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AP Computer Science * Mrs Dovi * Patrick Henry High School * Ashland VA Integer For these problems [ay special at- If this is a free response question

2016 AP® COMPUTER SCIENCE A FREE-RESPONSE QUESTIONS This question involves the implementation and extension of a RandomStringChooser

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Schram, Leon Multiple-Choice Free-Response Questions in Preparation for the AP Computer Science Examination 9th Ed New York: D&S Marketing, 2015

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AP Computer Science * Mrs Dovi * Patrick Henry High School * Ashland VA Integer For these problems [ay special at- If this is a free response question

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Exploring Computer Science is for students with little or no previous knowledge of programming AP Computer Science Principles is for students who have completed an introductory course, but are not yet ready for problems without seeking immediate assistance or getting frustrated Multiple-choice and free- response

Integer.MAX_VALUE

a constant holds the largest value for the int data type 2 31
-1

Integer.MIN_VALUE

a constant holds the smallest value for the int data type -2 31
static variables variable is connected to the whole class, not individual objects all variables of the class share this one variable with the keyword final it is used to create a constant also called: class variables AP Computer Science * Mrs. Dovi * Patrick Henry High School * Ashland VA final keyword used to make a variable a constant

ArrayList

resizable array uses the List interface only holds objects built in methods for inserting, delet- ing, etc

Can use the for-each loop or regu-lar for-loop

int x = 7; x = x /2; what does x equal? answer: 3

Remember that with integer divi-

sion the decimal is cut off, no rounding happens AP Computer Science * Mrs. Dovi * Patrick Henry High School * Ashland VA

String s = "turkey";

what is: s.substring(1, 4); answer: urk remember:

Strings start at 0

substring stops at the second number -1 int x = 3 % 7; what is x? answer: 3

7 divides into 3 zero times, leaving

3 as a remainder

Which is correct?

A. int x = (double) 4.5;

B. int x = (int) 4.5;

C. int x = 4.5;

answer: B

Since 4.5 is a double you must use

a numeric cast to change it to an int. 0 1 2 3 4 5 t u r k e y start stop 4-1 AP Computer Science * Mrs. Dovi * Patrick Henry High School * Ashland VA

String w = "ans: "+ 4 + 5;

What is w?

answer: ans 45 Since the first thing after the = is a String, java uses concatenation

so it changes 4 to a String, puts it on the end of ans , then changes 5 to a String and puts it at the end

String

concatenation

What Interface does it implement?

What methods are included?

What are:

A) \\ B) \n C) \" called escape sequences used for some characters that can- not be typed\displayed easily A) \

B) new line

C) " AP Computer Science * Mrs. Dovi * Patrick Henry High School * Ashland VA

Short circuit

evaluation Java doesn't always test both halves of a Boolean operation - sometimes it uses a shortcut

&& (and): if the first part is false, the whole thing will be false, so it never tests the second part

|| (or): if the first part is true, the whole thing will be true, so it never tests the second part

What is output?

for (int a = 1; a < 5; a++){ for (int b = a; b < 5; b += a)

System.out.print(b);

System.out.println();

answer:

1 2 3 4

2 4 3 4

Be careful! The first time a = 1, so the second loop counts by 1. Next time a = 2, so it counts by 2, etc.

For these problems [ay special at-tention to where they start, where they stop and what they count by.

Why is it best to

use .equals instead of == with objects? == only does a primitive test for equality in objects this means it is testing the memory address, not the values stored in the object AP Computer Science * Mrs. Dovi * Patrick Henry High School * Ashland VA overloading

In the same class, many methods

with the same name

Java tells them apart using the signature

Signature: the number a type of

parameters CANNOT use the methods return type to tell them apart

Integer

Wrapper class that holds ints

used to store ints in an ArrayList holds the MAX_VALUE and MIN_VALUE simple arrays array can hold primitive types or class types all elements are of the same type not resizable

Use the regular for-loop

Use .length to find the size

AP Computer Science * Mrs. Dovi * Patrick Henry High School * Ashland VA new creates a reference to an object in memory calls the constructor in the object constructor builds an object in memory sets up all the variables in the objects has the same name as the class

NEVER has a return type or void

can overload the constructor - Java tells them apart by the number and type of the parameters void means a method does not return a value AP Computer Science * Mrs. Dovi * Patrick Henry High School * Ashland VA public int [] doStuff()

What type does this method

return?

An array of integers

If this is a free response question

the first thing you should do is: int [] list = new int [10]; return list;

This is usually worth 1/2 to 1 point!

What is returned by the call

mystery(5)? public static int mystery(int num){ if (num ==0) return 0; return num + mystery(num - 1);

How about mystery (3)?

mystery(2)? answer: 15 mystery(3)? 6 mystery(2)? 3 int list [] = { 5, 7, 2, 4}; for (int i = 0; i < list.length; i++) list [i] = list[i] * 2;

What is stored in list

after this loop? answer:

0 1 2 3

10 14 4 8

AP Computer Science * Mrs. Dovi * Patrick Henry High School * Ashland VA null

What Interface does it implement?

What methods are included?

int stuff [] [] = new int [3][5];

How many columns?

How many rows?

