3. Convert 2E from hexadecimal to decimal. 4. Convert from binary to hexadecimal

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1 APCS A Midterm Review You will have a copy of the one page Java Quick Reference sheet. This is the same reference that will be available to you when you take the AP Computer Science exam. 1. n bits can represent exactly a) n different values b) 2n different values c) 2 n different values d) n 2 different values 2. Select the TRUE statement. a) 16 in base ten is equal to 1111 in base two. b) 16 in base 16 is equal to 10 in base ten. c) 16 in base ten is equal to 10 in base 16. d) 1111 in base ten is equal to 16 in base two. 3. Convert 2E from hexadecimal to decimal 4. Convert from binary to hexadecimal 5. How many assignment statements are in this code? 6. How many boolean expressions are in this code? int x; x = (int)(45*math.random()) - 7; if ( x == 12 ) System.out.println( "A" ); else if ( x > 10 ) x = x + 5; if ( x < 30 ) x = x + 2; else x = x - 11; System.out.println( x ); 7. Select the TRUE statement. a) The JVM compiles a java file into a class file. b) The JVM compiles a class file into a java file. c) The JVM translates the class file into Byte code and executes the program. d) The JVM translates Byte code in the class file into machine language and executes the program. 8. Select the FALSE statement. a) The JVM on a Windows computer is different than the JVM on an Apple computer. b) Mistakes detected by the compiler are called syntax errors or compile-time errors. c) An assignment statement assigns the value of the expression on the left to the variable on the right.

2 9. Select the FALSE statement. a) Some primitive data types are ints, doubles, and Strings. b) There are numeric literals and String literals. c) int17 is a valid variable name. d) A logic error will not be detected by the compiler. 10. What is the value of the expression on the right? a) 3 or 3.0 b) 4 or 4.0 c) 4.75 d) 5 or Select the TRUE statement. a) Line 2 generates a compiler error. b) Line 3 generates a compiler error. c) This compiles and runs. x equals 4, z equals 10. d) This compiles and runs. x equals 4, z equals What is the value of the expression on the right? a) 0.8 b) 2.0 c) 6 d) compiler error 13. What is the result of the code to the right? a) The code does not compile because you cannot assign a value of null to a string. b) The code does not compile because of a null pointer exception. c) The code compiles but throws a null pointer exception during run-time % 4 int x = 10; double z = 4; x = z; z = x; * 6 String s = null; double d = s.length(); System.out.println( d ); d) Code compiles and zero is displayed. 14. What is displayed? int count = 0; for ( int i = 3; i < 6; i++ ){ for ( int k = i; k < 8; k++ ){ count++; System.out.println( count ); 15. Select the FALSE statement. a) To use the Math class, you must first instantiate a Math object. b) The break statement is used to exit a loop early. c) To use the Math class, you do not need to instantiate a Math object. d) The abs methods of the Math class are an example of overloaded methods. 16. The value of n will be an integer so that a) 0 n 10 b) 0 n 11 c) 1 n 10 d) none of the above int n; n = (int) (10 * Math.random() );

3 17. Select the TRUE statement. a) 3.0 d < 50.0 b) 3.0 d 50.0 c) 3.0 d < 53.0 d) 3.0 d After the code executes, a) x = 7, y = 6, z = 5 b) x = 7, y = 6, z = 4 c) x = 7, y = 9, z = 5 d) x = 7, y = 9, z = After the code executes, a) a = 2, b = 8, c = 6 b) a = 3, b = 16, c = 4 c) Run-time error generated by c = c/a d) This is an endless loop. 20. What is printed by the last statement? a) 13 b) 14 c) The second line generates a compiler error. d) The last line generates a compiler error. double d; d = 50*Math.random() + 3; int x = 7, y = 6, z = 5; if (x!= 7 ) y++; else if (x = = y) y += 3; z - - ; int a = 0, b = 4, c = 12; while (b <= c) { a++; b *= a; c = c / a; int b = 10; for (int k = 5; k <=b; k++) { b -= 2; System.out.println(b + k); 21. Select the FALSE statement. a) Methods always have a return type and constructors never do. b) A class usually has instance variables but it doesn't have to. c) Class variables are created whenever an object is instantiated. d) "Garbage collection" refers to the process of deleting any unreferenced objects from memory. 22. The header for the clever method is: a) public void clever( int n ) b) public boolean clever() c) public boolean clever( int n ) d) There is not enough information to answer this question. Fox y = new Fox(); if ( y.clever( 5 ) ) y.win(); 23. Write a method named total that has two int parameters, begin and end, and returns the sum of the all integers between begin and end. For example, if the method is invoked with values of 5 and 7, then the method returns 18. You may assume that begin end.

