Unit 5: More on Classes/Objects Exercises

Transcription

1 Unit 5: More on Classes/Objects Exercises AP CS A 1. Select the TRUE statement. a) This does not compile. b) This does compile.. Select the TRUE statement. a) This does not compile. b) This does compile. 3. Select the TRUE statement. a) This does not compile. b) This does compile. 4. Select the one TRUE statement. a) The first line does not compile. b) The second line does not compile. c) The code compiles but generates a run-time error. d) Compiles, runs, and displays Select the TRUE statement(s). a) Two Rat objects were instantiated. b) Two Rat variables were instantiated. c) After the two lines are executed, there is no longer a reference to the first Rat object and it will be garbage collected. String a = null; boolean b = null; public String metty(){ returns null; Scanner c = null; int n = c.nextint(); System.out.println( n ); Rat a = new Rat( "dirty" ); a = new Rat( "rotten" ); 6. The code to the right compiles and runs. What is displayed? 7. After the Point object is instantiated, does its state ever change? 8. Why does the Point class has public instance variables? import java.awt.point; public class Runner { public static void main(string[] args) { Point pt = new Point( 4, 5 ); int a = pt.x; System.out.println( a ); pt.y = -33; System.out.println( pt.x + "," + pt.y ); 9. Circle the methods of the String class that are overloaded. length substring equals indexof touppercase 1

2 10. Select the TRUE statement. a) RunZoo1 compiles and runs. b) Line 1 causes a compiler error. c) Line causes a compiler error. d) Lines 1 and cause compiler errors. 11. Select the TRUE statement. a) RunZoo compiles and runs. b) Line 1 causes a compiler error. c) Line causes a compiler error. d) Lines 1 and cause compiler errors. 1 1 public class RunZoo1 { public static void main(string[] args){ Zoo z = new Zoo( 17 ); z.num = 7; public class RunZoo { public static void main(string[] args){ Zoo z = new Zoo(); z.inc(); The Zoo class compiles. public class Zoo{ public int num; public Zoo( int x ){ num = x; private void inc(){ num++; 1. Name two visibility modifiers. 13. Select the best answer. num is private because a) if it was public then client code could set it to any value. The obvious intent of the mutator method is to keep it at zero or positive. b) instance variables are always private c) my dumb teacher said it must be private. If it was up to me I d make it public. 14. The Bird class does NOT compile. Select the TRUE statement(s). a) It will compile if just the first get method is deleted. b) It will compile if just the second get method is deleted. c) It will compile if just the third get method is deleted. d) Two of the three methods must be deleted for this to compile. public class Bag{ private int num; public void add( int x ){ if ( x > 0 ) num += x; return num; public class Bird{ return 4; public double get() { return 3.; public int get(int x){ return x + 3;

3 15. The Trash class and the client code both compile. What does the print method display? // client code 16. Select the TRUE statement. a) C1 does not compile. b) Both compile but there is a runtime error. c) Both compile and run. 0 is printed. d) Both compile and run. null is printed. e) Both compile and run. 4 is printed. Trash t = new Trash(); t.print(); // client code public class Trash { private boolean b; private String s; C1 e = new C1( 4 ); public void print(){ System.out.println(b+", "+s); public class C1{ private int n; public C1( int x ){ System.out.println( n ); n = x; 17. Select the TRUE statement. a) C does not compile. b) Both compile but there is a runtime error. c) Both compile and run. 0 is printed. d) Both compile and run. null is printed. 18. Select the TRUE statement. a) C3 does not compile. b) The client code does not compile c) Both compile but there is a runtime error. d) Both compile and run. 0 is printed. // client code C c = new C(); c.mm(); // client code C3 c = new C3(); int k = c.get(); System.out.println( k ); public class C{ private String s; public void mm(){ int n = s.length(); System.out.println( n ); public class C3{ private int n; public C3( int x ){ n = x; return n; 19. Select the TRUE statement. If n has a value of 33 then a) 0 33 is printed b) null 33 is printed c) nothing because the Hat class does not compile. You cannot display the value of x before it has been initialized. public class Hat{ private int x; public Hat( int n ){ System.out.print( x ); x = n; System.out.println( x ); 3

