Java Arrays and Array Lists

8/6/2019 Java Arrays and Array Lists

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One variable to maintain an entire sequence of values of the same type


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J ava language has a built-in array structure found in java.lang that is has its own special syntax.

The array is used to store a list of values of the sametype.

The values or elements may be a primitive data type

such as int or double, or a Class objects such asRectangles or Strings.

Example: one array could contain 10 20 13 33 5 and adifferent array could contain swan pig dog


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B efore using the array, your code must use thenew operator to set aside space for the array

indicating the data type and maximumnumber of values or elements to be stored inthe structure.

variable-identifier = new data-type [ max-size];double [ ] price; // declaration of listprice = new double [5]; //allocate space


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A llocation of the array sets all of the arrayelement values to the default for the

specified data type.double [ ] price; // declaration of listprice = new double [5]; //allocate space

p0.0 0.0 0.0 0.0

reference

price

price[0] price[1] price[2] price[3] price[4]

0.0


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A rray indexing, like the positions of charactersin Strings, starts with O. A rrays allow access

to any element using its position called index.

price[0] = 4.95; // stores 4.95 in first price

4.95 0.0 0.0 0.0

reference

price


0.0


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U se the array elements as if they are variablesin their own right.

price[0] = 4.95; // stores 4.95 in first priceprice[2] = price[0] + 0.50;

4.95 0.0 5.45 0.0

reference

price


0.0


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U se the array-variable.length variable to getthe number of elements. The price.length is 5

for our price array.for(int index = 0; index < price.length ; index++)

{ price[index] = price[index] + 1.00; }

5.95 1.0 6.45 1.0

reference

price


1.0


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You can declare, allocate and initialize in onestatement.

int [] test = { 100, 90, 85, 92};

100 90 85

reference

test

test[0] test[1] test[2] test[3]

92


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U se the .clone() method to create a new arrayof the same size and copy the values into that

array. Note that you have to cast the clone.int [] test = { 100, 90, 85, 92};int [] twin= (int []) test.clone(); //ref is different

100 90 85reference2

twin twin[0] twin[1] twin[2] twin [3]

92

100 90 85reference1

testtest[0] test[1] test[2] test[3]

92


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You can use a loop to copy the number of values from onearray (called the source array) into another array (calledthe target array) or use System.arraycopy().

System.arraycopy(fromIndex, source, toIndex, target, number);

int [] test = { 100, 90, 85, 92};

int [] part = new int[5];System.arraycopy( 1, test, 3, part, 2);

0 0 0reference

partpart[0] part[1] part[2] part[ 3]

90

part[4]

85


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You can use a loop to copy the values from the source array into the targetarray using a loop, but it takes a lot more code!

int [] test = { 100, 90, 85, 92};int [] part = new int[5];int fromIndex = 1; // source array index to copy fromint toIndex = 3; // target array s index to copy toint number = 2; // number of elements to copyfor(int n = 0; n < number; n++){

part[toIndex] = test[fromIndex];toIndex++;fromIndex++;

}


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Rectangle [] boxlist = new Rectangle [3];boxlist[0] = new Rectangle(10, 20, 30, 40);

boxlist[1] = new Rectangle(50, 30, 10, 20);boxlist[2] = new Rectangle(15, 35, 20, 45);

reference

boxlist boxlist[0] boxlist[1] boxlist[2]

x=10, y=20,w=30,h=40

x=50, y=30w=10, h=20

x=15, y=35w=20, h=45


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A rrayL ist class manages a sequence of objects (not primitivedata types).The A rrayL ist template specifies the class of objects to bestored in the sequence.The template type is specified in angle brackets < >when theclass variable is declared.A rrayL ist indexes start with 0, just as String and arrays do.

A rrayL ist words = new A rrayL ist();A rrayL istboxes = new A rrayL ist();

A rrayL ist accts ;accts = new A rrayL ist();


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void .add(E element) adds element to end of list

void .add(E element, int index) inserts element into the list at indexposition and moves remaining elements over (re-indexes them).

boolean .contains(o bject o bj) returns true if element is in list and false,if not.

E .get(int index) returns the element found at index position

E .remo ve(int index) removes and returns element at index andmoves remaining elements back (re-ndexes them).

E .set(int index , E element) replaces the element at index positionwith the specified element and returns old element.

int .size() returns number of elements in the list


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String x = towns.remove(2); // indexed from OSystem.out.println("removed " + x);System.out.println(towns);String y = towns.remove(2);System.out.println( removed + y);System.out.println(towns);

Output:

removed L aurens[Hartsville, Columbia, Greenville]removed Greenville[Hartsville, Columbia]


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A rrayL ist boxlist = new A rrayL ist();boxlist.add(new Rectangle(10, 20, 30, 40) );Rectangle box = new Rectangle(20, 20, 30, 30);boxlist.add(box);boxlist.add(new Rectangle(4, 5, 6, 7));box = new Rectangle(1,1,1,1);boxlist.add(box);System.out.println(boxlist);

Output:[java.awt.Rectangle[x=10,y=20,width=30,height=40], java.awt.Rectangle[x=20,y=20,width=30,height=30], java.awt.Rectangle[x=4,y=5,width=6,height=7], java.awt.Rectangle[x=1,y=1,width=1,height=1]]

Java Arrays and Array Lists

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