Variables and objects
General rules:
-
The type of the variable (String, Long, Boolean…) start with a capital letter.
-
variable names start without a capital letter.
String variables store text
Creating a String variable and giving it a value
String title = "Welcome to my app"; (1)| 1 | Don’t forget the double quotes " "! |
Integer variables store round numbers
Creating an Integer variable and giving it a value
Integer classSize = 32;
int anotherClassSize = 25; (1)| 1 | int is like Integer. It takes less memory but is sometimes less convenient to use.
Also, note: no double quote!! Double quotes are just for String. |
Float and Double variables store decimals
Double is like Float but can store decimals with a lot more precision
Creating Float and Double variables
Float pi = 3.14f; (1)
Double piVeryPrecise = 3.141592653589793238462643383279502884197169d; (2)| 1 | Don’t forget the f letter at the end of your number. float or Float can be used, float takes less memory than Float. |
| 2 | Don’t forget the d letter at the end of your number. double or Double can be used, double takes less memory than Double. |
Long stores big round numbers
Long is like Integer but can store bigger numbers
Creating a Long variable
Long millisecondsSinceLastMonth = 364434030430414l; (1)| 1 | Don’t forget the l letter at the end of your number. long or Long can be used, long takes less memory than Long but can be inconvenient to use. |
Boolean stores true / false values
This seems not very useful but actually we use it quite often
Creating a Boolean variable
Boolean hasAStudentCard = true; (1)| 1 | A classic mistake is to write "true" (with double quotes, which is incorrect). Boolean values are true or false without double quotes " ".
boolean can be used instead of Boolean: less memory but also less convenient to use in some cases. |
Objects: to create and store a variety of things
A variety of objects exist - use them to create and store things
Creating an Object storing a Date
Date dateStartOfTheGame; (1)
dateStartOfTheGame = new Date(); (2)
Date dateEndOfTheGame = new Date(); (3)| 1 | An object dateStartOfTheGame of type Date is declared. It is null at the moment. |
| 2 | dateStartOfTheGame is instantiated: an instance of it is created. |
| 3 | Shortcut: a variable can be declared and instantiated in one line of code. |
Methods
Creating (defining) a method
Creating a method adding the VAT to a price
private Float addFrenchVAT(Float priceWithoutVAT) { (1)
Float priceWithVAT; (2)
//the regular rate of the VAT in France is 20% so we multiply the price by 1.20 to find the new price (3)
priceWithVAT = priceWithoutVAT * 1.20; (2)
return priceWithVAT; (2)
}(4)| 1 | title of the method you create, then the method start at the opening curly brace {. |
| 2 | the method itself |
| 3 | an explanation, not some code! The line starts with // to show this is some explanations for humans like you and me, not some code in our app. |
| 4 | this closing curly brace signals the end of the definition of the method. |
Creating a method which returns nothing
Float price = 5.99f; (1)
private void addFrenchVAT() { (2)
price = price * 1.20; (3)
} (4)| 1 | we have created a variable named price |
| 2 | now we define a method like the one before in this lesson, except that:
|
| 3 | the method does one thing: it multiplies the value of the variable price by 1.20 |
| 4 | this is the end of the method. There is no "return" statement. |
Classes
A class is just a file in your app. It contains the variables and the methods that you want. It looks like:
A simple example of what a class looks like
package net.clementlevallois.codapps.myfirstapp (1)
public class Form1 { (2)
Integer scorePlayer; (3)
public void addOneToScore() {
scorePlayer = scorePlayer + 1;
}
} (4)| 1 | a class always starts with the name of the package where it belongs |
| 2 | the name of the class (Form1) should have the same name as your file where it is writte (here the file would be Form1.java) |
| 3 | this is a variable which can be used anywhere in the class. |
| 4 | don’t forget the closing curly brace of the class! |
Instantiating a Form in MyApplication.java
public void start() {
Form1 myForm1 = new Form1(); (1)
myForm1.show(); (2)
}| 1 | We instantiate our Form1 |
| 2 | And now we can use methods of this Form1. Here, we use the method show() which has for effect to display the Form on screen. |
Another common way to instantiate an object is this one:
Getting the present time and storing it in a variable
public void start() {
LocalTime timeNow = LocalTime.now(); (1)
}| 1 | This stores the time at the moment when this line of code is executed, in the variable timeNow |
If… conditional statements
conditional statements for numbers
Different kinds of conditional statements about numbers
Float priceItemInEuros;
priceItemInEuros = 5.99f;
Label productLabel = new Label();
if (priceItemInEuros < 6) {
productLabel.setText("cheap product!");
}
if (priceItemInEuros == 5.99) { (1)
productLabel.setText("the price is exactly 5.99");
}
if (priceItemInEuros != 5.99) { (2)
productLabel.setText("the price is different from 5.99");
}
if (priceItemInEuros =< 6) {
productLabel.setText("the price is under or equal to 6!");
}
if (priceItemInEuros >= 7) {
productLabel.setText("the price is above or equal to 6!");
}conditional statements about text
It would be a mistake to write:
Mistake! Don’t do this!
String playerName1 = "Tristan";
String playerName2 = "Tristan";
if (playerName1 == playerName2) { (1)
messageLabel.setText("the two players have the same name!");
}| 1 | Using == to compare two Strings is incorrect.
|
Different kinds of conditional statements about text
String playerName1 = "Tristan";
String playerName2 = "Touni";
if (playerName1.equals(playerName2)) {
messageLabel.setText("the two players have the same name!");
}
if (!playerName1.equals(playerName2)) { (1)
messageLabel.setText("the two players have different names!");
}| 1 | note the ! in front |
conditional statements about several items
A statement with two conditions which need both to be true
Float priceItemInEuros;
priceItemInEuros = 5.99f;
Label productLabel = new Label();
if (priceItemInEuros < 6 & priceItem > 2) { (1)
productLabel.setText("relatively cheap product!");
}| 1 | the & means "and". The two conditions: priceItemInEuros < 6 and priceItem > 2 both need to be true for the statement productLabel.setText("relatively cheap product!"); to be executed. |
A statement with two conditions where just either one of the two needs to be true
Float priceItemInEuros;
priceItemInEuros = 5.99f;
Label productLabel = new Label();
if (priceItemInEuros < 6 | priceItem > 2) { (1)
productLabel.setText("relatively cheap product!");
}| 1 | the | means "or". Just one of the two conditions: priceItemInEuros < 6 or priceItem > 2 needs to be true for the statement productLabel.setText("relatively cheap product!"); to be executed. |
Loops
Writing a loop
for (int i = 0; i<100; i = i+1){
System.out.println("I looped " + i) +" times.";
}A loop with several ifs inside
for (int i = 0; i<100; i = i+1){
System.out.println("I looped " + i + " times.");
if (i == 0) {
System.out.println("We just started the loops. This is going to be a long journey.");
}
if (i == 50) {
System.out.println("Half way already!");
}
if (i == 99) {
System.out.println("This was the last loop. Bye!");
}
}ArrayLists
Creating a list and adding objects to it
ArrayList<Balloon> balloons = new ArrayList();
for (int i = 0; i<20000;i = i+1){
Balloon myBalloon = new Balloon();
balloons.add(myBalloon);
}Looping through a list to show the names of all players
for (String player: playerNames) { (1)
Label myLabel = new Label();
myLabel.setText(player);
myForm.add(myLabel)
}| 1 | here we assume that we had created before an ArrayList of player names.
|