University of Washington, CSE 142

Lab 9: Final Exam Practice

Except where otherwise noted, the contents of this document are Copyright 2013 Stuart Reges and Marty Stepp.

lab document created by Marty Stepp, Stuart Reges and Whitaker Brand

Basic lab instructions

Today's lab

Goals for today:

Exercise : reference mystery practice-it

What four lines of output are produced by the following program?

public class ReferenceMystery {
    public static void main(String[] args) {
        int y = 1;
        int x = 3;
        int[] a = new int[4];
        mystery(a, y, x);                                             // 2 3 [0, 0, 17, 0][^0-9,]+
        System.out.println(x + " " + y + " " + Arrays.toString(a));   // 3 1 [0, 0, 17, 0][^0-9,]+
        x = y - 1;
        mystery(a, y, x);                                             // 1 0 [17, 0, 17, 0][^0-9,]+
        System.out.println(x + " " + y + " " + Arrays.toString(a));   // 0 1 [17, 0, 17, 0][^0-9,]+
    }

    public static void mystery(int[] a, int x, int y) {
        if (x < y) {
            x++;
            a[x] = 17;
        } else {
            a[y] = 17;
        }
        System.out.println(x + " " + y + " " + Arrays.toString(a));
    }
}

Exercise : array simulation practice-it

Consider the following method:
public static void mystery(int[] array) {
    for (int i = 0; i < array.length; i++) {
        array[i] = i * array[i];
    }
}

Indicate in the right-hand column what values would be stored in the int[] array after mystery() executes with the int[] array in the left-hand column passed in as a parameter.

{}
{}
{7}
{0}
{3, 2}
{0, 2}
{5, 4, 3}
{0, 4, 6}
{2, 4, 6, 8}
{0, 4, 12, 24}

Exercise : inheritance mystery practice-it

Assume the following classes have been defined:

public class A extends B {
    public void method2() {
        System.out.println("a 2");
    }
}
public class D extends B {
    public void method1() {
        System.out.println("d 1");
    }
}
public class C {
    public String toString() {
        return "c";
    }

    public void method1() {
        System.out.println("c 1");
    }

    public void method2() {
        System.out.println("c 2");
    }
}
public class B extends C {
    public String toString() {
        return "b";
    }

    public void method2() {
        System.out.println("b 2");
    }
}

continued on the next slide...

Exercise - inheritance mystery

b
c 1
a 2
b
c 1
b 2
c
c 1
c 2
b
d 1
b 2

Consider the code below that uses these classes.
Write each line of its output in the boxes at right.

C[] elements = {new A(),
                new B(),
                new C(),
                new D()};
for (int i = 0; i < elements.length; i++) {
    System.out.println(elements[i]);
    elements[i].method1();
    elements[i].method2();
}

Exercise : printStrings practice-it

Write a method called printStrings that takes as a parameter a Scanner holding a sequence of integer/string pairs and that prints to System.out one line of output for each pair with the given string repeated the given number of times.

You should solve this problem in Practice-It!

Exercise : reverseLines practice-it

Write a method called reverseLines that takes a Scanner containing an input file as a parameter and that echoes the input file to System.out with each line of text reversed.

Example input Example output
If this method works properly,
the lines of text in this file
will be reversed.

Remember that some lines might be blank.
,ylreporp skrow dohtem siht fI
elif siht ni txet fo senil eht
.desrever eb lliw

.knalb eb thgim senil emos taht rebmemeR
Hint: check out lab 6 to review using Scanners over Files effectively. Check out lab 5 to review String processing.

Exercise : isAllEven practice-it

Write a method called isAllEven that takes an array of integers as a parameter and that returns whether or not all of the values are even numbers (true for yes, false for no).

Input
Output
{18, 0, 4, 204, 8, 4, 2, 18, 206, 1492, 42} true
{2, 4, 6, 8, 10, 208, 16, 7, 92, 14} false

Hint: check out lab 7 to review how to deal with arrays!
Another hint: When can we safely say that every number is even? When can we safely say that not every number is even?

Exercise : removeShorterStrings practice-it

Write a method called removeShorterStrings that takes an ArrayList of strings as a parameter and that removes from each successive pair of values the shorter string in the pair. If there is a tie (both strings have the same length), your method should remove the first string in the pair. If there is an odd number of strings in the list, the final value should be kept in the list.

Initial Contents of the ArrayList
Final Contents of the ArrayList
{"four", "score", "and", "seven", "years", "ago"} {"score", "seven", "years"}
{"here", "comes", "the", "sun", ":)"} {"comes", "sun", ":)"}

If you need reference to ArrayList methods, consult the cheat sheet! To review ArrayLists in general, check out Monday's slides!

Exercise : append practice-it

Write a method called append that accepts two integer arrays as parameters and that returns a new array that contains the result of appending the second array's values at the end of the first array.

For example:

public static void main(String[] args) {
    int[] list1 = {2, 4, 6};
    int[] list2 = {1, 2, 3, 4, 5};
    int[] combined = append(list2, list1);
    // combined stores {1, 2, 3, 4, 5, 2, 4, 6}
}

You can review arrays in lab 7!

Exercise : longestSortedSequence practice-it

Write a method called longestSortedSequence that accepts an array of integers as a parameter and that returns the length of the longest sorted (nondecreasing) sequence of integers in the array.

For example:

public static void main(String[] args) {
   int[] array = {3, 8, 10, 1, 9, 14, -3, 0, 14, 207, 56, 98, 12};
   int longest1 = longestSortedSequence(array);
   // longest1 is 4 (because of -3, 0, 14, 207)
   
   int[] array2 = {17, 42, 3, 5, 5, 5, 8, 2, 4, 6, 1, 19}
   int longest2 = longestSortedSequence(array2);
   // longest2 is 5 (because of 3, 5, 5, 5, 8)

You can review arrays in lab 7!

If you finish them all...

If you finish all the exercises, try out our Practice-It web tool. It lets you solve Java problems from our Building Java Programs textbook.

You can view an exercise, type a solution, and submit it to see if you have solved it correctly.

Choose some problems from the book and try to solve them!