Head First Java

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Chapter 1 Breaking the Surface

The Way Java Works

Source: Create a source document (filename.java)
Compiler: Run the document through a source code compiler.
1. this will check for errors and won't let you compile until it is satisfied
Output (code): Compiler creates new file containing bytecode
1. Java will be able to translate/interpret this file into something it can run
2. The compiled bytecode is platform independent
Virtual Machines: A java virtual machine is used to run java bytecode code
1. This is what makes java code machine independent

What you'll do in Java

Type your source code
Compile your source file

javac <sourcefile.java>

This compiles the code and creates a sourcefile.class file

this is the bytecode

Run the program

java sourcefile
This starts the Java Virtual Machine (JVM)
The JVM translates the bytecode into something the underlying platform understands

Code structure in Java

Put a class in a source file
Put methods in a class
Put statements in a method

What goes in a source file?

source code file holds one class definition

public class Dog {

}

What goes in a class?

a class has one or more methods
the method holds instructions on what that method should do
the method must be declared inside a class

public class Dog {
  void bark() {

  }
}

What goes in a method?

within the curly braces of a method, write the instructions for how that method should be performed
method code is basically a set of statements

public class Dog {
  void bark() {
    statement1;
    statement2;
  }
}

Anatomy of a class

When the JVM starts running, it looks for the class you gave it at the command line. Then is starts looking for a specially written method that looks exactly like:

public static void main (String[] args){
  // your code goes here
}

Next, the JVM runs everything between the curly braces of your main method. Every Java application has to have at least one class, and at least one main method (main per application not per class)

Writing a class with a main

everything goes into a class
running a program tells the JVM to load class, then start executing the main() method
keep running till all the code in the main method is finished
main() method is the entry point of the whole app

Remember:

Syntax Fun

each statement must end in a semicolon x = x + 1;
a single-line comment begins with two forward slashes // single-line comment
most white space does not matter x = 3 ;
variables are declared with a name and a type int weight
classes and methods must be defined within a pair of curly braces

public void go() {
  // code goes here
}

Looping and looping and...

Java jas three main looping constructs:

while
do-while
for

Concentrate on the while loop for now

while some condition is true, do everything inside the loop block
key to a loop is the conditional test

Simple boolean tests

do a simple boolean test by checking the value of a variable, using a comparison operator including:
- < (less than)
- > (greater than)
- == (equality)

int x = 4; // assign 4 to x
while(x > 3) {
  // loop code will run because x is greater than 3
  x = x - 1; // or we'd loop forever
}
int z = 27;
while(z == 17) {
  // loop code will not run because z is not equal to 17
}

while loop example: Loopy.java

public class Loopy {
  public static void main (String[] args){
    int x = 1;
    System.out.println("Before the loop");
    while(x < 4) {
      System.out.println("On the loop");
      System.out.println("Value of x is " + x);
      x = x + 1;
    }
    System.out.println("After the loop!");
  }
}

output

javac Loopy.java
java Loopy

Before the loop
In the loop
Value of x is 1
In the loop
Value of x is 2
In the loop
Value of x is 3
After the loop!

Conditional branching

An if test is basically the same as the boolean test in a while loop - except instead of saying " while this..." you'll say, " if there is..."

IfTest.java

class IfTest {
  public static void main(String[] args){
    int x = 3;
    if (x == 3){
      System.out.println("x must be 3");
    }
    System.out.println("This runs no matter what");
  }
}

In the example above, if the statement is true, then two statements will print out, but regardless, at least one statement will always print.

output

javac IfTest.java
java IfTest
x must be 3
This runs no matter what

Now, we can add an else to the condition, so that we can say something like " if this then do this else do this..."

IfTest2.java

class IfTest2 {
  public static void main(String[] args){
    int x = 2;
    if (x == 3){
      System.out.println("x must be 3");
    } else {
      System.out.println("x is NOT 3");
    }
    System.out.println("This runs no matter what");
  }
}

output

javac IfTest2.java
java IfTest2
x is not 3
This runs no matter what

BeerSong.java

public class BeerSong {
  public static void main(String[] args){
    int bottleCount = 99;
    String word = "bottles";

    while(bottleCount > 0){

      System.out.println(bottleCount + " " + word + " of beer on the wall.");
      System.out.println(bottleCount + " " + word + " of beer.");
      System.out.println("Take one down. Pass it around.");
      bottleCount = bottleCount - 1;

      if(bottleCount > 0){
        if(bottleCount == 1){
          word = "bottle";
        }

        System.out.println(bottleCount + " " + word + " of beer on the wall.");
      } else {
        System.out.println("No more bottles of beer on the wall.");
      }
    }
  }
}

Glossary

A definition list or a glossary:

First Term: This is the definition of the first term.
Second Term: This is the definition of the second term.

Last modified: 10 March 2024