Java setup & first programs

The Java toolchain

The supplied installation PDF is primarily a sequence of screenshots and historical download links. The JDK provides development tools, including javac, while java launches compiled classes. The original links mention JDK 15 and JDK 8; these are historical examples rather than a recommendation to install those versions today.

Use a maintained JDK compatible with your teaching environment. Confirm that both commands resolve before compiling. JAVA_HOME identifies a JDK directory for tools that use it; PATH determines which command executable the terminal finds.

1java -version2javac -version

Compile and run HelloWorld

The supplied HelloWorld.java is a minimal program with a public static void main(String[] args) entry point. The public class name and filename must agree. Compile from the source directory, then invoke the class name without .java or .class.

• javac creates bytecode in a .class file.
• java loads the class and invokes its main method.
• A compilation error must be fixed before the new bytecode exists.
• If you edit source after compiling, recompile before testing the change.

1javac HelloWorld.java2java HelloWorld

Static and instance methods

A static method belongs to the class and can be called without constructing an instance. An instance method acts through an object and may use the state stored in that object. The supplied StaticMethod.java and InstanceMethod.java show this distinction.

Method parameters carry inputs into a computation. A return value carries its result back to the caller. Printing a result and returning a result are different operations.

Arguments, standard input and dependencies

Euclid reads two values from args, converts them to integers and prints the greatest common divisor. Other course examples read from standard input using StdIn and write using StdOut. These are supplied library classes, not built-in Java keywords.

The algs4 reference library uses the edu.princeton.cs.algs4 package. Keep those classes in the corresponding package hierarchy or use the matching library when compiling clients. Do not flatten or silently remove their package declarations.

• Missing arguments can cause an array bounds exception.
• Non-integer arguments can cause a number format exception.
• Compile a selected example and its dependencies; the entire source collection includes duplicate class names and is not one flat Java application.

1javac Euclid.java2java Euclid 48 18

A repeatable experiment

Record the source version, input and runtime environment with each timing experiment. First establish that the program returns the expected result. Then measure performance using a clear input-size definition.

Original implementations

Read the complete original code and its documentation. Core algorithm pages add a walkthrough, complexity discussion and a worked example.

Check your understanding

Supplementary practice. Try each question before opening the answer.