Practice Problems: Static & instance methods

Transcription

1 Practice Problems: Static & instance methods The program and its execution are due via D2L. Late programs are not accepted. 1. Tracing programs a. For each program (total 3) below, show what is displayed on the screen when the code executes. If possible, analyze the result before you run the program and explain why the result comes out like that. public class HelloWorld public static void main(string [] args) String s = HelloWorld.getHello(); System.out.println(s); s = gethello(); System.out.println(s); public static String gethello() return "Hello, world!"; // call the unique static method // of class HelloWorld // call the unique static method // of class HelloWorld when this // call is inside the file/class public class NestedCalls public static boolean mystery1(int a) if(a > 0 && mystery2(-a)) return true; return false; public static boolean mystery2(int b) if(b + 2 > 0) return false; return true; public static void main(string [] args) System.out.println("main"); int x = 3; if(mystery1(x)) System.out.println("mystery1 returns true"); System.out.println("mystery2 = " + mystery2(x));

2 public class NamingAndScope public static void main(string [] args) int x = 3; x = x(); System.out.println(x); x = x(x+1); System.out.println(x); for(int i=0; i<2; i++) x = x(x+1); System.out.println(x); public static int x() int x = 5; return x + 7; public static int x(int x) // method overloading, compared with the above // return x + 1; b. What gets printed to the screen when we execute the MysteryClient class on the left? public class MysteryClient public static void main( String [] args) Mystery m = new Mystery("hi"); m.display(" again"); public class Mystery public String str = null; public Mystery(String s) str = s; public void display(string s) System.out.println(str + s); c. What gets printed to the screen when we execute the MysteryClient2 class on the left?

3 public class MysteryClient2 public static void main( String [] args) Mystery2 m = new Mystery2(15, 10); m.display(5); Mystery2 m2 = new Mystery2(15, 10); m2.reduce(5); m2.display(2); public class Mystery2 public int x = 0; public int y = 0; public Mystery2(int num1, int num2) x = num1; y = num2; public void display(int z) if(x%z == 0 && y%z == 0) System.out.println("divides"); else System.out.println("too bad"); public void reduce(int z) x = x / z; y = y / z; d. What are the values of the variables in main() at POINT 1? and at POINT 2? and 3? public class MysteryClient3 public class Mystery3 public static void main( public int [] arr = null; String [] args) Mystery3 m1 = new Mystery3(); Mystery3 m2 = new Mystery3(); // POINT 1 m1.setarr(2, 1); m2.setarr(4, 3); // POINT 2 public void setarr( int len, int val) arr = new int[len]; for(int i=0; i<len; i++) arr[i] = val; int x = m1.getval(1); int y = m1.getval(1); int z = m2.getval(0); // POINT 3 public int getval(int pos) arr[pos]++; return arr[pos]; 2. Develop MethodSignature.java includes the following static methods.

4 a. Write a method sum() that takes a positive nonzero integer value n1 and a positive nonzero integer value n2 as the argument. The program should return the sum of all the integers from 1 up to n1 (include n1) with a step size n2. Some examples... int r = sum(5, 1) will store 15 in r. Note that: this is equal to = 15 int r = sum(11, 3) will store 22 in r. Note that: this is equal to = 22 b. Write a method stair( ) that takes integer values of step, length of run, and height of rise. The method will print out the entire stairwell. For example, a call of stair(2, 5, 3) will print out c. Phonewords are mnemonic phrases represented as alphanumeric equivalents of a telephone number. In many countries, the digits on the telephone keypad also have letters assigned. By replacing the digits of a telephone number with the corresponding letters, it is possible to form a whole or partial word, an acronym, abbreviation, or some other alphanumeric combination. Toll-free telephone numbers are often branded by using phonewords; some firms use easily memorable vanity telephone numbers as the brands for their flagship products or names for the entire companies, such as Contacts, Flowers, FREE- 411, GOT-JUNK, BATTERY, etc. Here, the computer translates words to phone numbers. The user will enter a phone number as a String, such as Captial" for " Create a method ``phoneword ( ) that takes a phone word in String x as an argument and returns its digits (in the format of String). Example 1: String s = phoneword( Flowers"); will store " in String s. Example 2: String s = phoneword( Contacts");

