The Java programming language

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In the last class...

Our first introduction to programming

Scratch! (click)

What is a programming language?

A way of translating algorithms into instructions executable by a computer

public class Example1 { public static void main(String[] args) { int[] L = {10,0,99,1,255,1,3,2}; int MAX= L[0]; for(int n: L){ if(n>MAX){ MAX=n; } } System.out.println(MAX); } }

Syntax with instructions to be transformed into a machine code (i.e. binary), executable by a computer

How does our example's machine code look like?

Trying to understand a piece of machine code ~ Trying to understand a piece of DNA by looking at its atoms (Hofstadter, 1979)

Transforming Java text into machine code

• Java files end in .java

• Java code gets transformed into machine code (bytecode)

  javac Example1.java

  This creates a file called Example1.class, containing the bytecode

• Java bytecode can be executed by you computer

  java Example1

This week

1. Variables

   Primitive types

   Assignment of values

   Declarations

2. The structure of a Java program

   Everything lives inside a class

   Methods

   Basic Input and Output

3. Control statements

   Conditionals

   Loops

4. Data structures

   Arrays

Variables

Placeholders for data

Variables are how we represent data stored in memory

Variables

Placeholders for data

Variables are how we represent data stored in memory

Variables need to be declared before using them
boolean p; boolean q=true; int a=0,b=1; double x,y=3.14159;

How much space do variables take?

Variables are how we represent data stored in memory

Data is stored in binary

We use bytes to measure an amount of binary data

1 byte: 8 binary digits, or bits

8 bits can represent numbers from 0000 0000 to 1111 1111

Sizes of variables in Java

1. boolean → 1 bit (The actual size is not this)
2. int → 4 bytes
3. double → 8 bytes
4. char → 2 bytes
5. String of n characters → 2*n bytes

What is the range of numbers we can represent with an int variable?

How are real numbers represented with the bits of an double variable?

The structure of a Java program

public class Example2 { public static void main(String[] args) { System.out.println("Hello COMP 202, Section 3"); } }

• The name of the file that contains this code should be Example2.java
• Blocks of code are enclose in curly brackets { }
• Statements are lines of code. They must end with a semicolon ;
• public means "anyone" can load this class or execute this method (More on this later)
• public static void main defines the starting point of execution

Methods

public class Example3 { public static int sum(int x, int y){ return x+y; } public static void printSomething(){ int a = 1; int b = 2; System.out.println("The sum of "+x+" and "+y+" is "+sum(a,b)); } public static void main(String[] args) { System.out.println("Hello COMP 202, Section 3"); printSomething(); } }

• Methods define reusable blocks of code
• Methods can receive inputs: arguments
• Methods can give back an output: return value
• static on method declaration means "anyone" can execute this method from any part of the code
• public static void means the method won't return anything
• public static int means the method will return an integer number

Basic input and output

import java.util.Scanner; public class Example4 { public static int sum(int x, int y){ return x+y; } public static void main(String[] args) { Scanner sc = new Scanner(System.in); System.out.println("Type a number and press enter"); int i = sc.nextInt(); System.out.println("Type another number and press enter"); int j = sc.nextInt(); System.out.println("The sum of "+i+" and "+j+" is "+sum(i,j)); sc.close(); } }

• Variables can be things different to the primtitive data type
• Arbitrary datatypes can be defined as classes
• When using a class we create an object in memory
• sc is an object which corresponds to an instance of the Scanner class
• import java.util.Scanner allows us to use the Scanner code, written by someone else

Resources

• The Java data types:
  http://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html
• The Java operators:
  http://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html
• Suggested reading:
  How to think like a Computer Scientist, Chapter 1