4 24. Select the TRUE statement. a) x, y, and z are objects. b) x and z are objects. c) The objects created in lines 2 and 4 are equal. d) x and y reference the same object. 25. Given this method compiles, select the FALSE statement. a) This is an example of a mutator method. b) This is an example of an accessor method. c) value must be an instance or class variable. d) value is an int. 26. Select the TRUE statement. a) This method can change an object s state. b) This method cannot change an object s state. c) Not enough information was provided to determine if this method could or could not change an object s state. d) Moosylvania is a state. 27. Select the FALSE statement. a) This is code for a constructor. b) This is contained in a class named Xyz. c) y contains a reference to an object of the Abc class. d) x is a double; we have no clue what type y is. ClassA x, y, z; x = new ClassA(); y = x; z = new ClassA(); public int getvalue() { return value; public void met( int x ){ y = x; public Xyz(Abc g) { x = 0.0; y = g.action(); 28. Select the TRUE statement. a) Line 2 throws an IllegalArgumentException b) Line 2 throws an NullPointerException c) Line 4 throws an NullPointerException d) Line 3 throws an IllegalArgumentException public static void main( String [] args ){ String [] a = new String[4]; mz( a[2] ); public static void mz( String str ){ int n = str.length(); System.out.println( n ); 29. Suppose your design for a program includes two classes: Apple and Fruit. Which of the following is a good program design? a) Apple could be a subclass of Fruit. b) Fruit could be a subclass of Apple. c) It doesn t matter which one is the subclass and which one is the superclass. d) There is not enough information to say which could be a subclass of the other. 30. What range of values for x will cause code C to run? a) (x < -10) or (x > 0) b) (x < -10) or (0 < x < 5) c) (x < -10) and (x > 0) d) code C will never run 31. If x = 0, what code will run a) code A b) code B c) code D d) none of the above if (x >= 5 && x > -5) { if (x > 10) //code A else if (x <= 10) //code B else if ( x < -10 x > 0) // code C else // code D

5 32. Write the Mobile class that is a subclass of the Phone class. The constructor for the Mobile class has two parameters: the phone number (an int) and the battery life (an int). Mobile has one instance variable for the battery life. The Mobile class has a method named works that returns a boolean value. Every time the works method is called, the battery life decreases by 1. If the battery life is zero or negative, then the method returns false. Otherwise the works method behaves just like a regular phone. public class Phone { private int number; public Phone ( int n ){ number = n; public boolean works(){ // it s secret 33. Select the TRUE statement. a) This compiles. Under some circumstances it will run fine and the rest of the time there will be a run-time error. b) This does not compile. public void m5( Object x ){ String str = (String) x; int n = str.length(); System.out.println( n ); 34. What does this code display? for ( int n = 1; n < 4; n++ ) { for ( int k = n; k < 5; k++ ) System.out.print( "X" ); System.out.println(); 35. What does the code on the right do? a) z is the sum of all the elements in arr. b) z is the sum of all the indices in arr. c) z is the sum of the last two elements in arr. d) none of the above //arr is an array of ints of random values // the length of arr is an even number z = 0; for (int n = 0; n <arr.length - 1; n+=2) z = arr[n] + arr[n+1];

