cmsc132 exam #1 review

application programming interface (api)

description of what’s available in a class to the user

api examples

String, public features

data encapsulation

hides the representation of data from the user; the implementation details of a class are hidden from the public

procedural encapsulation

hides the algorithmic procedure; allows user to use system without worrying about specifics behind how the system works

autoboxing

Integer x = 7;

autounboxing

System.out.println(x+2);

comparable

interface that can be implemented to sort objects in “natural order”

comparable syntax

interface Comparable<T> {

public int compareTo(T other);

}

what are some java libraries that use comparable?

Collections.sort, Collections.max, Collections.min, Collections.binarySearch, Arrays.sort, Arrays.binarySearch

what similarities/differences to enumerations have to classes?

similarities: both have instance + static members

differences: constructors for enums are always private!

enumeration syntax

public enum Day {

SUN, MON, TUES, WED, THURS, FRI, SAT;

}

iterator

object that can sequentially access the elements of a collection

iterable interface

collections that implement this interface provide an iterator that can access its own elements

t/f: arraylists are iterable

true

write an iterator for the following arraylist: ArrayList<Cat> list = new ArrayList<>();

Iterator<Cat> iterator = list.iterator();

iterable interface syntax (include the three methods typically implemented in this interface)

interface Iterator<E> {

boolean hasNext();

E next();

void remove();

}

t/f: for-each loops are iterators

true

why do we need iterators/for-each loops?

1) the only way to cycle through elements of any java collection

2) efficient way to remove elements

immutable class/object

instance variables set during construction but cannot be modified after that

mutable class/object

instance variables set during construction and might be modified later

what happens when a mutable data member has been encapsulated?

• modification of its state within methods of the class is possible

• direct mutation by external code is not possible

what is a privacy leak? come up with an example that generates a privacy leak, and then practice correcting it

when private mutable data is accidentally exposed

what is inheritance?

creating new classes based on existing ones

keyword: extends

what is always at the root of the inheritance tree?

Object

what is polymorphism?

allows user to define one interface and implement it in various ways

suppose A extends B and B extends C. consider the method below:
void foo(B x) {...}
a. what classes of objects can be passed as arguments to this method?
b. what methods (from which classes) can be called on x inside this method?

what is method overloading?

methods in class w/ same name but different parameter types

what is method overriding?

subclass “replaces” inherited method (same parameter types)

what is “this”?

keyword used to refer to current object in a method/constructor

how is “this” used in constructors?

used to initiate a constructor in a current class

how is “this” used outside of constructors?

what is “super”?

used to invoke constructors/methods/instance variables of immediate parent class

how is “super” used in constructors?

how is “super” used outside of constructors?

early (static) binding

compiler makes decision (what is the type of variable?)

early binding example

Person p = new Student();

p.walk();

with early binding, walk() will be called via the Person class

late (dynamic) binding

decision made at runtime (what is class?)

late binding example

Person p = new Student();

p.walk();

with late binding, walk() will be called via the Student class

which type of binding does java use?

late (dynamic) binding

upcasting

casting “up the tree”; done implicitly

downcasting

• casting “down the tree”

• never done implicity

what is the format for an equals() method?

@Override

public boolean equals(Object other) {

if (!(other instanceOf Person) {

return false;

}

Person p = (Person)other;

return name.equals(p.name) && ID == p.ID;

}

Person p = new Person();

Student s = new Student();

Person tricky = new Student();

will the following code compile?

Person y = s;

yes, implicit upcast

Person p = new Person();

Student s = new Student();

Person tricky = new Student();

will the following code compile?

Student y = p;

no, attempted implicit downcast

Person p = new Person();

Student s = new Student();

Person tricky = new Student();

will the following code compile?

Student y = tricky;

no, attempted implicit downcast

Person p = new Person();

Student s = new Student();

Person tricky = new Student();

will the following code compile?

(Student)p

complies, but throws exception

Person p = new Person();

Student s = new Student();

Person tricky = new Student();

will the following code compile?

(Student)tricky

yes

Person p = new Person();

Student s = new Student();

Person tricky = new Student();

will the following code compile?

(Faculty)s

no, student + faculty on same level of tree

Person p = new Person();

Student s = new Student();

Person tricky = new Student();

will the following code compile?

(Faculty)tricky

compiles, but throws exception

which type of casting is done implicitly?

upcasting

what’s a safe form of downcasting (using the teacher + student classes)?

