Download Java to your
computer, download Eclipse or Android Studio, if use Eclipse, you must choose
Java 8.
Open Eclipse, go to
File > New > Java Project
On pop up window,
enter project name, finish.
Double click to project
name, right click to src folder, New > Class
On pop up window, name
it first, finish, screen will look like this
Copy these lines in to
class bracket.
public static void main(String args[])
{
String name="Hello
World !";
System.out.println(name);
}
Choose Run or press
green button, if it show this pop up.
Check to Always save
resources before launching, next time it will not show again.
Look in to Console
screen below, it ‘s our first Java programme.
If not see Console, go
to Window > Show View > Console to show it.
No need to see what public,
static,
void.
They are Java keywords, just ignore them. The line Sysem.out.println to show
some text to screen.
Short key for this
command, type sys and press Ctrl+Space, pop up window show Commands, press
Enter to choose.
Just look at the line:
String name="Hello
World !";
String mean data type
of variabel name, it is all characters we type from keyboard.
Now change name to nam, Eclipse
show an error.
The text name call variable, the line String name=”Hello World !”; declare a variable. When you
change to nam,
Eclipse doesn’t recognize the word name
because we have to declare it before use.
Change name to nam,
red mark will disappear.
Now change to String
nam=”Nguyen Van A”, add this line
Int age=20;
Change
System.out.prinlt to System.out.println( nam + " is "+ age + " years old!");
Run to see
result.
Look at
above picture, the line
System.out.println( nam) now look different, it
became comment.
If we have
long and complex code, comment make us easy to remember what we has done
yesterday.
The word int
before age declare a integer number.
Int, String are data types, String is text, int mean number.
When we code String
a=”8” it is a text, int a=8 it is number.
To change text to
number, we use this
int a=Integer.parseInt(String);
On the
contrary, to change number
to text
String b=String.valueOf(number);
Now delete one bracket
at last, an error show, add bracket to, it disappear. Programme must have close
brackets the same with open. When see red mark, move cursor to it, Eclipse will
show an notification.
Continue
String sex="Male";
if(sex.equals("Male")){
System.out.println( name + " is a
son ");
}
We compare String sex
with “Male”, to compare two strings, we must use .equals(), not == . If use ==,
it still run but sometime it become error, result not true but you can’t find
where. So always use .equals when compare to strings.
Make the line System.out.println( nam + " is
"+ age + " years old!"); become comment.
Type int a=12; int b =
15;
age=a+b;
System.out.println(age);
Now screen
show 27. On above it set age=20 but now it become 27
because we assign new value to it, age=a+b.
Computer
read code from up to down, if we assign new value to variable, old value will
lose.
Type continue to below
age=a+b;
if(a>b){
age=10;
}
else{
age=100;
}
Run to see number 100.
Now
delete else command, run, Console show 27.
We see the different
of if has else and no else.
If has else, when if
not true, command go to else, value will be 100. I no else, age still remain
value 27, value of a+b.
Continue to type.
for(int i=0;i<age;i++){
if(i>10)
System.out.println(i);
}
}
Screen Console show number from 11 to 26.
For loop
use to repeat a command, here we print number many time . Here we print number
from 11-26, it is one command, run many time.
Change if
to if(i%2==0), run, it will
print even numbers from 0 to 27. % character mean divide and get remainder.
Continue.
double c=12.5;
double d=c/7;
System.out.println(d);
double
mean decimal number.
Copy to
above print command, error show.
DecimalFormat f = new DecimalFormat("#.##");
Press
Ctrl+Shift+O to import library, it will show on top of programme. We don’t need
to remember libraries, just import them when needed.
Copy to
continue.
String
round=f.format(d);
System.out.println(round);
Console
print 1.71, DecimalFormat use to round decimal number. Change number repeat #
character to 3,4 to get 3,4 number after comma.
Excercise
Print odd
number from 1 to 100.
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