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README.md

Chapter 1 - Review Questions Attempt 4

Results:

Date: 02/01/2023
Score: 15/21 (71%)

Question # Correct
1 ✅
2 ✅
3 ✅
4 ✅
5 ✅
6 ✅
7 ✅
8 ❌
9 ✅
10 ✅
11 ✅
12 ❌
13 ✅
14 ❌
15 ❌
16 ✅
17 ❌
18 ✅
19 ✅
20 ❌
21 ✅

Question 1

❓What is the result of the following code❓

public class Employee {
    public int employeeId;
    public String firstName, lastName;
    public int yearStarted;
    @Override public int hashCode() {
        return employeeId;
    }
    public boolean equals(Employee e) {
        return this.employeeId == e.employeeId;
    }
    public static void main() {
        Employee one = new Employee();
        one.employeeId = 101;
        Employee two = new Employee();
        two.employeeId = 101;
        if (one.equals(two)) System.out.println("Success")
        else System.out.println("Failure");
    }
}
A. Success🎃
B. Failure🎃
C. The hashCode() method fails to compile🎃
D. The equals() method fails to compiler🎃
E. Another line fails to compile🎃
F. A runtime exception is thrown🎃

My Answer:

  • While equals method has not override the correct method in the Object class, it still works fine
  • It will print success since the ids are equivalent
  • A✅✅✅✅

Question 2

❓What is the result of the following code❓

public class Book {
    private int ISBN;
    private String title, author;
    private int pageCount;
    public int hashCode() {
        return ISBN;
    }
    @Override public boolean equals(Object obj) {
        if (!(obj instanceof Book)){
            return false;
        }
        Book other = (Book) obj;
        return this.ISBN == obj.ISBN;
    }
    // imagine setters and getters
}
A. The code compiles🎃
B. The code does not compile because hashCode() is incorrect🎃
C. The code does not compile because equals() does not override the parent method correctly🎃
D. The code does not compile because equals() tries to refer to a private field🎃
E. The code does not compile because the ClassCastException is not handled or declared🎃
F. The code does not compile for another reason🎃

My Answer:

  • A - true
  • B - false, it does compile
  • A✅✅✅✅

Question 3

❓What is the result of the following code❓

String s1 = "Canada";
String s2 = new String(s1);
if (s1 == s2) System.out.println("s1 == s2");
if (s1.equals(s2)) System.out.println("s1.equals(s2)");
A. There is no output 🎃
B. s1 == s2 🎃
C. s1.equals(s2) 🎃
D. Both B and C🎃
E. The code does not compile🎃
F. The code throws a runtime exception🎃

My Answer:

  • s2 is a completely different string so it will not print the first sysout
  • s1 and s2 are equal in content
  • C✅✅✅✅

Question 4

❓What is true about the following code❓

public class BaseballTeam {
    private String city, mascot;
    private int numberOfPlayers;
    public boolean equals(Object obj) {
        if (!(obj instanceof BaseballTeam))
            return false;
        BaseballTeam other = (BaseballTeam) obj;
        return (city.equals(other.city) && mascot.equals(other.mascot));
    }
    public int hashCode() {
        return numberOfPlayers;
    }
    // imagine setters and getters
}
A. The class does not compile🎃
B. The class compiles but has an improper equals() method🎃
C. The class compiles but has an improper hashCode() method🎃
D. The class compiles but has proper equals() and hashCode() methods🎃

My Answer:

  • The code does compile

  • C - true

  • C✅✅✅✅

  • The equals method can be anything the business demands!

  • It is the hashcode which needs to comply!!!!

Question 5

❓Which of the following statements are true, assuming a and b are String objects?❓

A. if a.equals(b) is true, a.hashCode() == b.hashCode() is always true 🎃
B. if a.equals(b) is true, a.hashCode() == b.hashCode() is sometimes but not always true🎃
C. if a.equals(b) is false, a.hashCode() == b.hashCode() can never be true🎃
D. if a.equals(b) is false, a.hashCode() == b.hashCode() can sometimes be true🎃

My Answer:

  • A - true
  • B - false
  • C - false, it is not required that two instances with the same hash are different!
  • D - true
  • A,D✅✅✅✅

