialized at the same time. int[] number = { 2, 4, 6, 8 }; double[] samplingData = { 2.443, 8.99, 12.3, , 18.2, 9.00, 3.123, , }; The capacity of the array is set to the number of elements in the list. number.length samplingData.length 4 9 Groeeet … out to reality … ArrayInit.java (Secretly, Java pre-initializes arrays to the default value for the type, e.g., 0 for int arrays.)

References are Pointers A reference variable points to an object So, arrays are like objects, but don't worry about that for now – we’ll come back to it. But it does mean you can Lose an array Have an array variable pointing nowhere Have multiple references to an array Not copy an array with =

Losing an Array Code State of Memory customer A. A. The variable is allocated in memory. A A int[] customer; B B customer = new int[5]; C C B. customer B. The reference to the new array is assigned to customer. Array in Memory Array in Memory C. customer. C. The reference to another array overwrites the reference in customer. Array in Memory Array in Memory

Garbage Collection An array that has no references is garbage collected by the java program Spaaocie … out to reality … ArrayGC.java

The array to no-where An array variable can be explicitly made to point to no data, using the null value A null array is different from an array of length zero. Spaaocie … out to reality … ArrayNULL.java

Having Two References to an Array Code State of Memory A A int[] clemens, twain; B B clemens = new int[5]; C C twain = clemens; A. A. Variables are allocated in memory. clemens twain B. clemens B. The reference to the new array is assigned to clemens. Array in Memory Array in Memory C. clemens customer. C. The reference in clemens is assigned to customer. Dooop … out to reality … ArrayDup.java

Cloning an Array An array can be copied using the clone method It's necessary to cast the clone to the right array type Babababoom … out to reality … ArrayClone.java

Two-Dimensional Arrays Two-dimensional arrays are useful in representing tabular information.

Declaring and Creating a 2-D Array Declaration [][] //variation 1 [][] //variation 2 Creation = new [ ][ ] Example double[][] payScaleTable; payScaleTable = new double[4][5];

Example Programs Ieeei … out to reality … ArrayMatrix.java Ieeei … out to reality … ArrayCalendar.java Ieeei … out to reality … ArrayCube.java

Multi-dimensional Arrays … NOT Java does not really have multi-dimensional arrays Java has arrays of arrays int[][] data = new int[3][5]; is shorthand for int[][] data = new int[3][]; data[0] = new int[5]; data[1] = new int[5]; data[2] = new int[5];

Multi-dimensional Arrays in RAM int[][] data = new int[3][5]; Zuuu … out to reality … ArrayRAMMatrix.java Zuuu … out to reality … ArrayRAMCube.java

Irregular Arrays int[][] weirdData = new int[3][]; weirdData[0] = new int[5]; weirdData[1] = new int[4]; weirdData[2] = new int[7];

Irregular Arrays int[][] weirdData = new int[3][]; weirdData[0] = new int[5]; weirdData[1] = new int[4]; weirdData[2] = new int[7]; weirdData.length == 3 weirdData[1].length == 4 Jioooul … out to reality … ArrayIrreg1.java

Irregular Arrays … null int[][] data = new int[3][]; data[0] = new int[5]; data[1] = new int[4]; data[2] = null; Jioooul … out to reality … ArrayIrreg2.java

Multi-dimensional Array Initialization Like 1D arrays, it is possible to declare and initialize a multi-dimensional array at the same time. Frong … out to reality … ArrayMDInit.java Frong … out to reality … ArrayCubeInit.java

Passing Arrays to Methods - 1 Code State of Memory minOne = searchMinimum(arrayOne); public int searchMinimum(float[] number)) { … } A A At before searchMinimum A A arrayOne A. A. Local variable number does not exist before the method execution

Passing Arrays to Methods - 2 Code State of Memory minOne = searchMinimum(arrayOne); public int searchMinimum(float[] number)) { … } arrayOne B B The address is copied at B B number B. B. The value of the argument, which is an address, is copied to the parameter.

arrayOne number While at inside the method C C Passing Arrays to Methods - 3 Code State of Memory minOne = searchMinimum(arrayOne); public int searchMinimum(float[] number)) { … } C C C. C. The array is accessed via number inside the method.

arrayOne number Passing Arrays to Methods - 4 Code State of Memory minOne = searchMinimum(arrayOne); public int searchMinimum(float[] number)) { … } D D arrayOne At after searchMinimum D D D. D. The parameter is erased. The argument still points to the same object.

Example Programs Flunot … out to reality … ArrayParamAvg.java Flunot … out to reality … ArrayParam1.java

Returning Arrays Array variables in methods exist until the method ends, but the array data lives while referenced An array variable can be returned from a method The receiving array variable then refers to the array data, and the array persists Wrrbbrack … out to reality … ArrayParam2.java

Local arrays Array variables in methods exist until the method ends The array data referred to by such an array variable is lost and garbage collected when the method ends Dessserts … out to reality … ArrayLocalGC.java