5 will store " in String s. d. Write a method lengthofmonth( ) that takes a year and a month as the argument. It will return how many days in that month. If it is February, the method needs to calculate if it is a leap year. In the Gregorian calendar, three criteria must be taken to identify leap years: I. The year is evenly divisible by 4; II. If the year can be evenly divided by 100, it is NOT a leap year, unless; III. The year is also evenly divided by 400. Then it is a leap year. The method must support both String input and numeric input. Overloading is needed. For example lengthofmonth(2000, 2) will return 29 but ``lengthofmonth( 1900, February ) will return 28. e. Write a method checkpalindrome( ) that takes a String s as the argument. It will return whether a word spelled in the textbox is palindrome (i.e., true or false). A palindrome is a word that is spelled the same forward and backward. For example, rotor" and redder" are palindromes, but motor" is not. f. Write a method myreverse( ) that takes a String s as the argument. It will return the reversed content of this string and append to the original one. The text will be a palindrome String. For instance, given a call myreverse( abc ) will return a String abccba. g. Write a method primelist( ) that takes a positive nonzero integer N as the argument. It will printout the first 10 prime numbers that are larger than N. For example, a call of primelist(1000) will print out 1009, 1013, 1019, 1021, 1031, 1033, 1039, 1049, 1051, and h. Write a method isperfect( ) that takes a positive nonzero integer N as the argument. It will return true/false and indicates whether the number N is perfect. A number is perfect if it is equal to the sum of all of its factors (not including itself as a factor, but including 1 as a factor). 6 is a perfect number, because its factors are 1, 2, and 3, and = 6. i. Write a method myluckynumber( ) that takes a positive nonzero integer N stored in the String format as the argument. It will return the digit sum of this number N in the String format. If this sum is over 10, repeat the calculation of that digit sum, until the final singleton digit. That is your lucky number today. For instance, a call myluckynumber( ) will return the answer 3 ( = 21 and 2+1 = 3). Test the program with a real big number like j. Write a method legalpin( ) that takes a String PIN as the argument and returns a Boolean value to indicate whether this PIN is legal. A legal PIN is:

6 (1) must be at least 8 characters in length (2) must have 3 of 4 characteristics below: lowercase character(s) uppercase character(s) numeric character(s) Special characters %, $, or # For example: Abc2 or ABCDEFG% is not, but ABCdef23$ and 0123ABC$ are legal. k. Write a method reverse( ) that takes a String s as the argument. It will return the reversed content of this string (in the backward direcdtion). For example, a call reverse( abc ) will return cba. l. (Bonus) Write a method myreplace( ) that takes a positive nonzero integer P and three Strings X, Y, and Z as the argument, and returns the String of X after locating the P th appearance of Y and replacing it by String Z. If there is no such P th appearance of Y inside X, return null. For example, myreplace( abcabcabc, abc, 123, 3) will return abcabc123 and myreplace( abcabcabc, abc, 123, 4) will return null. -- Hint: Play the role of the computer and think about how the problem can be solved step-by-step in the program. Run these testing cases. If your program cannot obtain the desired result, debug! System.out.println(sum(5,1)); System.out.println(sum(11,3)); stair(2, 5, 3); System.out.println(phoneword("1-800-Flowers")); System.out.println(phoneword("1-800-Contacts")); System.out.println(lengthOfMonth(2000,2)); System.out.println(lengthOfMonth("1900", "February")); System.out.println(checkPalindrome("rotor")); System.out.println(checkPalindrome("redder")); System.out.println(checkPalindrome("motor")); System.out.println(myReverse("abc")); primelist(1000); System.out.println(isPerfect(6)); System.out.println(myLuckyNumber("290172")); System.out.println(myLuckyNumber(" ")); System.out.println(legalPin("Abc2")); System.out.println(legalPin("ABCDEFG%")); System.out.println(legalPin("ABCdef23$")); System.out.println(legalPin("0123ABC$")); System.out.println(reverse("abc")); System.out.println(myReplace("abcabcabc", "abc", "123", 1)); System.out.println(myReplace("abcabcabc", "abc", "123", 2)); System.out.println(myReplace("abcabcabc", "abc", "123", 3)); System.out.println(myReplace("abcabcabc", "abc", "123", 4)); 3. Object & instance methods Write the Adder class so that the code in the Arithmetic class displays the correct sums:

7 4 + 3 = = = = public class Arithmetic public static void main(string [] args) Adder a1 = new Adder(3); System.out.println(" 4 + " + a1 + " = " + a1.add(4)); // 4+3 = 7 System.out.println(" 5 + " + a1 + " = " + a1.add(5)); // 5+3 = 8 Adder a2 = new Adder(7); System.out.println(" = " + a2.add(8)); // 7+8 = 15 System.out.println(" = " + a2.add(2)); // 7+2 =