Which relationship justifies class Manager extends Employee?
Which design is correct for a Car and its Engine?
The difference between composition and aggregation is mainly:
How many classes may a Java class directly extend?
Which members does a subclass inherit?
A subclass needs to read a private field of its superclass. It should:
Which access modifier makes a member visible to subclasses in other packages but not to unrelated classes?
If a constructor contains neither this(...) nor super(...), what does the
compiler insert?
A class Animal declares only public Animal(String name). Which statement is true
of a subclass Dog?
Where must super(...) appear in a constructor?
What is the output of new Child();?
class Parent { Parent() { System.out.print("P"); } }
class Child extends Parent {
{ System.out.print("B"); }
Child() { System.out.print("C"); }
}
What is printed?
class Ship {
protected int speed = 3;
Ship() { System.out.print(speed + ","); }
}
class Rocket extends Ship {
protected int speed = 5;
Rocket() { System.out.print(speed); }
public static void main(String[] a) { new Rocket(); }
}
In the previous question, what is the correct term for what Rocket's speed does
to Ship's speed?
Which statement about overriding is correct?
An overriding method may:
What does the @Override annotation do?
What is the output?
class Book {
public String material() { return "paper"; }
public void show() { System.out.print(material()); }
}
class Encyclopedia extends Book {
@Override public String material() { return "papyrus"; }
public static void main(String[] a) { new Encyclopedia().show(); }
}
Which member cannot be overridden?
What happens when a subclass declares a method with the same name as a superclass method but a different parameter list?
Which is true of final?
What is the result?
class Automobile {
private final String drive() { return "drive"; }
public String go() { return drive(); }
}
class ElectricCar extends Automobile {
public String drive() { return "charge and drive"; }
public static void main(String[] a) {
System.out.print(new ElectricCar().go());
}
}
Every class that does not declare a superclass implicitly extends:
Which method is inherited by every class in Java?
What is printed?
class A { void greet() { System.out.print("A"); } }
class B extends A { void greet() { super.greet(); System.out.print("B"); } }
// new B().greet();
What does super.method() call?
Which class heading correctly declares that Square is a special kind of
Rectangle?
A Library holds books that continue to exist if the library closes. This is:
Which is the strongest argument for composition over inheritance?
What is the output?
class Account {
protected double balance = 100;
public void withdraw(double a) { balance -= a; }
}
class Savings extends Account {
@Override public void withdraw(double a) {
if (balance - a >= 50) super.withdraw(a);
}
public static void main(String[] x) {
Savings s = new Savings();
s.withdraw(70);
System.out.print(s.balance);
}
}
In a three-level hierarchy A → B → C, creating a C object runs
the constructors in which order?
Which statement about constructors and inheritance is false?
A subclass overrides toString(). What does System.out.println(obj) print for an
object of that subclass?
Which of the following pairs shows overriding rather than overloading?
Explain, with a short example of your own, when you would model a
relationship with extends and when with a field, and give one problem caused by choosing
wrongly.
A class Shape has only the constructor
public Shape(String name). A student writes class Circle extends Shape { private double r; public Circle(double r) { this.r = r; } } and the compiler complains. Explain precisely what is wrong
and write the corrected constructor.
State the four rules an overriding method must satisfy, and explain
why @Override is worth writing even though it is optional.
Write a superclass Employee with name and baseSalary,
a constructor, and double computePay() returning the base salary. Write two subclasses:
SalesEmployee (adds a commission on sales) and HourlyEmployee (pay is rate times hours,
with overtime above 40 hours at 1.5 times the rate). Override computePay and toString
in both, use super where appropriate, and write a driver that prints the pay of one employee
of each kind.