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What is Java?

Java is not Javascript!

Java is programming a language and a platform.

  • platform = an environment for running other programs

The Java platform consists of:

  1. the JVM
  2. the Java language and compiler
  3. the Java standard libraries

These are all included in the JDK. The JRE only contains the JVM and standard libraries.

  • JDK = Java Development Kit

  • JRE = Java Runtime Environment

    • used primarily for RUNNING java programs, but does NOT allow for developing

  • runtime = The point at which a program is run, as opposed to the point at which it is created.

  • JVM = Java Virtual Machine

  • virtual = opposite of physical, i.e. does not really exist

  • virtual machine = A computer that doesn't physically exist. In this case, another program pretends to be ("emulates") the virtual computer.

  • compiler = A program that translates source code into machine code.

The Java compiler (the javac program) translates Java source code into machine code for the virtual machine, known as "Java bytecode".

  • library = A collection of software.

  • standard library = A library of basic routines that comes packaged with a programming language.

Exercise: Find the Java7 standard library documentation.

Exercise: Find the JDK documentation.

Command-line Java

  • command line = An environment that expects you to type commands, and that executes these one by one and prints out the result. For example, the Terminal app.

Terminal shows you a $ prompt and waits for your commands.

  • prompt = An indication that the computer is waiting for you to type something.

To use your Java7 JDK:

export PATH=/Library/Java/JavaVirtualMachines/jdk1.7.0_75.jdk/Contents/Home/bin:$PATH

This tells your command line to find the java and related programs in your JDK.

$ cd
$ mkdir java
$ cd java

Open TextEdit. Select Format > Make Plain Text. (Important!) Enter some Java code.

public class Hello {
    public static void main(String[] argv) {
        System.out.println("Hello, world!");
    }
}

Save the file as Hello.java in your java directory. (Important! The file name must match the class name in the program.) Back in the terminal:

$ ls
Hello.java
$ javac Hello.java
$ ls
Hello.class    Hello.java

You should see a new file named Hello.class. That contains the Java bytecode.

$ java Hello
Hello, world!

Exercise: Make it greet you three times (in one run of the program).

OK now we can go back to IntelliJ, which takes care of all of this for us.

Printing

For each exercise, create a new class!

  1. Select File > New > Java Class, or press ^⌥ N and select Java Class.
  2. Give the class a name. Important: Capitalize the class name!
  3. Add the main() function as from 'hello world'.

When you run your code, make sure you are running the correct class!

  • camel caps = Words smushed together with each one capitalized. For example, "My name is Alex" in camel caps is MyNameIsAlex.

Exercise: http://programmingbydoing.com/a/comments-and-slashes.html

  • comment = Text within source code that it ignored by the compiler; it's there only for humans reading the source.

Exercise: There's a second comment syntax in Java. Find it. Why two?

Exercise: http://programmingbydoing.com/a/letter-to-yourself.html

Exercise: http://programmingbydoing.com/a/your-initials.html

Strings in Java are always quoted with double quotes.

  • quoting = Surrounding text with quotes or other special markers to indicate that the text between the quotes should be interpreted specially.

Exercise: http://programmingbydoing.com/a/a-good-first-program.html

What if you want to print two strings on the same line?

  • Try adding the strings as if they're numbers.

  • Try changing some println to print. What's the difference?

Some Java string escapes:

Escape result
\n end the line
\b backspace
\" double quote
  • escape = escape sequence = A sequence of characters that has a special or unusual interpretation.

Exercise: Why do we need these?

Exercise: Find the full list of Java escapes.

Exercise: Write a Java program that prints out some Java source code! It should print out the 'Hello world' line.

System.out.println("Hello, world!");

So far we've printed only strings. You can print numbers too.

Exercise: Try these:

println(42)
println("42")

println(42.0)
println(42.0000)
println("42.0000")
println(41.999999999999999)    // fifteen 9's
println("41.999999999999999")

What's going on?

  • integer = A round number, which has no fractional part. It may be positive, negative, or zero.

  • floating-point number = A computer representation of a number with a fractional part, i.e. a decimal point.

Exercise: Try these:

println(17 + 25)
println("17" + "25")
println("17" + 25)
println(17 + "25")

What's going on?

Exercise: Using Java, compute the sum of the numbers one through ten.

  • literal = A fixed value (numerical, string, or otherwise) that you type in explicitly.

  • expression = A formula for a value computed by the computer.

Today, it's -4 C outside.

Exercise: Print the temperature in Farenheit. Can you get the output to look like this?

temperature: 25 F

Exercise: (hard) How about like this?

temperature: 25°F

There are a few other things, too: single characters, booleans,

  • character = A letter, digit, punctuation mark, or another single symbol.
  • boolean = A choice between true and false.
println(Hello, world!)

println('x')
println("x")
println('Hello, world!')

println(true)
println(false)

There's one more special thing: null.

Exercise: Try to print out null.

Exercise: (hard) Find println in the Java library documentation.

Exercise: http://programmingbydoing.com/a/numbers-and-math.html

Exit ticket

https://docs.google.com/forms/d/1LTrWmkDmxw1_9TRIw6e3Ihi68Y-OCX3Cx2dljYGeNI4/viewform