Test 1 Review Sheet

Building Basic Java Programs

• A Java source file must end with the extension .java and should have the exact same name as the public class name.
• To compile a class in a Java source code file (say Yadda.java) type:
  javac Yadda.java
• Each class (there may be more than one) in a source code file, is compiled into a .class file.
• To execute a java application (say named Yadda.class) type
  java Yadda
• Know how to create a jar file.
• The main method has the following signature
  public static void main(String[] args)
• Single line comments in Java start with //
  Block-style comments in Java start with /* and end with */

Basic Elements

• Identifiers are the user-defined names in programming. (ex: variables, constants, methods, classes, labels, packages, etc.)
  • Made up of characters letters, underscores (_), dollar signs ($), or digits.
  • Must NOT start with a digit as the first character
  • Must not be a reserved word
  • Case Sensitive
• Built-in (primitive) types (boolean, char, byte, short, int, long, float, double)
• Arithmetic operators
• Assignment operator =
• Shortcut Operators ( += -= /= *= %= ++ --)
• Know which type conversions are automatically allowed, and how to do casting for others.
• Comparison operators
• Boolean operators
  The short-circuit operators (conditional operators) are (&& and ||), know how these differ from the unconditional operators (& and |)

Control Structures (Chapters 4-5)

• Know the if statement
• Know the switch statement
  remember that a break statement is need at the end of each case statement, otherwise execution falls through to the next case statement. The default case is executed when no other case matches.
• Know the ternary operator or Conditional expressions
  booleanExpression ? expression1 : expression2
  If booleanExpression is true then the result is expression1 else the result is expression2
• Know for, while and do loops
  Given one of them be able to create identical behaving loops using the other two.
  Be able to write loop-based algorithms.
• break and continue statements -- know what effect each one has

Methods (Chapter 6)

• Know how to create and code a method
• Know how to call a method, given its definition (or more specifically, its prototype)
• Know the difference between passing a primitive type to (or returning from) a method vs. passing an array or an object to (or returning from) a method
• Know how to call the Math methods we've used so far (in examples or homeworks), like sqrt and random
• Understand (and be able to code) recursive methods

Arrays (Chapter 7)

• Understand arrays, this includes multi-dimensional arrays
• Know how to declare an array variable, and then create the array with the new operator
• 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. 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

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 (chapter 8)

• Objects and classes and the relationship between them
• Constructors: what they are and how to build them
• The modifiers public and private
• 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
• 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
• Class scope vs. local scope (like in a method)
• The keyword this
• Arrays of objects (array of reference variables, each can attach to an object

Basic GUI

• Understand the basic use of class JOptionPane
• Know that this is part of the javax.swing package
• Understand the basic types of JOptionPanes (message dialog, input dialog, confirm dialog (yes/no/cancel)).

Programming hints

• You may assume that MyInput.readInt() and MyInput.readDouble() are available to be used.
• Be able to write methods that do simple algorithmic tasks (like in the homeworks) and use parameters and returns appropriately
• Be able to write a recursive method, given the recursive definition.
• 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
• 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 the test will ask a question.

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
  

The method signature is:

public static int greater(int[] array, int number)