Student s = (Student)p;

if (p instanceOf Student) {

Student s = (Student)p;

}

what’s the purpose of safe downcasting?

prevents exceptions (in the case that p (Person) refers to object that does not satisfy type Student)

what is the visibilty of public int x?

x is visible anywhere in project

what is the visibilty of private int x?

x is visible only in current class

what is the visibilty of int x (package visibility)?

x is visible in any class within current package

what is the visibilty of protected int x?

x is visible in any class within the current package + any subclass of the current class

will the underlined method compile?

public class Base {

public void m (int x) { … }

}

public class Derived extends Base {

public int m (int x) {…}

}

no

will the underlined line of code compile?

public class Base {

public void m (int x) { … }

}

public class Derived extends Base {

public void m (int x) {…}

}

override

will the underlined line of code compile?

public class Base {

public void m (int x) { … }

}

public class Derived extends Base {

public void m (double x) {…}

}

overload

which of the numbered method is called?

public class Base {

public void m (int x) { … }

}

public class Derived extends Base {

public int m (int x) { … } (1)

public void m (int x) { … } (2)

public void m (double x) { … } (3)

}

Base x = new Base();

Base y = new Derived();

Derived z = new Derived();

x.m(5);

1 (superclass)

which of the numbered method is called?

public class Base {

public void m (int x) { … }

}

public class Derived extends Base {

public int m (int x) { … } (1)

public void m (int x) { … } (2)

public void m (double x) { … } (3)

}

Base x = new Base();

Base y = new Derived();

Derived z = new Derived();

y.m(6);

2 (override in subclass)

which of the numbered method is called?

public class Base {

public void m (int x) { … }

}

public class Derived extends Base {

public int m (int x) { … } (1)

public void m (int x) { … } (2)

public void m (double x) { … } (3)

}

Base x = new Base();

Base y = new Derived();

Derived z = new Derived();

y.m(7.0);

won’t compile

which of the numbered method is called?

public class Base {

public void m (int x) { … }

}

public class Derived extends Base {

public int m (int x) { … } (1)

public void m (int x) { … } (2)

public void m (double x) { … } (3)

}

Base x = new Base();

Base y = new Derived();

Derived z = new Derived();

z.m(8.0);

3 (overload in subclass)

x == y

true when x and y are the SAME object (aliases)

x.equals(y)

true when x and y are objects that “look” the same (same state)

equals() is called extensively throughout __________ framework

collection

is multiple inheritance allowed in java?

no!

can you override a variable that’s inherited

yes, it’s called (shadowing) (not recommended)

what are the benefits of abstract classes (in comparison of interfaces)

• instance variables (state)

• constructors

what is the benefit of interfaces (in comparison of abstract classes)

a class can implement more than one

what’s the purpose of the keyword final?

• final int x = 7; → x cannot be modified

• final Cat c = new Cat(); → c cannot refer to a different Cat

• final void foo {…} → cannot override/create subclass

abstract class

• cannot be instantiated directly

  • can’t use “new”

• purpose: to be extended in different ways

unchecked exceptions

• things that should never happen

• typically programming errors

• usually don’t require try/catch handlers

checked exceptions

• represent common events that can occur even w/ correct implementation

• normally require a catch block (catch/declare):

  • handle w/ catch block

  • declare method that throws it

unchecked exception class syntax

public class IllegalFishPositionException extends RuntimeException {…}

checked exception class syntax

public class NetworkTooSlowException extends Exception {…}

static nested classes syntax

public class A {

public static class B {

}

}

inner nested class syntax

public class A {

public class B {

}

}

what’s the difference between static nested classes and inner nested classes?

• static classes: essentially like having two ordinary classes

• inner classes: instance of inner class is always “attached to” an instance of the outer class

  • “shares” its members

  • no privacy between A + B

instantiating inner class objects syntax

Outer outer = new Outer();

Outer.Inner inner = outer.new Inner();

comparator interface

separate class (implementing Comparator) to define “additional orderings”

comparator interface syntax

interface Comparator<T> {

int compare(T obj1, T obj2);

}

anonymous inner classes

• class w/ no name

• declared + instantiated in the same spot

• must implement an interface or extend a class

• cannot have constructors

• if declared in static method, object in not “attached to” any instance of outer class

functional interface

interface w/ just one abstract method

functional interface examples

BinaryFunction, Comparator, Comparable, Runnable, ActionListener

lambda expressions

• objects that contain code for a single method

• no state, exactly "one behavior”

• can assign to variable whose type is a functional interface

lambda expressions syntax

BinaryFunction multiplication = (a, b) → {

return a*b;

};

what are some advantages lambda expressions have over anonymous inner classes?

• uses (slightly) less memory

• runs (slightly) faster

• sometimes fewer objects at runtime (reduces need for garbage collection)

what are two ways to copy an object

1) copy constructor

2) clone() method (if class implements Cloneable)

clone()

• inherited from Object class

  • returns a very fast shallow copy of entire object

  • class of new object is same as the class of the original

• implements Cloneable interface

  • no methods to implement

clone() syntax

public Triangle clone() throws CloneNotSupportedException {

return (Triangle)super.clone();

}

when does a static initializer block run?

runs once, when class is first loaded

static initializer syntax

public class Foo {

static {

}

}

instance initializer syntax

public class Foo {

{

}

}

when does a instance initializer block run?

• runs every time an object is instantiated

• runs before the constructor

• can eliminate redundancy in constructors

reference copy

two variables point to exact same object in memory

reference copy example

Person p = new Person();

Person refCopy = p;

shallow copy

make a new object, but make reference copies for the different instance variables

shallow copy example

int[] a = {1, 2, 3};

int[] shallowCopy = a.clone();