Question 6

❓What is the result of the following code❓

public class FlavorsEnum {
    enum Flavors {
        VANILLA, CHOCOLATE, STRAWBERRY
    }
    public static void main() {
        System.out.println(Flavors.CHOCOLATE.ordinal());
    }
}
A. 0 🎃
B. 1 🎃
C. 9 🎃
D. CHOCOLATE 🎃
E. The code does not compile due to a missing semi-colon🎃
F. The code does not compile for a different reason 🎃

My Answer:

  • An enum does not require a semi-colon
  • B✅✅✅✅

Question 7

❓What is the result of the following code❓

public class IceCream {
    enum Flavors {
        VANILLA, CHOCOLATE, STRAWBERRY
    }
    public static void main() {
        Flavors f = Flavors.STRAWBERRY;
        switch (f) {
            case 0: System.out.println("vanilla");
            case 1: System.out.println("chocolate");
            case 2: System.out.println("strawberry");
                break;
            default: System.out.println("missing flavor");
        }
    }
}
A. vanilla 🎃
B. chocolate 🎃
C. strawberry 🎃
D. missing flavor 🎃
E. The code does not compile 🎃
F. An exception is thrown 🎃

My Answer:

  • The case statements are wrong, so will not compile
  • E✅✅✅✅

Question 8

❓What is the result of the following code?❓

1:  public class Outer {
2:      private int x =5;
3:      protected class Inner {
4:          public static int x = 10;
5:          public void go() { System.out.println(x); }
6:      }
7:      public static void main(String[] args) {
8:          Outer out = new Outer();
9:          Outer.Inner in = out.new Inner();
10:          in.go();
11:      }
12:  }
A. The output is 5. 🎃
B. The output is 10. 🎃
C. Line 4 generates a compiler error. 🎃
D. Line 8 generates a compiler error. 🎃
E. Line 9 generates a compiler error. 🎃
F. An exception is thrown. 🎃

My Answer:

  • I think the output is 5

  • A❌❌❌❌

  • CORRECT ANSWER: C

  • Inner classes can NOT have static variables!!!

Question 9

❓What is the result of the following code?❓

1:  public class Outer {
2:      private int x = 24;
3:      public int getX() {
4:          String message = "x is ";
5:          class Inner {
6:              private int x = Outer.this.x;
7:              public void printX() {
8:                  System.out.println(message + x);
9:              }
10:         }
11:         Inner in = new Inner();
12:         in.printX();
13:         return x;
14:     }
15:     public static void main(String[] args) {
16:         new Outer().getX();
17:     }}
A. x is 10 🎃
B. x is 24 🎃
C. Line 6 generates a compiler error🎃
D. Line 8 generates a compiler error🎃
E. Line 11 generates a compiler error🎃
F. An exception is thrown🎃

My Answer:

  • Since the x in the inner class is referenced using Outer.this.x it gets assigened to the Outer class's value
  • So prints x is 24
  • B✅✅✅✅

Question 10

❓The following code appears in the file named Book.java. What is the result of compiling the source file❓

1: public class Book {
2:     private int pageNumber;
3:     private class BookReader {
4:         public int getPage() {
5:             return pageNumber;
6: } } }
A. The code compiles successfully, and one bytecode file is generated: `Book.class` 🎃
B. The code compiles successfully, and two bytecode files are generated: `Book.class` and `BookReader.class` 🎃
C. The code compiles successfully, and two bytecode files are generated: `Book.class` and `Book$BookReader.class` 🎃
D. A compiler error occurs on line 3 🎃
E. A compiler error occurs on line 5 🎃

My Answer:

  • No compiler error
  • Two bytecode is Book.class and Book$BookReader.class
  • C✅✅✅✅

Question 11

❓Which of the following statements can be inserted to make FootballGame compile?❓

package my.sports;
public class Football {
    public static final int TEAM_SIZE = 11;
}
package my.apps;
// INSERT CODE HERE
public class FootballGame {
    public int getTeamSize() { return TEAM_SIZE; }
}
A. import my.sports.Football; 🎃
B. import static my.sports.*; 🎃
C. import static my.sports.Football; 🎃
D. import static my.sports.Football.*; 🎃
E. static import my.sports.*; 🎃
F. static import my.sports.Football; 🎃
G. static import my.sports.Football;

My Answer:

  • It needs to statically import the TEAM_SIZE variable
  • A - false
  • B - false, this would not even compile
  • C - false, as above
  • D - true
  • E,G - false, wrong syntax
  • D✅✅✅✅

