AP CSA EXAM REVIEW

• a constant

• holds the largest value for the int data type

• 2^31 –1

Integer.MAX_VALUE

• a constant

• holds the smallest value for the int data type

• -2^31

Integer.MIN_VALUE

• 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

static variable

• keyword used to make a variable a constant

final

• resizable array

• uses the List interface

• only holds objects

• built in methods for inserting, deleting, etc

• Can use the for-each loop or regular for-loop

ArrayList

• answer: 3

• Remember that with integer division the decimal is cut off, no rounding happens

int x = 7;

x = x /2;

what does x equal?

answer: urk

String s = “turkey”;

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

• answer: 3

• 7 divides into 3 zero times, leaving 3 as a remainder

int x = 3 % 7;

• answer: B

• Since 4.5 is a double you must use a numeric cast to change it to an int.

Which is correct?

A. int x = (double) 4.5;

B. int x = (int) 4.5;

C. int x = 4.5;

• 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 w = “ans: “+ 4 + 5; What is w?

• the `+` operator which combines strings together

String concatenation

A) \\

B) new line

C) “

What are:

A) \\\\

B) \\n

C) \\”

• && (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

Short circuit evaluation

1 2 3 4

2 4

3

4

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();

}

• == only does a primitive test for equality

• in objects this means it is testing the memory address, not the values stored in the object

Why is it best to use .equals instead of == with objects?

• 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

overloading

Wrapper class that holds ints

• used to store ints in an ArrayList

• holds the MAX_VALUE and MIN_VALUE

Integer

• 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

simple arrays

• creates a reference to an object in memory

• calls the constructor in the object

new

• 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

constructor

• means a method does not return a value

void

• An array of integers

• If this is a free response question the first thing you should do is: int \[\] list = new int \[10\];

public int \[\] doStuff()

What type does this method return?

15

What is returned by the call mystery(5)?

public static int mystery(int num){

if (num ==0) return 0;

return num + mystery(num - 1);

}

answer:

0 1 2 3

10 14 4 8

int list \[\] = { 5, 7, 2, 4};

for (int i = 0; i < list.length; i++) l

ist \[i\] = list\[i\] \* 2;

What is stored in list after this loop?

`null` is a special value that can be assigned to any object reference variable to indicate that the variable does not refer to any object.

null

• 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;

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

How many columns? How many rows?

Write the for loops to initialize this to: 01234 12345 23456

Public means the variable or method can be accessed outside the class, private means it cannot

public vs. private

• Create a one-line comment

//

• Makes a comment

• Can block out several lines of code

/\* \*/

• answer: (x == 5 || y>0)

Simplify: !(x != 5 && y

int maxIndex = 0;

for (int i = 1; i < x.length; i++){

if (x\[maxIndex\] < x\[i\])

maxIndex = i;

}

int x\[\];

Assume this array has been initialized. Write a loop to find the index of the largest element.

• 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

static methods

• Used to define a set of behavior for a group of classes

• example: List interface, Comparable interface

interface

• 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

How are abstract classes different than interfaces?

• 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 todo list, once a child class has code for all the abstract methods, the abstraction is lifted

You have an abstract class called first. A child class called second extends first. What must be true for second to NOT be abstract?

• Used to show a class uses an interface

• example: public class Binary implements Comparable{

implements

• 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,

Bug b1 = null;

b1.act();

What’s wrong?

A

What is output if x = 7;

if( x % 2 ==1)

System.out.print(“A”);

else

System.out.print(“B”);

• Connects a child class to it’s parent

• The child class inherits all the features of the parent class

extends

• calls the constructor of the parent class

• must be the first line in the child class’ constructor

super

• 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

abstract class

• template for a object

• can include variables and methods

class

• 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 classe

Object

• a variable of a class type

• can hold data (variables) and have methods

object