6 36. What values will get A to be printed? a) x = 6 and y = 15 b) x = -1 and y = 0 c) x = 2 and y = 4 d) x = 3 and y = Add one line that changes the title to Mona Lisa. If it cannot be done, write IMPOSSIBLE. Art y = new Art( "Mona Liza" ); 38. Add one line that changes the cost to 300. If it cannot be done, write IMPOSSIBLE. Art y = new Art("Chewbacca Fighting Nazis"); if ( (x < y) && (y >= 3*x) ) System.out.println("A"); else System.out.println("B"); public class Art{ public String title; private int cost; public Art( String s ){ title = s; cost = 100; public void change(int n){ setcost( 2*n ); private void setcost(int n){ cost = n; 39. Complete the method below so that the largest value in the two-dimensional array. You must use the private helper method. The methods are overloaded. public int max( int[][] a ) { private int max( int [] a ){ returns the maximum value in the array (assume it works) 40. Select the true statement. Consider the two methods in problem 39. a) The first max method overrides the second max method. b) The second max method overrides the first max method. c) The two max methods are overloaded. d) There s a problem with these method headers and they will not compile. The simplest solution would be to rename one of the methods.

7 41. Select the TRUE statement. a) The first line creates 50 Rat objects. b) The first line creates an array of 51 Rat reference variables. c) A total of 50 Rat object are instantiated but garbage collection deletes the first 49. d) The third line causes a compiler error. 42. What is printed? a) 5 b) 11 c) 20 d) 25 Rat [ ] b = new Rat[50]; for (int n = 0; n <b.length; n++) b[0] = new Rat(); int [] a = { 5, 8, 7 ; int k = 5; for ( int n : a ) k += n; System.out.println( k ); 43. What is the purpose of the keyword final as used in the code to the right? public class TV{ public final int x = 7; 44. What is displayed? Bug bed = new Bug( 6 ); Bug stink = new Bug( 3 ); int x = stink.mx( bed ); System.out.println( x ); public class Bug{ private int ugly; public Bug( int u ){ ugly = u; public int mx( Bug b ){ return ugly + 10*b.ugly; Problems 45 and 46. int [ ] arr = { 5, 0, -3, 2; 45. What is the value of the expression arr[1 + arr[3] ] a) 0 b) -3 c) 2 d) This is an invalid expression. 46. What is the value of the expression arr[ arr.length ] a) 0 b) -3 c) 2 d) This is an invalid expression. 47. What is the value of the expression to the right? 9 / What is the value of the expression to the right? (double) (5/4)

8 49. Write a method that has one parameter, a two-dimensional array of ints with at least one value in it. The method returns a one-dimensional array of ints that contains all of the values in the parameter. The values in the first column will be at the start of the one-dimensional array, followed by the values in the second column and so on. You may assume that the length of each row is the same. For example, if the 2D array is { then the method returns { 1, 4, 2, 5, 3, public int [] convert( int [][] a ){ _ 50. Select the TRUE statement. The variables a) may be either instance or class variables. b) must class variables. c) must instance variables. d) must local variables. 51. After the top three lines are run and the method is executed, the values of x and array z are a) z is {2, 2, 2, 2, 2 and x is 6 b) z is {4, 5, 6, 7, 8 and x is 6 c) z is {2, 2, 2, 2, 2 and x is 33 d) z is {4, 5, 6, 7, 8 and x is 33 public static void md(){ System.out.println( a + b ); int [ ] z = {4, 5, 6, 7, 8; int x = 6; notypos(z, x); ***************************** public void notypos (int [ ] p, int q) { for (int i = 0; i < p.length; i++) p[i] = 2; q = 33;

9 52. In the box on the right, write the result of the code on the left. for (int n = 0; n <= 3; n++) { for (int k = 0; k <= n; k++) System.out.print(k + " "); System.out.println(); 53. Write a method that returns the total number of times that the TH appears in the array. Assume that all letters are uppercase. Write and use a private helper method that counts the number of times TH appears in a single string. public int count( String [] s ){ private 54. Select the TRUE statement. The method m77 a) must be a class method. b) may be a class or instance method. 55. Select the TRUE statement. The method m44 a) must be a class method. b) may be a class or instance method. 56. An advantage of inheritance is that it reduces the amount of redundant code. public static void m88() { int n = m77(); public void m55() { int n = m44(); TRUE FALSE

