1Reference Types10/24/20072Opening Discussion■Let's look at solutions to the interclass problem.■What people want to learn in Java.How to use it.Making a football or stats program.Pretty pictures (graphics).A scheduling/organizing program.Formal logic.Tracking spending.Flash movies.■Difference between theseScanner sc=new Scanner(System.in);Scanner kdb=new Scanner(System.in);■Difference between Java, C, and C++. What is windows written in?3Reference Types■The “built in” primitive types are not the only types in Java. Classes define types as well. These are called reference types.■We declare variables for reference types just like we would for primitive types.■Reference types can either refer to an object of the proper type, or they can be null.■Trying to use a null reference causes an exception.■You have already used a reference type variable when you use Scanner. Scanner is a class in the Java libraries.4What is a Reference?■Reference variables are called that for a reason. Instead of being an object, they refer to an object.■This is in contrast to a primitive variable which actually is the thing you declare it to be.■Because of this, a single reference variable could refer to several different objects over it's life.5Instantiating Objects■To make a new object we use the new keyword. The syntax for making an object is as follows:new Type(argumentList)■You will often see this on the right side of an assignment.■This tells Java to go out and get memory and set up an object of the specified type using that values that are passed in.■You have seen this already when working with a Scanner object to read values.6Calling Methods/Sending Messages■Once you have a reference variable that refers to a real object you can use the dot notation to call methods on that object.■This is what you have been doing with the Scanner object when you call methods like nextDouble().7The String Type■One of the most fundamental classes in Java is the String class. It represents any set of characters that you need to work with.■Let's look at String in the API.■The + operator is overloaded for String. Operators only work on primitives and, in this one instance, Strings.■Difference between null and empty Strings.■String literals are put in double quotes.■String objects are immutable. They don't change. Instead you build new String objects.■Checking equality.Use the equals method, not ==.8Wrapper Classes■There are some other classes that are very fundamental to Java that your book doesn't mention.■These are the wrapper classes in Java that have names like primitives, but start with capital letters.■For our purposes, the main use of these classes are the helpful methods they in them. These include things like converting strings to numbers.9Minute Essay■Write a little segment of code that has th user input a word and prints out how many letters are in the word.■Interclass Problem – Do problem 8.8. Warning: this isn't hard to program, but will take some
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