4 0. Select the TRUE statement a) This class will compile. b) This class will not compile because it has two methods with the same headers. b) This class will not compile because it has two methods with the same signatures. 1. The tie method is overloaded. TRUE FALSE public class Shoes{ public void tie(){ // code public double tie( int knots ){ // code. Select the TRUE statement a) This class will compile. b) This class will not compile. This is the error message: method getsize() is already defined in class Belt c) This class will not compile. This is the error message: Your code is bad and you should feel bad. public class Belt{ public int getsize(){ return 4; public double getsize(){ return 4.0; Go to java api (search the phrase Java API 8) to answer questions 3, The abs method in the Math class is overloaded. TRUE FALSE 4. The random method in the Math class is overloaded. TRUE FALSE Note. The term static is part of all the Math class method headers. We will cover the static keyword in a later unit. 4

5 1 6 public class RunToy{ public static void main( String [] args ){ Toy t1 = new Toy("yoyo", 4.50 ); double x = t1.met(); System.out.println( x ); Problems 5 and 6 refer to the above code public class Toy{ private String name; private double cost; public Toy(String s, double c){ name = s; cost = c; public double met(){ int n = name.length(); double r = cost*n; return r; 5. Show the contents of the stack and heap just before line is executed. Be sure to label all stack frames. Stack Heap 6. Show the contents of the stack and heap just before line 5 is executed. Be sure to label all stack frames. // Client code Barrel b = new Barrel(); Monk m1 = new Monk(5,b); Monk m = new Monk(,b); Monk m3 = new Monk(6,b); public class Barrel{ private int k; public void in( Monk m ){ this.k += m.get(); System.out.print(k+" ")); public class Monk{ private int age; public Monk(int a,barrel b){ age = a; b.in( this ); return this.age; 7. Everything compiles and runs. What is displayed? 8. What is the data type of the keyword this in the Monk class? 9. What is the data type of the keyword this in the Barrel class? 5

6 30. The Person class does NOT compile. Which line causes the problem and why? 1 public class Person{ private final int MIN_AGE = 1; public boolean isadult( int age ){ return age >= MIN_AGE; 3 public void change( int limit ){ MIN_AGE = limit; 31. An instance variable must be public in order to use the keyword final. TRUE FALSE 3. What is displayed? public class Runner{ public static void main( String [] args ){ Robot x1 = new Robot( 1 ); Robot x = new Robot( ); Robot what = x1.regen( x ); System.out.println( what.get() ); Robot who = x.regen( x1 ); System.out.println( who.get() ); public class Robot{ private int id; public Robot( int i ){ id = i; public Robot regen( Robot r ){ int idx = 10*this.id + r.id; return new Robot( idx ); return this.id; 33. What is displayed? public class Runner{ public static void main( String [] args ){ Thing a = new Thing( "A" ); Thing b = new Thing( "B" ); Thing c = new Thing( "C" ); a.met( b ); b.met( c ); c.met( a ); b.met( a ); System.out.println(a.toString()); System.out.println(b.toString()); System.out.println(c.toString()); public class Thing{ private String s; public Thing( String str ){ s = str; public void met( Thing t ){ this.s += t.s; public String tostring(){ return s; 6

7 34. The Bird class compiles. Does the Cage class compile? public class Cage{ public int met( Bird b1, Bird b ){ return b1.age + b.age; public class Bird{ private int age; public Bird( int a ){ age = a; return age; 35. The errere method returns the number of times the letter e appears with an r before and/or after it. For example, if s is ere then the method returns. If s is rer the method returns one. Write the hasr helper method so that the rrrrr method works correctly. public int errere( String s ){ int count = 0; int loc = s.indexof( "e", 0 ); while ( loc!= -1 ){ if ( hasr( s, loc ) ) count++; loc = s.indexof( "e", loc+1 ); return count; private 7

8 36. The highlighted sections are very similar. Use functional decomposition to rewrite the met method (i.e. write a helper method that does what a highlighted section does. Assume that a and b are always positive. public void met( int a, int b ){ public void met( int a, int b ){ for ( int k = 1; k <= a; k++ ) System.out.print(k + " "); System.out.println(); if ( a > b ) for ( int k = 1; k <= b; k++ ) System.out.print(k + " "); else System.out.println(); if ( a > b ){ for ( int k = b; k <= a; k++ ) System.out.print(k + " "); System.out.println(); else { for ( int k = a; k <= b; k++ ) System.out.print(k + " "); System.out.println(); private public class RunTank{ public static void main( String [] args ){ Tank t1 = new Tank(); int z; z = t1.m1( ); System.out.println( z ); 37. The main method is running. The m1 method is called which calls m. Show the contents of the stack and heap just before line A is executed. Be sure to label all stack frames. A Stack public class Tank{ public int m1( int a ){ int b = * m( a ); return b; private int m( int c ){ int d = 3*c ; return d; Heap 8