AP Computer Science Study Guide

Welcome to the AP Computer Science Study Guide. Let's jump right in.

Table of Contents

Comments

• Long Comment

 /* * This is * a long comment. */ 

• Short Comment

// This is a short comment.

Intro to Strings

• String is the line of text, object in java, defined by the String class.
• String concatenation - Use + operator which adds multiple strings or number
• Escape Sequence

\n and \" will be the escape sequence you will used most frequently.

Variables

• Variables in math and science represents something useful. In Computer Science, a variable is an identifier of a memory location.
• Location's value can be modified.

Primitive Data Types

int = integer (-2, 147, 9, . )

double = floating point number, twice as much memory than float

char = single character

All primitive types and memory cost

• String is not a primitive type.

Using Mathematical Operators

• PEMDAS
• Operators: + , - , * , / , % (modulus - gets the remainder), = (assign NOT EQUAL)
• Parentheses overrides the operation order
• Defining a variable:

• Casting is converting from one data type to another.

  double square = (int)98.28; // Becomes 98

  += add then assign

  -= subtract then assign

  *= multiply then assign

  /= divide then assign

  %= get the reminder then assign

  The Math Class

  • Java has a built in math class that can deal with mathematical operations that are more complex than simple operation.

  int abs(int num) // returns absolute value

  double pow(double num, double power) // returns number to the power

  double sqrt(double num) // returns square root of positive number

  double random() // returns number between 0.0 & 1.0

  Random Class

   Random generator = new Random(); generator.nextInt(n); //return 1 to n-1 generator.nextDouble(); // returns same as math double random() 

  Intro to Conditionals

  • if/else
  • switch
  • while
  • do-while
  • for

  Equality Operators

  == means equals (= is the assignment operator)

  != means not equal to

  Relational Operator

  Logical Operators

  ! means not

  Example: !true // means not true or false

  && means and

  || means or

  Example: if (i < 0 || i >1) // i has to be less than 0 or greater than 1 to be true

  The if statement

  • This statement will do the next thing if the contents in the parentheses is found to be true. Otherwise, the statement will be skipped.

  Example: if (cuteness > 5)

  The else statement

  • This statement provides a secondary path of execution when an `if` clause evaluates to false.

  Example: if (housing != livesWithParents) < girl.giveRealNumber(); >else

  The while statement

  • If the statement is true, it will continue to run the code until it is false.
  • Beware of infinite loops.

  Example: int n = 1; int endPoint = 10; while (n

  Sentinel Value

  • Sentinel, or "flag value" is a special value. It is usually provided to end a program.

  The for statement

  • The for statement provides a compact way to iterate over a range of values.

   for (initialization; termination; increment)

  • When using this version of the for statement, keep in mind that:
    • The initialization expression initializes the loop; it's executed once, as the loop begins.
    • When the termination expression evaluates to false, the loop terminates.
    • The increment expression is invoked after each iteration through the loop; it is perfectly acceptable for this expression to increment or decrement a value.

    Examples: for (int i = 1; i for (int i = 0; i

    The switch statement

    • The switch statement can have a number of possible execution paths.

    Example: int month = 8; String monthString; switch (month) < case 1: monthString = "January"; break; case 2: monthString = "February"; break; case 3: monthString = "March"; break; case 4: monthString = "April"; break; case 5: monthString = "May"; break; case 6: monthString = "June"; break; case 7: monthString = "July"; break; case 8: monthString = "August"; break; case 9: monthString = "September"; break; case 10: monthString = "October"; break; case 11: monthString = "November"; break; case 12: monthString = "December"; break; default: monthString = "Invalid month"; break; >System.out.println(monthString); 

    Objects

    

    • What's the point?
      • Reuse code, break down problem solutions, divide and conquer, avoid errors.
      • A good object will be a thing, not an action. Example: Triangle, Snowman
    • Anatomy of a Class
      • Class Name
      • Class Data
      • Constructor (sets the initial data)
      • Method declarations (what the class does)
    • Visibility (aka scope)
      • public: anything outside of the class
      • private: can only be used within the class
      • protected: not used at this time
  • Methods do the work and are usually verbs.

    Naming Conventions

    • With multiple words:

    Advanced Conditional Statements

    String Comparison

    Remember String is an object.

    string1 == string2 tests for reference equality and is for testing whether two strings are the same object. Use string1.equals(string2) to test for value equality.

    The Comparable interface contains only one method, compareTo, which takes an object as a parameter and returns an integer. If you need comparison functionality, you should implement the comparable interface. The String class implements the Comparable interface. Strings are compared character by character. Each character is encoded and stored in binary.

