Explore the concept of exceptions in Java.

Understand what exceptions are and how to use them to handle errors in Java programs.


When running a Java program, if the JVM detects an error it will generate an error condition called an Exception. An Exception is actually an object that contains information about the error and is available for your code to capture and handle as needed. Generating an exception is called throwing, since all Exception objects are subclasses of the Java Throwable object. when an exception occurs, execution of your program stops at that point and the JVM will look for special code that handles exceptions. If an exception is not explicity handled by your code, the JVM will abort your program and report the exception details to the console.

An Exception object identifies the type of exception, indicated by the specific Exception object thrown. There are many pre-defined Exception objects descended from Exception such as IOException or ArithmeticException. An Exception object will contain the location in your program where the exception occurred (stack trace) and may include a description (message).

The most common exception you will encounter is the NullPointerException. This occurs when you attempt to use an object reference variable that has not been set to a valid object reference. Here is an example of this exception in CodingPoint. Compile and run the program to see the exception abort the program and report its information to the console. You can then comment out line 7, compile and run again. This will demonstrate how the stack trace information shows the where in a hierarchy of method calls the exception occurred.

What if you would like to catch exceptions and handle them in some other manner than aborting your program? Java provides a way to do that with try/catch/finally blocks. The general form of a try/catch/finally block is:

This says try executing the code in the try block and if an exception occurs, pass the exception to the catch block, which executes the statements in the catch block (your error handling code). If there is no exception, execution passes to the next statement after the catch block. The code in the optional finally block is always executed exception or not, and execution proceeds to the next statement after the finally block.

Here is an example in CodingPoint of catching an exception. You can compile and run the example and then uncomment the finally block and compile and run again to see how the finally block works. You can also comment out the call to myMethod to see how finally works when there is no exception.

Note that the exception occurred in myMethod but the try/catch block in the main method handled the exception. This is because Java will work its way back through a method call hierarchy until it finds a try/catch block that can handle the exception. Notice we said "finds" a try/catch block that can "handle" the exception. This is because a catch can specify a specific Exception it will handle. If the exception being caught matches an Exception class specified on a catch statement, that catch will process the exception. This is coupled with the fact that you can have multiple catch statements and so tune your exception processing by Exception type.

Here is an example in CodingPoint showing multiple catch statements handling the NullPointerException differently than all other exceptions.

When designing your programs you can use Exceptions for your own error handling. You can trigger exception handling just like Java with the throw statement. You simply throw the exception you want handled:

This will throw the standard Java Exception with your text as it's message.

You can also extend the Exception class to create your own Exceptions. Here is an example in CodingPoint showing how to use exceptions to handle your own error processing. Compile and run to see the generic Java exception used. Then comment the first throw out and uncomment the second. This will show the use of a custom Exception. Finally you can uncomment the catch for the MyException class and see how you can trap custom exceptions.

Note that if you throw exceptions in a method, the throws Exception specifier must be added to the method definition.

Here is a series of videos (video1, video2, video3, video4) about Exceptions and here is a detailed discussion of Exceptions.



Explore some of the advanced data types available with Java.
Explore how Java converts between different data types.

Understand Java's data conversion features which are called Casting.


Java provides the ability to convert from one data type to another within a set of rules. Some conversions are made automatically and some you have to explicitly request.

Here is a discussion of converting between primitive data types.

Here is a discussion of converting between reference (object) data types.

Here is a video about casting.



Explore basic variable collections in Java.

Understand basic collections, what they are, how they work and how to use them.


A Collection is an object that stores lists of other objects allowing the group of stored objects to be manipulated in many powerful ways. A Collection may sound like an array or ArrayList and while a Collection is quite different than an array, ArrayList is in fact one implementation of the Collection concept. Java has a large number of specific implementations of the Collection concept you can use. Here are the most commonly used types of Collection:

  • set - A list of objects with no duplicates.
  • list - A list of objects duplicates allowed. (ArrayList is an implementation of the list general type)
  • map - A list of objects with key values (no duplicates).
  • queue - A list of objects with features that support sequential processing of the elements.