10 57. What is displayed? public class Runner{ public static void main( String [] args ){ int [] a = { 5, 6, 7 ; String s = "bab"; mx( s, a ); System.out.println( s + "," + a[0] ); public static void mx( String s, int [] b ){ s += "y"; b[0] = 9; 58. What is displayed? public class Runner{ public static void main( String [] args ){ Ax y = new Ax( 23 ); int h = 70; my( y, h ); System.out.println( y + "," + h ); public static void my( Ax x, int z ){ x.set( 99 ); z = 33; public class Ax{ private int z; public Ax(int n){ z = 8; public void set(int n){ z = n; public String tostring(){ return "" + z; 59. If the array is {8, 5, 2, 7, what does the method return? 60. If the array is {9, 1, 17, 14, what does the method return? public int mm( int[] a ){ int z = a[0]; for ( int k = 1; k < a.length; k++ ){ if ( a[k] < a[k-1] ) z = a[k]; return z; 61. Line 6 generates the following compiler error: Cannot find symbol variable p. Why? for ( int k = 0; k <= 5; k++ ){ int p = 1; System.out.println( k + "," + p ); p++; System.out.println( p ); 62. What is the final value of n? String s = "needed"; int n = s.indexof("e", 0 ); n = s.indexof("e", n );

11 Problems 63 to 68 all refer to the code to the right. Assume the code compiles and runs. Manny moe = new Jack(); moe.methoda(); 63. Manny may be an abstract class. True False 64. Manny may be a superclass of Jack. True False 65. Manny may be a subclass of Jack. True False 66. Jack may be an abstract class. True False 67. Jack must be a class that implements, or inherits, methoda(). True False 68. Manny must have a methoda True False 70. What is displayed? String s = "washer"; s.substring( 3 ); System.out.println( s ); 71. If the string is SESSION, what does the method return? 72. Under what circumstances with there be a run-time error, or will there never be a runtime error? public int mx( String s ){ for ( int k = 0; k < s.length(); k++ ){ String s1 = s.substring( k, k+1 ); String s2 = s.substring( k+1, k+2 ); if ( s1.equals( s2 ) ) return k; else return k+ 1; return -1; 73. In the client code call the sort method and use array b as the argument. Then display the largest value in the returned array. Do not write any loops. // client code public class Clerk{ double [] b = new double[ 900 ]; private Clerk(){ for ( int i = 0; i < 900; i++) b[ i ] = 50000*Math.random() ; public static int [] sort( double [] a ){ /* this returns a copy of the parameter except all the numbers have been rounded and the contents have been sorted and put into increasing order */

12 74. If the string is BARN, what does the method return? public String mx( String s ){ String x = ""; for ( int k = 0; k < s.length(); k++ ){ String temp = s.substring( k, k+1 ); x = temp + x; return x; public class ABC { private int w; private static int x = 7; public ABC( int n) { x++; w = n; public String tostring() { return w + " " + x; Problems 75 and 76 refer to the above code. public class Runner { public static void main( String [] args ){ ABC h1 = new ABC( 14 ); ABC h2 = new ABC( 45 ); ABC h3 = new ABC( -2 ); System.out.println( h1 ); What is displayed when the main method is executed? 76. At the time that line 4 of the main method is being executed, how many w variables had been created? How many x variables had been created? 77. What does the client code display? If there s an error, write ERROR. AA ab = new BB(); System.out.println( ab.meth3() ); 78. What does the client code display? If there s an error, write ERROR. BB bb = new BB(); System.out.println( bb.meth3() ); 79. What does the client code display? If there s an error, write ERROR. AA ab = new BB(); System.out.println( ab.meth2() ); public class AA { public int meth1() { int x = 3 * meth2(); return x; public int meth2() { return 8; public class BB extends AA{ public int meth2() { return 7; public int meth3() { return 2*meth1();