"... a person does not really understand something until after teaching it to a computer, i.e., expressing it as an algorithm."
          - D. Knuth

Computers are good at following instructions, but not at reading your mind."
          - D. Knuth

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"source" code in Java --> compile -->

bytecode --> start up JVM -->

Program output- problem solution

    .java file                                                 .class file                   
                                                                

• The compiler creates a new document - in machine independent bytecode. 
• Bytecode is like machine code, but will run on any chip - PowerPC, Pentium, etc.
• To run a program, the bytecode in the .class file is given to an interpreter (the Java Virtual Machine, or JVM) which converts the bytecode into machine language for a specific chip   
  
•  The Java Virtual Machine (JVM) is software (a program) that runs on the computer, and translates the bytecode into low level code the computer can understand.
• Here's what you do:
• Type your source code - your Java program - in a file.
• Compile it (either using the javac compiler or bluej) - translate your java program into bytecode.
• Run the resulting bytecode on a JVM.   
  

---

public class Hello {
    public static void main(String[] args) {
        System.out.println("Hello, world!");
        System.out.println();
        System.out.println("This program produces");
        System.out.println("four lines of output");
    }
}

• This class is stored in a file called Hello.java

• Its output:

    Hello, world!

    This program produces
    four lines of output

---

public class SomeJavaClass
{
   public static void main(String[] args)
   {
       statement;
       statement;
       ...
       statement;
   }
}


This class is stored in a source file called SomeJavaClass.java.

An application program: one or more classes + (a single main) method (in one of the classes).

• The body of the class is contained in curly braces.
• The class header indicates the accessibility of the class and its name.

A Java class - one way to think of it: a unit of code that is the basic building block of Java programs.
A method: a named segment of code that can be called or invoked to carry out some operations or actions. 

• Methods, like classes, have a header and a body.
• The header:
• accessibility (usually public)
• type of data returned by method (void --> no data returned)
• parameter list, indicating what information will be passed to the method
• 
  

A statement: a command to be executed.
The main method is where program execution begins.

One method is called in our main method: System.out.println.

---

• A statement that prints a line of output on the console.
• pronounced "print-linn" or "print-line"
• sometimes called a "println statement" for short

• Two ways to use System.out.println :
• System.out.println("text");
• Prints the given message as output.
• System.out.println();
• Prints a blank line of output.

---

• You must give your program a name.

        public class GangstaRap {

• Naming convention: capitalize each word (e.g. MyClassName)
• Your program's file must match exactly (GangstaRap.java)
• includes capitalization (Java is "case-sensitive")

• identifier: A name given to an item in your program.
• must start with a letter or _ or $
• subsequent characters can be any of those or a number
• legal: _myName     TheCure     ANSWER_IS_42     $bling$
• illegal: me+u      49ers      side-swipe      Ph.D's

• Use meaningful identifier names:
• Use max instead of m
• Use currentItem instead of c

---

keyword: An identifier that you cannot use because it already has a reserved meaning in Java.


abstract    default        if                     private          this
boolean      do              implements     protected     throw
break         double       import             public            throws
byte            else           instanceof       return           transient
case            extends    int                    short             try
catch          final          interface          static             void
char            finally      long                   strictfp         volatile
class           float         native                super            while
const         for             new                    switch
continue    goto          package             synchronized

• You cannot name a class short or const, because those are keywords.
• Keywords are always all lowercase.
  

---

• syntax: The set of legal structures and commands that can be used in a particular language.
• Every basic Java statement ends with a semicolon ;
• The contents of a class or method occur between { and }

• syntax error (compiler error): A problem in the structure of a program that causes the compiler to fail.

• Examples:
• Missing semicolon
• Too many or too few { } braces
• Illegal identifier for class name
• Class and file names do not match

---

1   public class Hello {
2       pooblic static void main(String[] args) {
3           System.owt.println("Hello, world!")_
4       }
5   }

• Compiler output:

    Hello.java:2: <identifier> expected
        pooblic static void main(String[] args) {
                ^
    Hello.java:3: ';' expected
    }
    ^   
    2 errors

• The compiler displays the line number where it found an error.
• The error messages can be difficult to understand.

---

• string: A sequence of characters to be printed.
• Starts and ends with a " quote " character.
• The quotes do not appear in the output.
• Examples:
• "hello"
• "This is one crazy looonnnnggg string!!"

• Restrictions:
• May not span multiple lines.

  "This is not
  a legal String."

• May not contain a " character.
• "This is not a "legal" String either."

---

Two character sequences that represent other characters

• \n : the new line escape sequence
• \t : the tab escape sequence
• \" : double quote
• \\ : backslash

Example:
System.out.println("\\hello\nhow\tare \"you\"?\\\\");

Output:
\hello
how         are "you"?\\

---

What is the output of the following println statements?

• System.out.println("\ta\tb\tc");
• System.out.println("\\\\");
• System.out.println("'");
• System.out.println("\"\"\"");
• System.out.println("C:\nin\the downward spiral");

Write a println statement to produce this output:
/ \ // \\ /// \\\


Question 1: What is an example of secondary storage?
A. monitor
B. CPU
C. USB flash drive

Question 2: Every Java statement must end in:
A.  }
B.  )
C.  ;
D.  .

Question 3: What is the output?
System.out.println("Hello\nWorld");
A.
Hello
World
B.
Hello World
C. Hello\nWorld
D. Hello       World