Java programming homework 4 and project 4


  1. What are the differences between constructors and methods?
  2. What is wrong with each of the following programs?
    1. 1: public class ShowErrors { 2:    public static void main ( String [] args ) { 3:       ShowErrors t = new ShowErrors( 5 ); 4:    } 5: }
    2. 1: public class ShowErrors { 2:    public static void main ( String [] args ) { 3:       ShowErrors t = new ShowErrors(); 4:       t.x(); 5:    } 6: }
    3. 1: public class ShowErrors { 2:    public void method1 () { 3:       Circle c; 4:       System.out.println( “What is radius ” 5:          + c.getRadius() ); 6:       c = new Circle(); 7:    } 8: }
    4. 1: public class ShowErrors {  2:    public static void main(String[] args) {  3:       C c = new C(5.0);  4:       System.out.println(c.value);  5:    }  6: }  7:   8: class C {  9:    int value = 2; 10: }
  3. Which of the following statements are valid?
    1. int i = new int(30);
    2. double d[] = new double[30];
    3. char[] r = new char(1..30);
    4. int i[] = (3, 4, 3, 2);
    5. float f[] = {2.3, 4.5, 6.6};
    6. char[] c = new char();
  4. Given an array of doubles, write Java statements to do the following:
    1. Assign the value 5.5 to the last element in the array.
    2. Display the sum of the first two elements of the array.
    3. Write a loop that computes the sum of all elements in the array.
    4. Write a loop that finds the minimum element in the array.
    5. Randomly generate an index and display the element of this index in the array.
    6. Use an array initializer to create another array with the initial value 3.5, 5.5, 4.52, and 5.6.
  5. Use the following illustration as an example, show how to apply the binary search approach to a search first for key 10 and then key 12, in the list:
    [2, 4, 7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79].
    key is 11          0   1   2   3   4   5   6   7   8   9  10  11  12 11<50  [ 2,  4,  7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79]        low=0                   mid=6                   hi=12  11>7   [ 2,  4,  7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79]        low=0    mid=2      hi=5  11=11  [ 2,  4,  7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79]                    low=3    hi=5                        mid=4
    (Note how binary search eliminates half of the list from further consideration after each comparison.)
  6. What types of array can be sorted using the Java.util.Arrays.sort method?  Does this sort method create a new array?
  7. Which of the following statements are valid?
    1. int[][] r = new int[2];
    2. int[] x = new int[];
    3. int[][] y = new int [3][];
    4. int[][] z = {{1, 2}};
    5. int[][] m = {{1, 2}, {2, 3}};
    6. int[][] n = {{1, 2}, {2, 3}, };
  8. Describe the difference between passing a parameter of a primitive type and passing a parameter of a reference type.  Then show the output of the following program: 1: class Test {  2:     public static void main ( String [] args ) {  3:         Count myCount = new Count();  4:         int times = 0;  5:         for ( int i = 0; i < 100; i++ )  6:             increment( myCount, times );  7:         System.out.println( “count is ” + myCount.count );  8:         System.out.println( “times is ” + times );  9:     } 10:     public static void increment ( Count c, int times ) { 11:         c.count++; 12:         times++; 13:     } 14: } 15:  16: class Count { 17:     public int count; 18:     public Count ( int c ) { 19:         count = c; 20:     } 21:     public Count () { 22:         count = 1; 23:     } 24: }
  9. What is wrong in the following code?1: public class Test { 2:    public static void main ( String [] args ) { 3:       java.util.Date[] dates = new java.util.Date[10]; 4:       System.out.println( dates[0] ); 5:       System.out.println( dates[0].toString() ); 6:    } 7: }
  10. If a class contains only private data fields and no “set” methods, is the class considered to be immutable?
  11. If a class contains only data fields that are both private and primitive, and no “set” methods, is the class considered to be immutable?
  12. What is wrong in the following code? 1: public class C {  2:     private int p;  3:   4:     public C () {  5:         System.out.println( “C’s no-arg constructor invoked” );  6:         this(0);  7:     }  8:   9:     public C ( int p ) { 10:         p = p; 11:     } 12:  13:     public void setP ( int p ) { 14:         p = p; 15:     } 16: }
  13. What is wrong in the following code?1: public class Test { 2:     private int id; 3:     public void m1 () { 4: = 45; 5:     } 6:     public void m2 () { 7: = 45; 8:     } 9: }



For this project, you will create a class called TextKit containing several utility methods that can be used in different applications.  This class is not intended to be a complete application by itself!  (It has no main method.)  Your class will be put into a package called utils.  The package will then be documented using the javadoc tool to create HTML documentation for your package.  Finally, you will create a small stand-alone non-GUI Java program that tests the methods of your utils.TextKit class.

(Your next project will use this package, and you won’t be allowed to make any changes to TextKit once submitted.)


Create a public Java class named “TextKit” in a package called “utils” that contains the following public static methods (at least):

  • lineOfStars
    This method will create and return a String containing a line of asterisks (or stars).  This method must take a single parameter only, an int which says how many stars to draw.  For example, the code:

       System.out.println( utils.TextKit.lineOfStars(4) );
    Should print a line that looks like:

    The intent is that lineOfStars should be a generally useful method that given a single number returns a String of that many stars.  (Such a method could easily be reused in another project someday.)

  • pad
    This method will format integers by adding spaces (called padding) to the left end, to make the resulting String a certain minimum length.  (If the number contains more digits than the specified width, then no padding is added.)  This method must take two int arguments, the first is the number to format, and the second is the desired minimum String length.  The resulting String is returned.  For example, the code:

       int num = 17;    System.out.println( “*” + utils.TextKit.pad(num, 4) + “*” );
    Should print a line that looks like:

       *  17*
    (Notice there are two leading blanks added by pad, to make the field length 4.)

To facilitate such reuse, these methods must be public static methods of a public class called TextKit, which must be in a package called utils.  (Someday you might add other text utility methods to this class or add other classes to this package.)

Be sure to add appropriate Java doc comments throughout your code!

For full credit, your methods must check for invalid arguments (for example, inappropriate negative numbers).  If you detect invalid arguments passed to a method, the method must throw an appropriate java.lang.IllegalArgumentException.

Next, create a testing application.  Your test program (containing just a main method) should not be in the utils package, but rather in the default, nameless package.  You can name the class anything you like; something likeTextKitApp is fine.  This test program should invoke each method of the utils.TextKit class at least once, to verify those methods work.  You can test the resulting String object returned from each method, against the expected value.  Or you can simply print a message that says something like “You should see five stars here: ”, followed by the output of the method call with (in this example) the argument 5.  (Such a main method is sometimes referred to as a test driver.)  A good test driver will have many test cases, to more thoroughly check the methods.  Having failing cases (in a try...catch block of course) is also a good idea, but not required for this project.

Finally, you must use the javadoc tool to create HTML documentation for your package.  This documentation should be placed in a directory called “docs”.  (The docs directory should not be placed inside of the utilsdirectory.)  Note, only the code in the package needs to be documented with Java doc comments; your test driver only needs regular style comments.

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