Java/Android Notes

• How is Java program created/built/executed?
• Create Hello.java.
• Create class Hello.
• Inside the Hello class, create a static method main with the signature:
• public static void main(String[] args)
• Inside the main method, use System.out.println("Hello world!"); to write to standard output.
• Use javac Hello.java to create Hello.class.
• Use java Hello to run Hello.class, notice no .class extension when using java to execute a class.
• Write a Java program that defines/uses package.
• Assume current directory is hello.
• mkdir -p a/b/c && cd a/b/c
• Inside a/b/c, create a file Add.java.
• Inside Add.java file, declare the package it belongs to:
• package a.b.c;
• Inside Add.java file, create a public class Add.
• Inside class Add, create a public static method add with the signature:
• public static int add(int a, int b)
• Go back to hello directory: cd hello.
• Create a file AddTest.java.
• Inside the AddTest.java file, import class Add from package a.b.c:
• import a.b.c.Add;
• Inside the AddTest.java file, create a public static main method with the signature:
• public static void main(String[] args)
• The main method:
• Read arguments from the command line.
• Use Integer.parseInt to convert them to int.
• Call Add.add to compute the sum.
• Call System.out.println to print the result to standard output.
• In directory hello, compile the AddTest.java file by javac AddTest.java, it generates AddTest.class.
• Run the program: java AddTest 1 2, should see the result 3 printed to the standard output.

• Java's member access rule:

Modifier    | Class | Package | Subclass | World
————————————+———————+—————————+——————————+———————
public      |  y    |    y    |    y     |   y
————————————+———————+—————————+——————————+———————
protected   |  y    |    y    |    y     |   n
————————————+———————+—————————+——————————+———————
no modifier |  y    |    y    |    n     |   n
————————————+———————+—————————+——————————+———————
private     |  y    |    n    |    n     |   n

y: accessible
n: not accessible

• How to import .jar in ADT
• Download the library .jar file x to folder p.
• In the `Package Explorer', find libs folder (if not exists, create one: New -> Folder).
• Right click libs, Import -> General -> File System.
• Navigate to folder p, find x in the right panel. Check the check-box. Click Finish.
• If libs is already in the build path, we are done. Otherwise, Right click the project, Build Path -> Configure Build Path..., select Libraries tab. Click Add JARs..., find the x, click ok.

• How to setup jni? (TODO)
• How to call c/c++ procedure from Java through jni? (TODO)
• How to call Java procedure from c/c++ through jni? (TODO)