Within each type, there are a number of actual implementations you can use. Each implementation has specific features or performance aspects that you consider when choosing an implementation to use for your programs. Here is an example:

This prints out:
second string
third string
first string
The example creates a Set of String objects stored in a HashSet implementation. A HashSet is a high performance Set but does not guarantee any particular order when retrieving elements from the Set. Note that since this is a Set, the second attempt to add "first string" to the collection is ignored since there is already a duplicate element in the Set.
Collections have methods for adding, deleting and retrieving elements and much more. One useful feature of Collections is the Iterator. An Iterator is an object you can retrieve from the Collection that provides methods for navigating and modifying the list. Using an Iterator you can move forward and backward on a Collection using next and previous. Note the for keyword in the example. The Java for keyword understands Iterators and supports using them to access Collection elements.  Here we we are saying do a for loop for all of the elements in mySet, type the elements as String and give me access to each element through the variable s.
Here is an example of a List type Collection:

This prints out:
new first string                                                                                                                                       
first string                                                                                                                                           
second string                                                                                                                                          
third string
second string
second string
third string
second string
first string
new first string

Here we create a List type Collection using the ArrayList implementation and add some elements. Note we added an element using an index (position) and it inserted the element at that location, moving all subsequent elements up. The ArrayList allows us to add a duplicate element. Finally we use the Iterator type ListIterator (a specialized Iterator for List collections) to manually list the elements in forward order and then reverse order. Note that the ListIterator to go in reverse order is created with it's starting position set to the last element in the list by using the list length field to identify the last element's position.

Due to the many types of Collections and the many implementations of the types of Collections, Collections can seem daunting and overly complex. Collections are very powerful tools for manipulating data sets but most cases can be handled by the ArrayList Collection type.

Here is a video on the ArrayList Collection type. Here is a detailed discussion of Collections starting with an introduction and moving through the specific implementations of the various Collection types.

Here is the example code in CodingGround. Add code to the example to use the iterator for myList to locate the element containing "third string" and remove it from the list. Print out the modified myList to confirm the removal.



Explore variable arrays in Java.

Understand what arrays are and how to use them.


An array is a special object used to store a list of variables of the same data type. An array is defined like this:

This statement defines and then creates an array of 3 integer variables (or elements) which will be addressed as a list. The new keyword defines the size of the array. We can then put values in the array and access them with an index value (position) in the array. Arrays are indexed starting at zero:

Note that we can initialize array values with the new keyword:

Arrays may have more than one dimension:

For loops are especially useful in processing arrays:

This will print out:

row 0 col 0 = 5
row 0 col 1 = 10
row 1 col 0 = 15
row 1 col 1 = 20

Notice the array has a built-in field called length that tells the size of the array.

Arrays are fixed in their dimensions once created so the array size can't be changed. If you need dynamic array sizing, that is, you want to change the size of the array as your program proceeds, you can use a class called an ArrayList. The ArrayList is defined in the java.util package. An ArrayList has methods that allow you to add and remove elements on the fly:

String s will contain "A different String object". Why? Because when we removed element zero, the rest of the elements shifted down.

ArrayLists have a number of methods you can use to manipulate the array. Note that the ArrayList can only contain object instance references (no primitives). Also note that when we created the ArrayList, we specified the type of object that would be contained in the ArrayList.

The for statement has a special case called for-each that applies to arrays and collections (next lesson). This special for statement will automatically provide each element in an array to your code in the for statement or block:

ArrayList is just one of many types of Lists (called Collections) available in the Java API.

Here is a video about single dimension Arrays. Here is a video about multi-dimension Arrays. Here is a detailed discussion of Arrays.

Here is the example code on CodingGround. Add code to the example to add up all the elements in array x1 using a for loop and print the result. Add another for loop to print the strings in ArrayList a1.



Explore Java enum (enumerations) class.

Understand Java enum class and how to use it.


Lots of times when programming we need to assign constant values to track the various states of a data item. For example, in a program we have an integer variable that indicates the day of the week. We can define a convention where the integer value zero is assigned to mean Sunday, the value of 1 to mean Monday, 2 to mean Tuesday and so on. When coding our program we have to remember that 2 means Tuesday. This tracking of numeric values and what they mean for various variables can get cumbersome and error prone in more complex programs. Another way to track the values associated with various states of a variable is with enums.

An enum or enumeration, is a special data type (an object) defined by Java. It is a list of constant numeric values known at compile time by names. When a variable of the enum type is created it is assigned one of it's pre-defined constant value names and can only have one of it's constant names as its value at a time. Because they represent constants, the names assigned are by convention in upper case. Here is an example:

Internally, Sunday is assigned a value of zero by the compiler, Monday a value of 1 and so on. The key concept is using names instead of constant values, so instead of having to remember what 1 or 6 or whatever some number means in the context of some variable, you use a name. Using names improves readability and reliability of code by making things more obvious. So how do we use enums? Here are some examples:

Here we created a variable day of enum type DayOfWeek and set it to TUESDAY. We can then use day in various ways. enums are best used to represent a fixed set of constants, like day of week, planets of the solar system, menu choices, any list of constants whose values are all known at compile time. Internally by default, each enum name is assigned a constant integer value starting at zero, but in the simple enum case, we don't care as we want to use the names not the numbers. Data types other than integers may be used (see below).
To make things more interesting, remember that an enum is a special form of a class. Therefore it can have fields and methods. By default, enum variables don't tell you their underlying constant value, only the name the variable is set to. We can extend the enum in the above example to track and return the underlying constant value of the enum to demonstrate extending the enum with fields and methods:

