Introduction to Java

ArrayLists

Write a program that reads 3 integers and displays them

for (int i = 0; i < 3; i=i+1) {int val = in.nextInt();System.out.println(val);

}

• Write a program that reads integers contained in a file and display them in reverse order– Input: 5 2 76 8 1– Output: 1 8 76 2 5

• Observations:– Can not start displaying until we have read the last

value • The last value must be displayed first

• Can not forget any values

• Write a program that reads integers contained in a file and display them in reverse order

• Will not work!!– Don’t know how many variables do we need

• Possible solution:– ArrayLists

int v1, v2, …, vn-1,vn

v1= in.nextInt();v2= in.nextInt(); …vn-1 = in.nextInt();vn= in.nextInt();

System.out.println(vn);system.out.println(vn-1);…system.out.println(v1);

ArrayList

• A sequence of 0 or more values under a common name– Values have the same types– Values are distinguished by their position

1 5 3 2 18 11

0 1 2 3 4 5

a

Operations

• createnew ArrayList<Type>();

Creates an ArrayList with size 0;type of each value in the ArrayList is Type

ArrayList <Integer> a = new ArrayList<Integer>();a

Operations

• add (x)

– Increase the size of the ArrayList by 1– Place value x at the end of ArrayList

• size()

– Returns the current size of the ArrayList

add(x)ArrayList <Integer> a = new ArrayList<Integer>();

a.add(1);

a.add(5);

a.add(3);

a.add(2);a.add(18);a.add(11);

1 5 0 1

a a.size() is 2

1 5 3 0 1 2

a a.size() is 3

a a.size() is 0

1 5 3 2 18 11 0 1 2 3 4 5

a a.size() is 6

a 1 a.size() is 1 0

get(i)• return the value at position i of the ArrayList• 0 <= i < size()

• a.get(2); returns 3• a.get(0); returns 1• a.get(12);– Error: 12 is larger than a.size();

1 5 3 2 18 11 0 1 2 3 4 5

a

set(i,x)• sets the value at position i of the ArrayList to x• 0 <= i < size()

• a.set(2, 5);

• a.set(5, 15);

• a.set(6, 7);– Error: 6 is larger than a.size();

1 5 3 2 18 11 0 1 2 3 4 5

a

1 5 5 2 18 11 0 1 2 3 4 5

a

1 5 5 2 18 15 0 1 2 3 4 5

a

remove(i)• removes the value at position from the ArrayList• 0 <= i < size()• Size of the ArrayList is decreased by 1

• a.remove(2);

• a.remove(0);

• a.remove(4);– Error: 4 is larger than a.size();

1 5 3 2 18 15 0 1 2 3 4 5

a

1 5 2 18 15 0 1 2 3 4

a

5 2 18 15 0 1 2 3

a

add(i,x)• Increases the size of the ArrayList by 1• Shifts values at positions i, i+1, … size()-1 to postions i+1, i+2, …

size()• The value x is inserted at position i of the ArrayList• 0 <= i <= size()

• a.add(0, 1);

• a.add(2, 3);

• a.add(6, 12);

• a.add(8);– Error: 8 is larger than a.size();

1 5 2 18 15 0 1 2 3 4 5

a

5 2 18 15 0 1 2 3 4

a

5 2 18 15 0 1 2 3

a

1

3

1 5 3 2 18 15 0 1 2 3 4 5 6

a 12

• Write a program that reads integers contained in a file, input.txt and display them in reverse order– Input: 5 2 76 8 1– Output: 1 8 76 2 5

ArrayList<Integer> numbers = new ArrayList<Integer>();File inpFile = new File(“input.txt");Scanner in = new Scanner(inpFile); while (in.hasNext()) { int val = in.nextInt(); numbers.add(val); } for (int i=numbers.size()-1; i>=0; i=i-1) System.out.println(numbers.get(i));

Write a that reads and executes a sequence of commands as follows

a read an integer value and add it to the end of an ArrayList

d display the content of the ArrayList

f read an integer value and display its position in the arrayList. If the input value is not in the ArrayList, then display a message

r read an integer value and remove it from the ArrayListif the value is not in the ArrayList, display a message

s sort the content of the ArrayList

q terminate execution of the program

• Write a method that sorts the values of an ArrayList in the ascending order– Before sort: 8 2 7 3 6 1– After sort: 1 2 3 5 6 7

• Step through the positions of the ArrayList one by onepositions 0 to size() – 1

• Find the smallest value in the rest of the ArrayListin positions i+1 to size()

• If the smallest value is found at position jMin– If value at position i is larger than the value at position

jMin• Exchange the two values

Exchange

8 2 7 3 6 1 0 1 2 3 4 5

a

i jMin

i jMin

a 1 2 7 3 6 8 0 1 2 3 4 5

Don’t exchange

i jMin

a 1 2 7 3 6 8 0 1 2 3 4 5

Exchange

Exchange

1 2 3 7 6 8 0 1 2 3 4 5

a

i jMin

i jMin 0 1 2 3 4 5

a 1 2 3 6 7 8

Don’t exchange

a 1 2 3 6 7 8 0 1 2 3 4 5

i = 4 = size(); done!