To Lecture Notes

IT 313 -- Feb 20, 2020

Review Exercises

1. Look up some interesting instance methods of the HashMap class in the Java 11 Class Reference. Try them out in a Java JUnit test class.

IntelliJ Debugger

• An editor like IntelliJ is useful to help you find compiler errors.
• Even more, IntelliJ can help you fix compiler errors, for example, with Alt-Enter.
• Unfortunately, even after all compiler errors are fixed, logic errors may remain in your code. They are often hard to find.
• One way of debugging programs with logic errors in a high level language such as Java is to insert extra print statements so that you can compare the actual values of variables with their expected values.
• Another way to find logic errors is to use a debugger, in our case the IntelliJ debugger.
• Use the IntelliJ to find the compiler errors in this BreakfastBeverage class.
• Now use the IntelliJ debugger to step through the BreakfastBeverage program to find the logic errors:
  
  1. Set a break point by clicking in the gray area to the right of the line numbers. (If you click in the white code area, you are clicking to far to the right.) Set a breakpoint at Line 8. The breakpoint appears as a maroon disk.  Click on the breakpoint to remove it.
  2. Run the main method with debugging by clicking the green triangle to the left of the main method. Select Debug BreakfastBeverage.main instead of Run BreakfastBeverage.main. At the bottom of the IntelliJ display, there are two windows:
     
     1. The Frames window that shows all currently executing methods, in this case the main method in the BreakfastBeverage class.
     2. The Variables window that shows the values of all active variables, in this case args, which is an empty array.
  3. Then move forward though the code line by line using these buttons or control keys:
     

     Button	Tooltip	Description
     	Rerun (Control-F5)	Rerun the code in current class.
     	Step Over (F8)	Go to the next line, stepping over method calls.
     	Step Into (F7)	Go to the next line, stepping into a method, if there is one.
     	Force Step Into (Alt-Shift-F7)	Go to the next line, stepping into a method, if there is one, even if it is into the Java Class library.
     	Step Out (Shift-F8)	Go to the next line, skipping over method calls.
     	Run to Cursor (Alt-F9)	Run program from current line to cursor position.
     	Evaluate Expression (Alt-F8)	Evaluate expression.

     
     
  4. Select the Console window to enter keyboard input or select Debugger to show the debugger buttons, the Frames window, and the Variables window.
  5. Here is a video entitled Debugging in IntelliJ
          https://www.youtube.com/watch?v=1bCgzjatcr4

Project 6b

• Discuss Project 6b.

HashMap Iterators

• Two HashMap iterator styles -- K is the datatype of the key; V is the datatype of the value:

  // First style:
  for(Map.Entry<K, V> pair : hm.entrySet( )) {
      System.out.println(pair.getKey( ) + " " + pair.getValue( ));
  }
  
  // Second style:
  for(V value : hm.values( )) {
      System.out.println(value);
  }
  

• Practice Problem: Show the contents of the HashMap in Review Exercise 11 using these two iterator styles.

Serialization

• Serialization is the process of converting an object into a format in which it can be stored on a disc file.
• Deserialization is the reverse process of converting the object data stored on disc into a conventional in-memory object.
• Serializing an object is also called marshalling an object; deserializing is also called unmarshalling.
• Reasons to use serialization:
  
  1. it provides the ability to persist objects, that is writes the object data to disc.
  2. it allows objects to be transmitted over a network using a protocol like SOAP.
  3. it provides a method for distributing objects in the software community, for example CORBA or COM.
  4. it provides a method for detecting changes in time-varying data.
• In Java and other languages, serialization is performed automatically without requiring any user formatting code.
• Java binary serialization stores object data in binary .ser files.
• Other forms of serialization are possible:
  XML    YAML    JSON    SOAP
• See the Serialization1, Serialization2, and Serialization3 Examples.
• An object to be serialized must implement the Serializable interface, or have one of its base classes implement it.
• To implement the Serializable interface, no additional methods are required; the class must simply include this header:
  

  public class ClassName implements Serializable {
  

• All Java collection classes implement the Serializable interface.
• Important: only one object at a time can be serialized. If you wish to serialize multiple objects, put them into a collection class object and serialize that single object instead.
• Look at the EmployeeFind Example.

Software Design Patterns

• Software Patterns Summary
• We will discuss these software design patterns:
  Factory  Singleton  Observer  Iterator  Strategy  State