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Introduction to Java

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

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Introduction to Java

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  1. Introduction to Java ArrayLists

  2. Write a program that reads 3 integers and displays them for (inti = 0; i < 3; i=i+1) { intval = in.nextInt(); System.out.println(val); }

  3. 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

  4. 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); • 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

  5. ArrayList • A sequence of 0 or more values under a common name • Values have the same types • Values are distinguished by their position a 1 5 3 2 18 11 0 1 2 3 4 5

  6. Operations • create new ArrayList<Type>(); Creates an ArrayListwith size 0; type of each value in the ArrayList is Type ArrayList <Integer> a = new ArrayList<Integer>(); a

  7. 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

  8. add(x) a a.size() is 0 ArrayList <Integer> a = new ArrayList<Integer>(); a.add(1); a.add(5); a.add(3); a.add(2); a.add(18); a.add(11); a 1 a.size() is 1 0 a 1 5 a.size() is 2 0 1 a.size() is 3 1 5 3 a a 1 5 3 2 18 11 0 1 2 0 1 2 3 4 5 a.size() is 6

  9. 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(); a 1 5 3 2 18 11 0 1 2 3 4 5

  10. 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(); a a a 1 5 3 2 18 11 1 5 5 2 18 11 1 5 5 2 18 15 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5

  11. 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(); a 1 5 2 18 15 0 1 2 3 4 a 1 5 3 2 18 15 a 5 2 18 15 0 1 2 3 4 5 0 1 2 3

  12. 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 a 5 2 18 15 0 1 2 3 4 a 1 5 2 18 15 a 5 2 18 15 3 0 1 2 3 4 5 0 1 2 3 12 a 1 5 3 2 18 15 0 1 2 3 4 5 6

  13. 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()) { intval = in.nextInt(); numbers.add(val); } for (inti=numbers.size()-1; i>=0; i=i-1) System.out.println(numbers.get(i));

  14. 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 ArrayList if the value is not in the ArrayList, display a message s sort the content of the ArrayList q terminate execution of the program

  15. 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 one positions 0 to size() – 1 • Find the smallest value in the rest of the ArrayList in 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

  16. a 8 2 7 3 6 1 0 1 2 3 4 5 Exchange Exchange i i i jMin jMin jMin a 1 2 7 3 6 8 0 1 2 3 4 5 Don’t exchange a 1 2 7 3 6 8 0 1 2 3 4 5

  17. a 1 2 3 7 6 8 0 1 2 3 4 5 Exchange i i jMin jMin a 1 2 3 6 7 8 0 1 2 3 4 5 Don’t exchange 1 2 3 6 7 8 a 0 1 2 3 4 5 i = 4 = size(); done!

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