Test 2 Review Checklist

Methods (i.e. functions)

Method Basics

• Know what a method is
• Reasons for writing methods (divide-and-conquer, reusability)
• Perspective: builder vs. caller

Using Methods

• Know how to call a method
  • syntax
  • passing arguments
  • using the returned value
• Understand how they are like mathematical functions
• Predefined methods
  • methods in libraries
  • static vs. instance -- know the difference in the call syntax

Building Methods

• Prototype -- name, return type, parameter list
  • Know the format
  • Know what to specify in the parameter list
• Header includes modifiers (like public and static), as well as prototype
• Definition -- header, along with method body
  • the keyword return -- returning values from methods
  • using the formal parameters in the method
• Scope (and how it relates to methods)
  • Meaning of scope
  • Variables local to function blocks
  • Variables local to internal blocks (like loop bodies)
  • Scope of method parameters
• void as a return type
• Empty parameter lists
• Method overloading - methods with same name and different parameter lists

Strings

• Know how to construct a String object
• Know the common String methods: length(), concat(), equals(), compareTo(), charAt(), substring(), trim(), toLowerCase(), toUpperCase(), replace(), valueOf(), and the concatenation operator +
• Know the difference between String and StringBuilder
• Know the common StringBuilder methods: append(), insert(), delete(), along with common methods that work the same as for String (charAt(), length() for example), and know how to construct one

Arrays

• Understand arrays, this includes multi-dimensional arrays
• Know how to declare an array variable, and then create the array with the new operator
  int[] array1 = new int[size];

• Understand how array indexing is used, this includes multi-dimensional arrays
  Remember array indexing starts at zero and NOT one.
  given
  array =
{
  {1, 2, 3, 4},
  {5, 6, 7},
  {8}
}
  Then array[1][2] is equal to 7.
• Know that the length of an array (say named array) can be found by using array.length
• Know that the reference (name) of an array can point to any array of that type
  (two array references can point to the same array)
• Understand all the ways to declare an array variable. All the following are the same and are legal
  (the first is the preferred way):
int[][][] thingy;
int[][] thingy[];
int[] thingy[][];
int thingy[][][];
• Understand how to declare, create, and initialize an array all at once
  int[] bubba = {2, 5, -56};
• Know how to pass arrays in and out of methods
• Know how to interpret and how to write an enhanced for-loop, especially for use with arrays
• Understand how to interpret and how to write a variable-length parameter list on a method. Know when it is legal to do so

Some practice array algorithms to try (coding practice)

• Compute and print the sum (product, average) of the elements of a numerical array.
• A method that returns the maximum element of an array
• A method that prints all array elements that are between two given values (parameters)
• A method that counts up all the even numbers in an array
• A method that returns an array that contains all odd elements from an original array (incoming parameter)

Classes and Objects

• Objects and classes and the relationship between them
• Understand how to declare an object reference variable and then create the object with new, along with how to invoke the chosen constructor
• Understand how objects are passed in and out of methods
• Constructors:
  • Know what a constructor is for
  • Know how to identify a constructor in a class definition (i.e. the constructor has the same name as the class and no return type)
  • Know how to invoke a constructor (when building an object with new
• The modifiers public and private
• Accessor and mutator methods
• The special toString() method
• The static modifier, and the difference between static variables and methods vs. instance variables and methods, as well as which variables can be accessed from which methods
• The keyword this
• Class scope vs. local scope (like in a method)
• Arrays of objects (array of reference variables, each can attach to an object)

Programming hints

• You may assume that java.util.Scanner is available to be used
• Be able to write methods that do simple algorithmic tasks (like in the homeworks) and use parameters and returns appropriately
• Given an array be able to do a variety of array processing tasks -- a few examples are: compute or find and print out the sum of the elements, the product of the elements, the average of the elements, the maximum element, the minimum element, etc.
• Be able to write a method that modifies the array sent to it. A method that return the array but with the elements sorted would be an example
• Be able to write a method that modifies an object sent to it. Also be able to write a method that returns a new object (based perhaps on some relation to a parameter object or a calling object).
• Be able to write code statements that involve relationships between base and derived types
• Be able to write code that uses calls to the common String and StringBuffer class methods discussed in class
• Given an array be able to write a method that will create a new array based on the old array. For example: a method that will return a new array that has only the even elements in the original array in it.
  Example: Here's a method that returns a new array that contains just the positive elements of the original array.

    int [] firstHalf(int[] array)
    {
       // count the number of positive elements
       int count = 0;
       for (int i = 0; i < array.length; i++)
          if (array[i] > 0)
             count++;
  
       // create new array and copy positives into it
       int j = 0;
       int[] resultArray = new int[count];
       for (int i = 0; i < array.length; i++)
          if (array[i] > 0)
             resultArray[j++] = array[i];
  
       return resultArray;	// return the new array
    }
  
    

Here is a sample programming problem similar to how I might ask a question on the test.

Sample Exercise

Write a method that will return the number of elements in an array that are greater than a given number.

Given this sample main method:

public static void main(String[] args)
{
    int[] array = {14, 6, 3, 2, 8, 5};
    System.out.println("Given the following: array = ");
    printArray(array);
    System.out.println();
    System.out.println("The number of elements greater than 5 is " +
                       greater(array, 5));
}
  

The output is:

Given the following: array = {14, 6, 3, 2, 8, 5}
The number of elements greater than 5 is 3