Validação JSF

<f:view>
        <h:head>
            <title>Facelet Title</title>
        </h:head>
        <h:body>
            <h:form>
                <h:outputText value="Nome: " />
                <h:inputText id="txtNome" required="true" requiredMessage="Obrigatório" value="#{agendaContatoBean.contato.nome}"/>
                <h:message for="txtNome" style="color: blue"></h:message>
                <br/>
                <h:outputText value="Endereço: " />
                <h:inputText value="#{agendaContatoBean.contato.endereco}"/>
                <br/>
                <h:outputText value="Sexo: " />
                <h:inputText value="#{agendaContatoBean.contato.sexo}" />
                <br/>
                <h:commandButton
                    actionListener="#{agendaContatoBean.incluirContato}"
                    value="Incluir" />
                <h:commandButton
                    actionListener="#{agendaContatoBean.novoContato}"
                    value="Novo" />
                <br/><br/>
                <h:dataTable id="oi" var="obj" value="#{agendaContatoBean.contatos}"
                             border="1" width="100%">
                    <h:column>
                        <h:commandLink value="Editar" actionListener="#{agendaContatoBean.editarContato}">
                            <f:param value="#{obj}" name="contato" id="contato"></f:param>
                        </h:commandLink>
                    </h:column>
                    <h:column>
                        <f:facet name="header">
                            <h:outputText value="Nome"/>
                        </f:facet>
                        <h:outputText value="#{obj.nome}" />
                    </h:column>
                    <h:column>
                        <f:facet name="header">
                            <h:outputText value="Endereço"/>
                        </f:facet>
                        <h:outputText value="#{obj.endereco}" />
                    </h:column>
                    <h:column>
                        <f:facet name="header">
                            <h:outputText value="Sexo"/>
                        </f:facet>
                        <h:outputText value="#{obj.sexo}" />
                    </h:column>
                </h:dataTable>
            </h:form>
        </h:body>
    </f:view>

package bean;

import java.util.ArrayList;
import java.util.List;
import javax.faces.bean.SessionScoped;
import javax.faces.component.UIParameter;
import javax.faces.event.ActionEvent;

@javax.faces.bean.ManagedBean
@SessionScoped
public final class AgendaContatoBean {

    private List<Contato> contatos;
    private Contato contato;

    public AgendaContatoBean() {
        this.setContatos(new ArrayList<Contato>());
        this.setContato(new Contato());
    }

    public void incluirContato(ActionEvent event) {
        if (!this.getContatos().contains(this.getContato())) {
            this.getContatos().add(this.getContato());
        }
    }

    public void editarContato(ActionEvent event) {
        contato = (Contato)((UIParameter)event.getComponent().findComponent("contato")).getValue();
    }

    public void novoContato(ActionEvent event) {
        contato = new Contato();
    }

    public List<Contato> getContatos() {
        return contatos;
    }

    public void setContatos(List<Contato> contatos) {
        this.contatos = contatos;
    }

    public Contato getContato() {
        return contato;
    }

    public void setContato(Contato contato) {
        this.contato = contato;
    }
}

package bean;

public class Contato {
private String nome;
private String endereco;
private String sexo;

    public String getEndereco() {
        return endereco;
    }

    public void setEndereco(String endereco) {
        this.endereco = endereco;
    }

    public String getNome() {
        return nome;
    }

    public void setNome(String nome) {
        this.nome = nome;
    }

    public String getSexo() {
        return sexo;
    }

    public void setSexo(String sexo) {
        this.sexo = sexo;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        final Contato other = (Contato) obj;
        if ((this.nome == null) ? (other.nome != null) : !this.nome.equals(other.nome)) {
            return false;
        }
        return true;
    }

    @Override
    public int hashCode() {
        int hash = 7;
        hash = 79 * hash + (this.nome != null ? this.nome.hashCode() : 0);
        return hash;
    }
}