Write the for loops to initialize this to:

01234
12345
23456

2-D arrays are row-major - > the

rows come first. rows = 3, columns = 5 for(int r = 0; r < stuff.length; r++) for (int c = 0; c < stuff[r].length; c++) stuff[r][c] = r + c; public vs. private

Public means the variable or

method can be accessed outside the class, private means it cannot On the AP exam all variables in a class are expected to be private AP Computer Science * Mrs. Dovi * Patrick Henry High School * Ashland VA

Create a one-line comment

Makes a comment

Can block out several lines of code

Simplify:

!(x != 5 && y <=0) answer: (x == 5 || y>0)

This is an example of DeMorgan's Law

To simplify you distribute the ! and take the opposite of each operation

Be careful! The opposite of > is <=

Review this - there are always a few of these in the MC section - this should be a fast and easy question

AP Computer Science * Mrs. Dovi * Patrick Henry High School * Ashland VA

What is

1101
2 in base 10? answer: 13

You should know binary, octal and

hex. They are in the AP course description. int x[];

Assume this array has been

initialized. Write a loop to find the index of the largest element. int maxIndex = 0; for (int i = 1; i < x.length; i++){ if (x[maxIndex] < x[i]) maxIndex = i;

Why do we set maxIndex to 0 before the loop?

Why does the loop start at 1?

static methods method connected to the class, not an object ex: Math.random You do not declare a variable of Math type to get to the random method AP Computer Science * Mrs. Dovi * Patrick Henry High School * Ashland VA interface Used to define a set of behavior for a group of classes example: List interface, Comparable interface

How are abstract

classes different than interfaces?

Interfaces cannot have variables or

code in methods. They can only have constants and a list of abstract methods Abstract classes can have some code along with the abstract methods

You have an abstract

class called first. A child class called second extends first.

What must be true for

second to NOT be abstract? second must include code for all of the abstract methods in first. If second does not have code for all the abstract methods in first, second is also abstract

The abstract methods are like a to-do list, once a child class has code for all the abstract methods, the abstraction is lifted

AP Computer Science * Mrs. Dovi * Patrick Henry High School * Ashland VA implements

Used to show a class uses an interface

example: public class Binary implements Comparable{

Bug b1 = null;

b1.act();

What's wrong?

Since b1 is null, it does not point to

a location in memory. We cannot use any methods on a null object. throws a NullPointerException,

What is output if x = 7;

if( x % 2 ==1)

System.out.print("A");

else

System.out.print("B");

answer: A % finds the remainder %2 finds if a number is even or odd AP Computer Science * Mrs. Dovi * Patrick Henry High School * Ashland VA extends

Connects a child class to it's parent

The child class inherits all the

features of the parent class

Example:

public class RockMonster extends Monster super calls the constructor of the parent class must be the first line in the child class' constructor abstract class

A class set up to be a parent to a

subclass can have abstract methods, these are methods with no code an abstract class cannot be instantiated (cannot create an object from it) If the child class does not implement all the abstract methods it is also abstract AP Computer Science * Mrs. Dovi * Patrick Henry High School * Ashland VA class template for a object can include variables and methods

Hints:

According to the AP Java subset all

variables should be private! On some old free response ques-tions there has been a 1/2 deduc-tion for not using private

Object

All classes are children of this class

has a toString and equals method toString returns the variable's memory address equals tests if two variables memory addresses are the same the equals and toString methods are usually rewritten in child classes object a variable of a class type can hold data (variables) and have methodsquotesdbs_dbs20.pdfusesText_26

[PDF] [PDF] Flash Cards

Integer.MAX_VALUE

Integer.MIN_VALUE

ArrayList

Can use the for-each loop or regu-lar for-loop

Remember that with integer divi-

String s = "turkey";

Strings start at 0

7 divides into 3 zero times, leaving

3 as a remainder

Which is correct?

A. int x = (double) 4.5;

B. int x = (int) 4.5;

C. int x = 4.5;

Since 4.5 is a double you must use

String w = "ans: "+ 4 + 5;

What is w?

String

What Interface does it implement?

What methods are included?

What are:

B) new line

Short circuit

What is output?

System.out.print(b);

System.out.println();

1 2 3 4

Why is it best to

In the same class, many methods

Java tells them apart using the signature

Signature: the number a type of

Integer

Wrapper class that holds ints

Use the regular for-loop

Use .length to find the size

NEVER has a return type or void

What type does this method

An array of integers

If this is a free response question

This is usually worth 1/2 to 1 point!

What is returned by the call

How about mystery (3)?

What is stored in list

0 1 2 3

10 14 4 8

What Interface does it implement?

What methods are included?

How many columns?

How many rows?

Write the for loops to initialize this to:

2-D arrays are row-major - > the

Public means the variable or

Create a one-line comment

Makes a comment

Can block out several lines of code

Simplify:

This is an example of DeMorgan's Law

Be careful! The opposite of > is <=

What is

You should know binary, octal and

Assume this array has been

Why do we set maxIndex to 0 before the loop?

Why does the loop start at 1?

How are abstract

Interfaces cannot have variables or

You have an abstract

What must be true for

Used to show a class uses an interface

Bug b1 = null;

What's wrong?

Since b1 is null, it does not point to

What is output if x = 7;

System.out.print("A");

System.out.print("B");

Connects a child class to it's parent

The child class inherits all the

Example:

A class set up to be a parent to a

Hints:

According to the AP Java subset all