Pi j4.1 packages
- 1. Programming in Java 5-day workshop The Java SDLC Matt Collison JP Morgan Chase 2021 PiJ4.1: Java Packages
- 2. Packages A package is a group of related classes, interfaces and resources: • Built-in packages • java.lang (imported automatically) Object, String, Math, primitive wrapper classes, ... • java.util Scanner, List, ArrayList, Random, HashSet, HashMap, ... • java.io File, FileReader, FileWriter, InputStream, OutputStream, IOexception, ... ... • User-defined packages (including the ‘no-name’ package)
- 3. Advantages of using packages 1. Reusability 2. Better organisation 3. Name conflicts: we can define two classes with the same name in different packages package package.name;
- 4. User defined packages package shape; //package declaration at the beginning of the program public class Rectangle implements Shape { ... } • A class can have only one package declaration. • Package name is all lowercase, by convention.
- 5. Compiling user-defined packages • Compile the code: >>javac -d destination_folder fileName.java • Example: >> javac -d . Shape.java Rectangle.java • After compilation, the byte codes will be located at: ./shape/Shape.class ./shape/Rectangle.class Note the dot referring to the current directory. ./shape is added as this represents the package
- 6. Using user-defined packages • A package name corresponds to the directory structure for storing the class files. import shape.Triangle; //compiler will look for shape/Triangle.class • In general, a company uses its reversed Internet domain name for its package names. //compiler will look for com/apple/computers/*.class import com.apple.computers.*;
- 7. Using user-defined packages Three options for using the package scope: 1. No import – use fully qualified name without the import keyword: shape.Shape rt = new shape.Rectangle(); 2. Import the class – only import the specific class you are interested in using: import shape.Shape; 3. Import the package – a wildcard import for all classes and interfaces in the package: import shape.*; Note: It is generally not good practise to import package name.* unless you really need many package members — Why?
- 8. Filesystem and packages • Packages in Java are often groups in subdirectories but are not hierarchical (although they may seem to be by looking at them). For example: • javax.crypto • javax.crypto.interfaces • javax.crypto.spec • import javax.crypto.*; • will import all the members of javax.crypto • but not javax.crypto.interfaces or javax.crypto.spec package members
- 9. Filesystem and packages • It is common to arrange your source and class directories separately. • Example: • The package source code: ./src/shape/*.java • The package class files: ./bin/shape/*.class • Q: What is the command for the above directory structure? >>javac -d bin/ src/shape/*.java
- 10. Classpath • To use this shape package, you may either copy the shape/*.class into the current working directory • or: >>javac -classpath bin/ ShapeApp.java >>java -classpath bin/:. ShapeApp or: Set CLASSPATH System Variable to locate where this package is.
- 11. The ant build tool • Ant enable the configuration of your project classpath, manifest file, target directory, etc in a structured xml >> ant compile >> ant jar >> ant run
- 12. The maven (mvn) build tool • Similar to ant but a stricter build lifecycle phases: • Maven enables: • dynamic dependency crawling • Plugin execution for fully operational devops
- 13. Packages summary • Keywords: package, import • Declare a package package shape; • Import a package member import shape.Rectangle; • Package management is best handled with maven
- 14. Documentation • Java supports three types of comments. During compilation, all the comments are ignored. • single line comment // ... • multiple lines comment /* ... */ • documentation comment /** ... */ • JavaDoc: A JDK tool automatically creates fancy HTML-based documentation based on /** ...*/ in your source files. >> javadoc NaturalLanguageNumbers.java
- 15. Javadocs import java.util.Scanner; //not used import java.util.Random; //not used /** * must put the class documentation directly before * the class definition. */ public class NaturalLanguageNumbers { /** * And method documentation directly before the method definition. */ public static String intToString( int number ){ … } … }
- 16. Javadoc tags In addition, you can include special javadoc tags in the documentation comments that provide specific information used by JavaDoc to format the documentation pages. • @author Provides information about the author. • @version Indicates the version number. • @since Indicate the version. • @param Provides the name and description of a method. • @return Provides a description of a method’s return value. • @throws Indicates exceptions that are thrown by a method.
- 17. Javadocs tags import java.util.Scanner; import java.util.Random; /** * A class for converting numbers to natural language representation. * @author Matt Collison * @version 1.0 */ public class NaturalLanguageNumbersTags { /** Defines the language */ public static final String language = “English”; /** * A method to convert integer values into natural language expressions. * @param number an integer value * @return the number expressed as words in natural language */ public static String intToString( int number ){ … } … }
- 18. Java Documentation • >> javadoc –d docs NaturalLanguageNumbersTags.java
- 19. Annotations • An annotation always starts with the symbol @ followed by the annotation name. • Different from the doc tag, annotations are checked by Java compiler. • Q: Where have you seen annotations before?
- 20. Pre-defined annotations Commonly used pre-defined annotations: • @Deprecated • @Override • @SuppressWarnings Custom annotations could be defined by using @interface. • More details see: https://docs.oracle.com/javase/tutorial/java/annotations/index.html
- 21. Javadoc tags vs annotations example • @Deprecated - indicates a marked element should no longer be used. It is usually also documented using the Javadoc @deprecated tag /** * @deprecated ← a doc tag, not checked by compiler * explanation of why it was deprecated */ @Deprecated ← an annotation, checked by compiler void anyMethod() { ... }
- 22. Learning resources The workshop homepage https://mcollison.github.io/JPMC-java-intro-2021/ The course materials https://mcollison.github.io/java-programming-foundations/ • Session worksheets – updated each week
- 23. Additional resources • Think Java: How to think like a computer scientist • Allen B Downey (O’Reilly Press) • Available under Creative Commons license • https://greenteapress.com/wp/think-java-2e/ • Oracle central Java Documentation – https://docs.oracle.com/javase/8/docs/api/ • Other sources: • W3Schools Java - https://www.w3schools.com/java/ • stack overflow - https://stackoverflow.com/ • Coding bat - https://codingbat.com/java