Question 12

❓What is the result of the following code❓

public class Browsers {
    static class Browser {
        public void go() {
            System.out.println("Inside Browser");
        }
    }
    static class Firefox extends Browser {
        public void go() {
            System.out.println("Inside Firefox");
        }
    }
    static class IE extends Browser {
        public void go() {
            System.out.println("Inside IE");
        }
    }
    public static void main() {
        Browser b = new Firefox();
        IE e = (IE) b;
        e.go();
    }
}
A. Inside Browser 🎃
B. Inside Firefox 🎃
C. Inside IE 🎃
D. The code does not compile 🎃
E. A runtime exception is thrown 🎃

My Answer:

  • Does not compile. The firefox instance is not related to the IE class

  • D❌❌❌❌

  • CORRECT ANSWER: E

  • A runtime exception is thrown

  • It does actually compile but the casting throws exception at runtime

Question 13

❓Which is a true statement about the following code❓

public class IsItFurry {
    static interface Mammal { }
    static class Furry implements Mammal { }
    static class Chipmunk extends Furry { }
    public static void main() {
        Chipmunk c = new Chipmunk();
        Mammal m = c;
        Furry f = c;
        int result = 0;
        if (c instanceof Mammal) result += 1;
        if (c instanceof Furry) result += 2;
        if (null instanceof Chipmunk) result += 4;
        System.out.println(result);
    }
}
A. The output is 0 🎃
B. The output is 3 🎃
C. The output is 7 🎃
D. c instanceof Mammal does not compile 🎃
E. c instanceof Furry does not compile🎃
F. null instanceof Chipmunk does not compile🎃

My Answer:

  • c is an instance of Mammal and Furry
  • The output is 3
  • B✅✅✅✅

Question 14

❓What is a true statement of the following code (choose all that apply)❓

import java.util.*;
public class IsItFurry {
    static class Chipmunk { }
    public static void main() {
        Chipmunk c = new Chipmunk();
        ArrayList<Chipmunk> l = new ArrayList<>();
        Runnable r = new Thread();
        int result = 0;
        if (c instanceof Chipmunk) result += 1;
        if (l instanceof Chipmunk) result += 2;
        if (r instanceof Chipmunk) result += 4;
        System.out.println(result);
    }
}
A. The code compiles, and the output is 0 🎃
B. The code compiles, and the output is 3 🎃
C. The code compiles, and the output is 7 🎃
D. c instanceof Chipmunk does not compile 🎃
E. l instanceof Chipmunk does not compile 🎃
F. r instanceof Chipmunk does not compile 🎃

My Answer:

  • The code will not compile

  • F is true

  • F❌❌❌❌

  • CORRECT ANSWER: E

  • newArrayList<Chipmunk> instance of Chipmunk will not compile

  • ArrayList is a concrete class, the compiler knows that this will never be compatible!!!

  • If it were to be referenced via an interface (like with r) then it will compile fine!!!!

Question 15

❓Which statements are true about the equals() method (choose all that apply)❓

A. if equals(null) is called, the method should throw an exception 🎃
B. if equals(null) is called, the method should return false 🎃
C. if equals(null) is called, the method should return true 🎃
D. If equals() is passed the wrong type, the method should throw an exception 🎃
E. If equals() is passed the wrong type, the method should return false 🎃
F. If equals() is passed the wrong type, the method should return true 🎃

My Answer:

  • A - false

  • B - true

  • C - false

  • D - false

  • E - false

  • F - false

  • B❌❌❌❌

  • CORRECT ANSWER: B,E

  • null AND INCORRECT types should return false

Question 16

❓Which of the following can be inserted in main❓

public class Outer {
    class Inner { }

    public static void main(String[] args) {
        // INSERT CODE HERE
    }
}
A. Inner in = new Inner(); 🎃
B. Inner in = Outer.new Inner(); 🎃
C. Outer.Inner in = new Outer.Inner(); 🎃
D. Outer.Inner in = new Outer().Inner(); 🎃
E. Outer.Inner in = new Outer().new Inner(); 🎃
F. Outer.Inner in = Outer.new Inner(); 🎃

My Answer:

  • A - false, you cannot instantiate inner directly
  • B - false as above
  • C - false wrong syntax
  • D - false wrong syntax
  • E - true
  • F - false, wrong syntax
  • E✅✅✅✅