Here we added an int field to the enum and defined a constructor to set the value of the field. We also added a method to get the day number and made the field public so we can access it directly. Note that when using a constructor, we are required to explicitly define the constants for each name. Note in this example the numeric values assigned to the enum names are not in sequence. They don't have to be.
You can assign any primitive or String constant value you wish to the names and you can have more that one data value for each name. However, when you want to use a constant value other than integer or have more that one constant value associated with a name, you must use a constructor to define the data types. See the detailed discussion for more about this.
Enums are very useful in their basic form and given that they have the full capability of classes, they can be extended to be very powerful as in the Planets example in the detailed discussion.
Here is a video about enums. Here is a detailed discussion of enums.
Here is the example code in CodingGround for you to experiment with. Complete both enums for a full 7 days and test. After the next lesson on Arrays, return here and modify the example to print out a list of all of the values in the enums. You will need to view the video above to see how.


Explore Java classes for basic numeric data types.

Understand what the Number classes are and how they are used, including the concepts of boxing and unboxing.


We explored Java's primitive data types in an earlier lesson. While we use primitive numeric data types directly most of the time, there are times when we need to treat a numeric primitive as an object. For this reason Java provides the Number Classes. There is a subclass of Number for each numeric primitive:

  • Byte (byte)
  • Integer (int)
  • Short (short)
  • Long (long)
  • Double (double)
  • Float (float)

These classes wrap the primitive data type in an object. Often the compiler wraps the primitive for you. If you use a primitive where an object is expected, the compiler "boxes" the primitive in its wrapper class for you. If you use a Number object where a primitive is expected the compiler will convert or "unbox" the object into the primitive value.

There are three main reasons why you would want to use a Number object instead of the corresponding primitive type:

  • As an argument of a method that expects an object.
  • To use constants defined by the class, such as MIN_VALUE and MAX_VALUE, that provide the upper and lower bounds of the data type.
  • To use class methods for converting values to and from other primitive types, for converting to and from strings, and for converting between number systems (decimal, octal, hexadecimal, binary).

The last item is the most common reason.  The Number classes provide many methods for converting, parsing and otherwise manipulating numbers. Many of the methods are static, meaning you can use them without an actual instance of a Number class. A typical use is converting (parsing) a string of digits into a Number object or the primitive type. The various methods can be quite useful.

You can also use instances of Number classes just like you would a primitive type. Here is an example showing an Integer variable being used just like a primitive int and also showing one of the static methods of Integer to convert a String to an integer value:

Lets look at what is happening in the example. First we create an Integer object and set it's value to 3. Then we perform a math operation on that Integer object. To do this, Java unboxes the Integer object i to a primitive integer and adds 2 to it. The statement returns the primitive integer value 5 and Java sees that we want to put a primitive 5 into the Integer object i and so Java boxes the primitive integer back into an Integer object, which now contains 5 as it's value.

Next we print out the value of the Integer object i. Java sees we are using i like a primitive value and so unboxes the Integer object to a primitive value which println() knows how to handle. We also see that since i is an Integer object, we can use the methods defined for the Integer object, one of which is toString() which returns a string representation of the numeric value. The println() function sees a string and knows how to handle it.

Next we use a static method of the Integer class, parsetInt(n), to parse a string of digits into an int value and store that int value into a primitive int variable. You can see that the methods of the Integer object can be used to work with both primitive int values and Integer objects.

Finally we print the value of the primitive int j. Again, println() knows how to handle primitive int values so the first j is no problem. However, the next display of j tries to use the toString() method. This will fail since the variable j is a primitive int, which does not have methods.

Here is this example on CodingGround. Compile to see the error generated by Java when you try to use a method on a primitive data type variable. Fix the problem and get the program to run.

Here is a detailed discussion of the Number Classes.



Explore advanced topics related to objects and how they are used.
Explore the difference between passing primitive parameters and reference parameters.

Understand the difference in behavior of primitive data type parameters and reference data type parameters in methods.


In the lesson on passing parameters we learned that primitive parameters are passed by value, that is, a copy of the calling code's variable value is given to the method for it's internal use. Reference variable (object instance pointer) parameters are passed the same way but there is an important distinction to make in how reference parameters (objects) are used in methods.

For this lesson, we are going to use the String object and a new object called StringBuffer. StringBuffer is part of the standard Java API library. We will use String and StringBuffer to illustrate show Java manages object instances and reference variables.

