Advanced Class Design

Question 1

a instanceof B

Answer 1

Returns true if the reference to which a points is an instance of class B, a subclass of B (directly or indirectly), or a class that implements the B interface (directly or indirectly).

Question 2

What is the output?

abstract class Animal { 
     String name = "???"; 
     public void printName() {
         System.out.println(name); 
     }
}
class Lion extends Animal {
      String name = "Leo"; 
}

public class PlayWithAnimal {
       public static void main(String... args) {
           Animal animal = new Lion();
           animal.printName(); }
}

Answer 2

”???”

This is because name is an instance variable, so it uses the version that is in the class it is used in.

When it comes to methods, an overridden method will be used even from a superclass.

Question 3

@Override

Answer 3

Can be put before a method to indicate that it will be overriding a method from a superclass or interface. If the method that follows does not in fact override another method then you’ll get a compiler error.

Question 4

toString()

Answer 4

A method in the Object class that many classes override in order to print out the details of an object in a readable format. 
When you do System.out.println(obj) it will automatically try to call the toString() method on obj

Question 5

Rules for the equals() method

Answer 5

1) It is reflexive: x.equals(x) should return true.

2) It is symmetric: x.equals(y) should return
true if and only if y.equals(x) returns true.

3) It is transitive: if x.equals(y) returns
true and y.equals(z) returns true, then x.equals(z) should return true.

4) For any non‐null reference value x, x.equals(null) should return false. NOT NullPointerException.
5) It is consistent: For any non‐null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.

Question 6

Rules for the hashCode() method

Answer 6

1) Within the same program, the result of hashCode() must not change. This means that you shouldn’t include variables that change in figuring out the hash code.
2) If equals() returns true when called with two objects, calling hashCode() on each of those objects must return the same result. This means hashCode() can use a subset of the variables that equals() uses.
3) If equals() returns false when called with two objects, calling hashCode() on each of those objects does NOT have to return a different result.

Question 7

enum

Answer 7

A class that represents a fixed set of constants

Question 8

How to declare an enum

Answer 8

public enum testEnum {
FIRST, SECOND, THIRD
}

Question 9

Enum method values()

Answer 9

Returns an array of all of the enum values

Example:

public enum testEnum {
FIRST, SECOND, THIRD
}

testEnum.values() // [FIRST, SECOND, THIRD]

Question 10

Enum method name()

Answer 10

Returns the name of the enum constant

public enum testEnum {
FIRST, SECOND, THIRD
}

for (testEnum e: testEnum.values()) {
System.out.print(e.name()); //FIRST, then SECOND, the THIRD
}

Question 11

Enum method ordinal()

Answer 11

Returns the ordinal value of the enum constant

public enum testEnum {
FIRST, SECOND, THIRD
}

for (testEnum e: testEnum.values()) {
System.out.print(e.ordinal()); //1, then 2, then 3
}

Question 12

Can you extend an enum?

Answer 12

Noooo

Question 13

Nested Class

Answer 13

A class that is defined within another class

Question 14

Inner Class

Answer 14

A nested class that is not static

Question 15

Member Inner Class

Answer 15

Defined at the member level of a class (the same level as the methods, instance variables, and constructors).

• Can be declared public, private, or protected or use default access
• Can extend any class and implement interfaces
• Can be abstract or final
• Cannot declare static fields or methods
• Can access members of the outer class including private members

Question 16

If a class called Outer contains an inner class called Inner, what are the two class files generated?

Answer 16

Outer.class

Outer$Inner.class

Question 17

Local Inner Class

Answer 17

A nested class defined in a method

• They do not have an access specifier.
• They cannot be declared static and cannot declare static fields or methods.
• They have access to all fields and methods of the enclosing class.
• They do not have access to local variables of a method unless those variables are final or effectively final. If the code could still compile with the keyword final inserted before the local variable, the variable is effectively final.

Question 18

Anonymous Inner Class

Answer 18

A local inner class that does not have a name. The block with “SaleTodayOnly sale = …” is considered the anonymous inner class

public class AnonInner {
     abstract class SaleTodayOnly {
           abstract int dollarsOff(); 
     }
    public int admission(int basePrice) { 
           SaleTodayOnly sale = new SaleTodayOnly() {
                 int dollarsOff() { return 3; } 
           };
           return basePrice - sale.dollarsOff();
     }
}

Question 19

Static Nested Class

Answer 19

A static nested class is a static class defined at the member level. Since it is static, it can be instantiated without an object of the enclosing class, so it can’t access the instance variables without an explicit object of the enclosing class.

Question 20

How to import a static nested class?

Answer 20

Either with a regular import or a static import. Either will work.

Question 21

Method Signature of .equals() from Object

Answer 21

public boolean equals (Object obj)

Question 22

What is the rule involving a semicolon when declaring an enum?

Answer 22

The semicolon at the end of the enum values is optional only if the only thing in the enum is that list of values.

public enum Season {
WINTER(“Low”), SPRING(“Medium”), SUMMER(“High”), FALL(“Medium”);
private String expectedVisitors;
private Season(String expectedVisitors) {
this.expectedVisitors = expectedVisitors;
}
}

VS

public enum Season {
WINTER, SPRING, SUMMER, FALL
}

Question 23

When does the enum constructor run?

Answer 23

The first time that we ask for any of the enum values, Java constructs all of the enum values. After that, Java just returns the already‐constructed enum values.

This means the constructor will only run once no matter how many instances of the enum you create.

Question 24

What must you do if adding an abstract method to an enum?

Answer 24

Implement the method for each of the enum values

public enum Season {
WINTER {
public void printHours() {
System.out.println(“9am-3pm”);
}
}, SPRING {
public void printHours() {
System.out.println(“9am-5pm”);
}
}, SUMMER {
public void printHours() {
System.out.println(“9am-7pm”);
}
}, FALL {
public void printHours() {
System.out.println(“9am-5pm”);
}
};

public abstract void printHours();
}

Question 25

What level of access may an enum constructor have?

Answer 25

default or private

Question 26

Virtual Method Invocation

Answer 26

The subclass method gets called at runtime rather than the type in the variable reference

class Animal {
    public void eat() {
        System.out.println("kibble");
    }

class Lion extends Animal {
    public void eat() {
        System.out.println("steak");
    }
}

  Animal a = new Lion();
  a.eat();   
}

//Prints “steak”

Question 27

What is printed here?

public class FourLegged {
    String walk = "walk,";
    static class BabyRhino extends FourLegged{
        String walk = "toddle,"; 
    }
    public static void main(String[] args) {    
          FourLegged f = new BabyRhino(); 
          BabyRhino b = new BabyRhino(); 
          System.out.println(f.walk); 
          System.out.println(b.walk);
    }
}

Answer 27

“walk, toddle”

When it comes to instance variables, it’s the reference type that matters