    I the first characters are the same, then the next ones are compared and son. This basically does an alphabetization. Think of the string in the parenthesis as "the OTHER string".

    Example: String s1, s2; // s1 and s2 are assigned values int result = s1.compareTo( s2 ); if ( result > 0 ) < System.out.println( s1 + " is greater" ); >else if ( result < 0 ) < System.out.println( s2 + " is greater" ); >else

    Float Comparison

    Character Comparison

    Character values are numbers in the Unicode character set. They are in order, but upper and lowercase is seperator, so try not to use a>b.

    Arrays

    

    • Arrays are objects used to group and organize data. They are a sequence of values of the same type.
    • The values held in an array are called array elements.
    • An array stores multiple values of the same type (the element type).
    • The element type can be a primitive type or an object reference.
    • Thus, we can create an array of integers or an array of characters, or an array of string, objects, etc.
    • In Java, the array itself is an object.
    • Mistakes
      • Bound Errors - modified elements beyond the array
      • Uninitialized arrays
      • Unfilled arrays null reference
    • Use constants to show array length

      final int POWER = 5;
      int[] powerLevels = new int[POWER];

      Example: for (int index = 0; index < array.length; index++) < // do something here > 

      • The array starts at 0 (remember value 1 is at location 0)
      • The array goes to the length of the array
      • Bound error is avoided by using less than evaluation

      Example: // for each digit in the array numbers, goes through the entire array for (int digit: numbers) < // do something here > 

      A more in depth look at strings

      Primitive Data Type - Char

      • Char is a primitive data type. Example: char letter = 'a';

      String is an array of chars.

      Methods of the String Class

      • int length() - length of the string
      • String substring(int from, int to) - returns String from to to-1
      • String substring(int from) - return string from all the way to the end of the string
      • int indexOf(String str) - returns index of first found str. returns -1 if not found.
      • char charaAt(int index) - returns character at given index
      • int compareTo - compares two strings

      2D Arrays

      • A two-dimensional array is really nothing more than an array of arrays (a three-dimensional array is an array of arrays of arrays).

      Try the following: int[][] multi = new int[5][10]; . which is a short hand for something like this: int[][] multi = new int[5][]; multi[0] = new int[10]; multi[1] = new int[10]; multi[2] = new int[10]; multi[3] = new int[10]; multi[4] = new int[10]; Note that every element will be initialized to the default value for int, 0, so the above are also equivalent to: int[][] multi = new int[][]< < 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 >, < 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 >, < 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 >, < 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 >, < 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 >>; 

      • Looping through 2d arrays:

       for(int i = 0; i < array.length; i++) < for(int j = 0; j < array[0].length; j++) < // do something here >> //I'd recommend j < array[i] here. Nothing //changes on this case, but it would bring //some trouble if the second dimension of the //array was not the same for every case. 

      • The first for iterates over the array of five int arrays, that's why i goes from 0 to 4 and the array's length is 5. The second for iterates over the indexes of the int contained on the first array (the one with length 5). This arrays have a length of 10 elements.

      Array List

      • Dynamically changes size depending on what is needed
      • Allows you to specialize on the ArrayList or ArrayList etc.
      • The Wrapper Class:
        • Each of Java's eight primitive data types has a class dedicated to it. These are known as wrapper classes, because they "wrap" the primitive data type into an object of that class. So, there is an Integer class that holds an int variable, there is a Double class that holds a double variable, and so on. The wrapper classes are part of the java.lang package, which is imported by default into all Java programs.
        • As you are aware, In Java not everything is an Object. There are primitive types such as int ,double float etc. So if you wanted to use a primitive type in place of Object i.e. if a method expects an Object but you need to send in a primitive type, without Wrapper classes this wouldn't be possible. Take for instance the Map interface. The put(Object,Object) method takes Objects as both key and value. If you, for example wanted to store a mapping between an integer value 1 (int i=1) to an Object, without wrapper classes this wouldn't be possible. Wrapper classes are used to represent primitive values when an object is required.

        Inheritance and Polymorphism

        • Subclass - class that is extension of the larger class
        • Superclass - class above a class
        • Subclasses - define more specific behaviors, Is-A-Relationship
        • To inherit the functionality of superclass, use the key word extends

        Example: public class Object extends SuperClass < // do something here > 

        Recursion

        What is recursion?
        • Recursion is when the method directly or indirectly invokes itself.

        Example: public int factorial (int x) < if (x >1) < //recursive case: return factorial(x-1) * x; >else /*base case*/ return 1; > > 

        

        Following the code when it is factorial(3):

      • Without a base case in recursion the stack overflows