Question 17

❓What is the result of the following code (choose all that apply)❓

1:  public enum AnimalClasses {
2:      MAMMAL(true), FISH(Boolean.FALSE), BIRD(false),
3:      REPTILE(false), AMPHIBIAN(false), INVERTEBRATE(false)
4:      boolean hasHair;
5:      pubic AnimalClasses(boolean hasHair) {
6:          this.hasHair = hasHair;
7:      }
8:      public boolean hasHair() {
9:          return hasHair;
10:     }
11:     public void giveWig() {
12:         hasHair = true; 
13:     }}
A. Compiler error on line 2 🎃
B. Compiler error on line 3 🎃
C. Compiler error on line 5 🎃
D. Compiler error on line 8 🎃
E. Compiler error on line 12 🎃
F. Compiler error on another line 🎃
G. The code compiles successfully 🎃

My Answer:

  • There needs to be seperation between the enums and the fields of the class

  • B❌❌❌❌

  • CORRECT ANSWER: B,C

  • Both lines 3 and 5 do NOT compile

  • A semi-colon is needed after the enum values

  • The constructor can NOT have a public modifier, it can either have no modifier or be private only

Question 18

❓What is the result of the following code (choose all that apply)❓

public class Swimmer {
    enum AnimalClasses {
    MAMMAL, FISH {
        public boolean hasFins() { return true; }},
    BIRD, REPTILE, AMPHIBIAN, INVERTEBRATE;
    public abstract boolean hasFins();    
    }
    public static void main() {
        System.out.println(AnimalClasses.FISH);
        System.out.println(AnimalClasses.FISH.ordinal());
        System.out.println(AnimalClasses.FISH.hasFins());
        System.out.println(AnimalClasses.BIRD.hasFins());
    }
}
A. fish 🎃
B. FISH 🎃
C. 0 🎃
D. 1 🎃
E. false 🎃
F. true 🎃
G. The code does not compile

My Answer:

  • The code does not compile as not all the enums implement the abstract class
  • G✅✅✅✅

Question 19

❓Which of the following can be inserted to override the superclass method (choose all that apply)❓

public class LearnToWalk {
    public void toddle() {}
    class BabyRhino extends LearnToWalk {
        // INSERT CODE HERE
    }
}
A. `public void toddle() {}` 🎃
B. `public void Toddle() {}` 🎃
C. `public final void toddle() {}` 🎃
D. `public static void toddle() {}` 🎃
E. `public void toddle() throws Exception {}` 🎃
F. `public void toddle(boolean fall)` 🎃

My Answer:

  • A - true
  • B - false typo
  • C - true
  • D - false
  • E - false, an implementation cannot throw checked exception
  • F - false
  • A,C✅✅✅✅

Question 20

❓What is the result of the following code❓

public class FourLegged {
    String walk = "walk,";
    static class BabyRhino extends FourLegged {
        String walk = "toddle,";
    }
    public static void main() {
        FourLegged f = new BabyRhino();
        BabyRhino b  = new BabyRhino();
        System.out.print(f.walk);
        System.out.print(b.walk);
    }}
A. toddle,toddle, 🎃
B. toddle,walk, 🎃
C. walk,toddle, 🎃
D. walk,walk, 🎃
E. The code does not compile. 🎃
F. A runtime exception is thrown 🎃

My Answer:

  • Both instances are babyRhinos
  • A❌❌❌❌❌
  • CORRECT ANSWER: C
  • We get walk,toddle, printed
  • Instance variables DO NOT get overrided by subclass
  • As we are referencing the FoureLegged for the first object, we get walk, printed
  • As we are referencing BabyRhino for the second object, we get ,toddle

Question 21

❓Which of the following could be inserted to fill in the blank (Choose all that apply)❓

public interface Otter {
    default void play() { }
}
class RiverOtter implements Otter {
    ____________________________
}

A. @Override public boolean equals(Object o) { return false; }
B. @Override public boolean equals(Otter o) { return false; }
C. @Override public int hashCode() { return 42; }
D. @Override public long hashCode() { return 42; }
E. @Override public void play() { }
F. @Override void play() { }

My Answer:

  • A - true, it correctly overrides the equals(Object) method in the Object class
  • B - false
  • C - true
  • D - false hashCode returns int
  • E - true
  • F - false, it restricts visibility
  • A,C,E✅✅✅✅