How to implement this Class Diagram in JAVA?

Question

I'm following a Class Diagram of a project. and I got confused with this part:

How am I supposed to create a variable containing a Class? (I don't know if it's clear, but none of these classes are the main one)

Please, bare in mind that I'm a student, and I've never had to follow any Class Diagram before.

Answer 1

Class diagram shows a composition between Historico and Usuario. It's a has relationship between (Historico has a Usuario).

You can define zero parameterized constructor like this:

 class Usuario {
    public Usuario() {
   }
  }

more ever compiler will add zero parameter constructor if you don't define while byte code generation if no constructor defined.

You can define compostion like this :

public class Historico {
  private Usuario responsavel;
}

Complete code:

public class Usuario {
    String login;
    String senha;

    /**
     * @return the login
     */
    public String getLogin() {
        return login;
    }

    /**
     * @param login the login to set
     */
    public void setLogin(String login) {
        this.login = login;
    }

    /**
     * @return the senha
     */
    public String getSenha() {
        return senha;
    }

    /**
     * @param senha the senha to set
     */
    public void setSenha(String senha) {
        this.senha = senha;
    }

}

class Historico:

 public class Historico {
    private Usuario responsavel;
    private String dataatualizacao;
    private String descricao;

    /**
     * @return the dataatualizacao
     */
    public String getDataatualizacao() {
        return dataatualizacao;
    }

    /**
     * @param dataatualizacao the dataatualizacao to set
     */
    public void setDataatualizacao(String dataatualizacao) {
        this.dataatualizacao = dataatualizacao;
    }

    /**
     * @return the descricao
     */
    public String getDescricao() {
        return descricao;
    }

    /**
     * @param descricao the descricao to set
     */
    public void setDescricao(String descricao) {
        this.descricao = descricao;
    }

    public void setResponsavel(Usuario responsavel) {
        this.responsavel = responsavel;
    }
}