 String objects are immutable, meaning that once they are created they can't be changed. So if we do this:

The first statement created a String object instance with the value "abc". Java allocates computer memory and places a String object containing the characters abc into that memory and places a reference (pointer) to that memory location in the variable myString. The second statement creates a new String object instance with the value "xyz" and places a reference to that new object in the same variable, myString. At this point the reference (pointer) to the original object containing "abc" is now no longer available to us. "abc" still exists in memory but we overlayed the reference to "abc" with a reference to "xyz". The String object "abc" is now orphaned, meaning it no longer is connected to a variable in our program and will be discarded by Java. This is called garbage collection.

Note that the String object has no methods that change it's contents. Strings are modified by making a new String object from the old one. Here is another example:

Here again, we did not modify the string "abc" with the second statement. A new string is created containing the characters myString references, "abc" in this case, and concatenates the string "xyz" to create a new string containing "abcxyz" and a reference to this new string is stored in myString. The original string containing "abc" is no longer referenced by this code and will be garbage collected.

Now the StringBuffer class does provide methods to change it's contents without creating a new object. So if we do this:

The first statement creates a new StringBuffer object instance and places "abc" into it. The second statement calls a method on the StringBuffer object that adds "xyz" to the "abc" already inside the StringBuffer object instance without creating a new instance. When done, mySB is still a reference to the same StringBuffer object which now contains "abcxyz".

So with all of that out of the way we can discuss the nuances of using reference parameters in a method.

Consider this example:

The intent of the method is to append "xyz" to whatever is in the String str. After the call to myMethod(), what is in the String myStr? The answer is "abc". Why? The parameter myStr is passed as a copy and placed into the method variable str. In the method, we change the contents of str. Str now points to a new String object instance containing "abcxyz". However, this new String object instance reference is not passed back to the calling code. myStr still points to "abc". Now consider this example:

After the call to myMethod1(), what does mySB contain? The answer is "abcxyz". Why? Because we called a method on the mySB object instance using the copy of mySB passed into the method's sb variable. This means that sb also references or points to the same object instance as mySB. With the StringBuffer object's append method, we change the internal state of the object instance jointly pointed to by sb and mySB.

After the call to myMethod2(), what does mySB contain? The answer is still "abcxyz". Why? Because we created a new StringBuffer object instance containing "xyz" and stored the reference to it in sb. For the rest of the method sb contains a reference to "xyz". Since we changed the object instance that sb points to, the method lost its pointer to mySB and can't reference it after that point. Note that at the end of the method sb, which contains "123456", will be released since variables created in a method only exist while that method is executing.

As a final point, in our first example the method wanted to append "xyz" to whatever string was passed into it, but it does not change the string that was passed in. Here is how you could fix that method:

Here the method concatenates the two strings using the + operator and creates a new string with it's reference deposited into str. The method then returns this new string reference and the calling code replaces the original string reference with the returned string reference and so myStr ends up pointing to the string instance "abcxyz".

A final note about garbage collection. In the last example, str will go out of scope when the method ends. You might think the new string pointed to by str would be orphaned and then garbage collected. However, since a reference to the actual string object instance in memory was passed back to the calling program and placed into myStr, the string object is not orphaned and will continue to exist. When a reference to an object instance is stored in a variable, the JVM keeps track of that by incrementing the object instance's reference count. When a variable that references an object goes out of scope or the variable has something new stored in it, the JVM decrements the object instance reference count. This is how the JVM knows when an object instance is orphaned, when its reference count goes to zero (no variable is pointing to it). When no variable is pointing to an object instance in memory, that object instance is no longer accessible and the memory can be released.

Here is a video about the difference between primitive and reference variables and how they are passed to methods.

Here are the examples on CodingGround.



Explore method overriding when extending classes.

Understand how subclass methods can override methods in the superclass.


We have seen how we can extend a class to have additional fields and methods and build a more specialized object. But what if we want to change the behavior of one of the superclass methods? we can do this with Overriding.

Overriding is simply defining a method in the new (sub) class that has the same signature (method name and parameter list) of a method in the super (parent) class. This method in the subclass is said to override the same method in the superclass. When you call this method on a subclass instance, you will execute  the code defined in the subclass. Note that if you call the method on an instance of the superclass, you will execute the code defined in the superclass.

Lets look at an example. Say we have a base class of Animal and subclasses of Dog and Cat. All three have the same method, whatIsMyName():

This code will print out:

I am an Animal named roger
I am a Dog named rover
I am a Cat and I am an Animal named kitty

So each class has the same method signature whatIsMyName() but the result of the method is different for each class. This concept is called polymorphisim. Note in the Cat version of whatIsMyName() we called the superclass method of the same name with the super keyword. So we can replace superclass functionality or modify the result of the superclass method in a subclass.

Here is a video with more about Overriding.

Here is the example above in CodingGround. Give it a try!