How to create a java class?

Question

I would like to know how to create a class in the java, how to use this class in the main program and how is the variable created in this class.

Answer 1

Initially it is called the main method of the main class (program start), but no instance of the class (object) is created just by doing this.

To create an instance of a class, it is necessary to call the constructor of this class (if there is no one, the compiler will create a constructor without default parameters).

After instantiation it is possible to assign values to object attributes, or else pass them as parameters to the constructor and assign them within the constructor.

In this assignment one must be cautious, because depending on the class of the object being used (in this example the main class), a compilation error will occur if there is an attempt to violate the rules of visibility.

Attribute modifiers, constructors and methods are: protected, package (also called default, which is the absence of modifier), private and public.

The only difference of protected for package (no modifier) is that protected allows subclasses (heirs classes) outside the package to have access.

More details on this question from Soen

Obs: In Java 8, the keyword "default" is a valid modifier for interface methods, and in such a context has no relation to package visibility (default, no modifier).

class Programa {
  public static void main(String[] args) {
    Conta minhaConta = new Conta();

    minhaConta.dono = "Duke";
    minhaConta.saldo = 1000.0;

    System.out.println("Saldo atual: " + minhaConta.saldo);
  }
}

Class counts with its methods and attributes:

class Conta {
   double saldo;
   String dono;

   void saca(double quantidade) {
     double novoSaldo = this.saldo - quantidade; 
     this.saldo = novoSaldo;
   }
 }

Complementing the subject of modifiers: To ensure greater security and greater encapsulation (among other features) it is necessary to restrict access to variables (usually using access modifiers protected and private) disabling direct access to the attribute (check the access level of each modifier), to provide access is used Access methods get and set:

private double x; 

public double getX() { 
      return x; 
} 

public void setX(double x) { 
    this.x = x; 
}

So for example it is always possible to validate the values to be assigned.

Answer 2

Classes that can be called as main for Java-SE applications are classes that serve as the input/execution point of their publication, these classes are those that have the public and static method main, this method expects to receive a array of Strings that represents the arguments offered when the class was called with the Java command from the command line.

Even if you use an IDE to run your application, the IDE will call this method stating the parameters that can be passed through the execution parameterization screen.

In the body of the method main you can reference static variables of your class, instantiate the class itself through one of its constructors, and then reference their variables if they are visible, which will depend on their scope. You can also instantiate other classes, manipulate the visible variables and call your visible methods according to the scope.

@Rodolfo, from my point of view your question is very broad and may involve various knowledge concerning the construction of classes and small programs in java.

Answer 3

class NomeDaClasse {

      // Atributo
      public int atributo;

      // Método construtor
      public NomeDaClasse() {
      }

      // Metodo
      public void metodo() {

      }
}

To create a class instance:

NomeDaClasse objeto = new NomeDaClasse();

Answer 4

class AlgumaCoisa 
{
   /* Alguma variável qualquer */
   bool ClasseLegal = true;

   int Arg1 = 0; //primeiro argumento
   int Arg2 = 0; //segundo argumento

   /* O método construtor deve ser o nome da classe */
   public AlgumaCoisa(int Argumento1, int Argumento2) //Pode remover os argumentos se quiser
   { Arg1 = Argumento1;
     Arg2 = Argumento2; }

   /* Propriedade somente leitura qualquer */
   public readonly int CodigoDaClasse()
   { return 20; }

   public int SomaDeles() { return Arg1 + Arg2; }
   public int MultDeles() { return Arg1 * Arg2; }
   public int DivsDeles() { return Arg1 / Arg2; }
   public int SubtDeles() { return Arg1 - Arg2; }
}

to declare it use:

AlgumaCoisa nomeAqui = new AlgumaCoisa(ValorDoArgumento1, ValorDoArgumento2)

Here’s an example with this class:

AlgumaCoisa nomeAqui = new AlgumaCoisa(10, 10)
int Soma          = nomeAqui.SomaDeles(); // 20
int Multiplicação = nomeAqui.MultDeles(); // 100
int Divisão       = nomeAqui.DivsDeles(); // 1
int Subtração     = nomeAqui.SubtDeles(); // 0