Histroy of Robotic in art and science

Back to Physical and Embedded Computer

Portable programs (multiple platforms, Windows, MacOS, Linux, etc.)

Object-oriented language

Everything is an object (class).

Java virtual machine

Get it from http://www.java.com/

Development process

• Compile (javac turns a xxx.java file into xxx.class file)
• Execute (java runs the xxx.class file)

Simple hello world example using,

System.out.println("Hello World!");

GUI example using Swing facility

A simple HelloWorld example. The name of the program is

HelloWorld.java

/*
   This is a simple Java program. 
*/
class HelloWorld {

// This is the main program.

  public static void main(String[] args) {
    System.out.println("Hello World!");
  }
}

Comment can be either

/*
Multiple lines comment. 
*/

or

// Single line comment. 

Capital and small letters matter.

HelloWorld does not equal to helloWorld

Dot syntax to denote object, properties and methods.

e.g. System.out.println

Everything is inside a Class.

The name of the file HelloWorld.java should match the major class name HelloWorld.

It needs a main function to let the program run.

Note that there are three words in front of the main function.

public - indicates that the main function can be accessed outside the class HelloWorld. The opposite of public is private.

static - allows the main function to be called before an object of the HelloWorld class is created.

void - tells us that the main function does not return anything after it is called.

Now we try another example.

UseVar01.java

class UseVar01 {
  public static void main(String[] args) {
    int var01, var02;
    float var03, var04;
    double var05, var06;
    var01 = 5;
    var02 = var01 / 3;
    var03 = 5;
    var04 = var03 / 3;
    var05 = 5;
    var06 = var05 / 3;
    System.out.println(var02);
    System.out.println(var04);
    System.out.println(var06);
  }
}

Note the 3 numbers printed.

1
1.6666666
1.6666666666666667

int, float and double are 3 different numeric data types

Try another example with the String data type.

UseVar02.java

class UseVar02 {
    public static void main(String[] args) {
        String s1, s2;
        s1 = new String("Bryan");
        s2 = "Chung";
        System.out.println(s1);
        System.out.println(s2);
        System.out.println(s1 + " " + s2);
    }
}

          

Note the result,

Bryan
Chung
Bryan Chung 

char data type is a single character

char ch = 'Q';

Condition

if - else statement

int a, b;
a = 3;
b = 4;
if (a==b) { 
  System.out.println("equal");
} else {
  System.out.println("not equal");
}

switch statement

char myChar = 'b';
switch (myChar) {
  case 'a':
    System.out.println("This is a");
    break;
  case 'b':
    System.out.println("This is b");
    break;
  case 'c':
    System.out.println("This is c");
    break;
  default:
    System.out.println("None of the above");
}

Loop - repetition

for statement

int i;
for (i=0;i<10;i++) {
  System.out.println("i = " + i);
}

for (int i=0;i<10;i++) {
  System.out.println("i = " + i);
}

while statement

char myChar;
myChar = 'a';
while (myChar<='z') {
  System.out.println(myChar);
  myChar++;
}

do - while statement

int i = 10;
do {
  System.out.println("i = " + i);
  i--;
} while (i>0);

break statement

for (int i=0;i<10;i++) {
  if (i==5) {
    break;
  }
}

continue statement

for (int i=0;i<10;i++) {
  if (i==3) {
    continue;
  } else {
    System.out.println(i);
  }
}

Create an Object from a class.

Remember each Java program has a class definition.

A simple one first

public class SimpleOne {
  public static void main(String[] args) {
    SimpleOne so = new SimpleOne();
  }
}

so will be an object of the class SimpleOne.

Properties can be added to the class.

public class SimpleOne {
  String name;
  int id;

  public static void main(String[] args) {
    SimpleOne so = new SimpleOne();
    so.name = "bryan";
    so.id = 167;
    System.out.println(so.name);
  }
}

Methods can also be added to the class.

public class SimpleOne {
  String name;
  int id;
  
  public void ChangeID(int i) {
    id = i;
  }

  public static void main(String[] args) {
    SimpleOne so = new SimpleOne();
    so.name = "bryan";
    so.id = 167;
    System.out.println(so.id);
    so.ChangeID(168);
    System.out.println(so.id);
  }
}

Majority of the Java programs have a number of objects either pre-defined or user-defined.