What is the peek method in Java Stream API and how to use it?

The peek method in Java’s Stream API is an intermediary operation. This method provides a way to inspect the elements in the stream as they’re being processed by other stream operations. However, it’s important to note that peek should only be used for debugging purposes, as it can be quite disruptive to the stream’s data flow, particularly when it’s used in parallel streams.

The key point of the peek operation is that it’s not a terminal operation (i.e., it doesn’t trigger data processing), instead, it integrates nicely within the operation chain, allowing insights to be gained during the processing phase.

Here’s a simple way of using it with Java:

package org.kodejava.util;

import java.util.stream.Stream;

public class PeekMethodStream {
    public static void main(String[] args) {
        Stream.of(1, 2, 3, 4, 5)
                .peek(i -> System.out.println("Number: " + i))
                .map(i -> i * i)
                .forEach(i -> System.out.println("Squared: " + i));


Number: 1
Squared: 1
Number: 2
Squared: 4
Number: 3
Squared: 9
Number: 4
Squared: 16
Number: 5
Squared: 25

In this code snippet:

  • We create a stream with Stream.of(1, 2, 3, 4, 5).
  • Then we use peek to print each element in its current state: “Number: 1”, “Number: 2”, etc.
  • After that, we use map to square each element.
  • Finally, we use forEach to print the squared numbers: “Squared: 1”, “Squared: 4”, etc.

Remember, use peek carefully and preferably only for debugging purposes.

How do I debug application that uses JavaMail APIs?

To ask Java mail to print out a debug message when you run a Java mail program you can add the mail.debug and set the value to true into the properties used to create the session object.

The other way is to set the debug setting of the session using the session.setDebug() method and pass true as the parameter. This will also turn on the debug message in your Java mail program.

package org.kodejava.mail;

import javax.mail.*;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;
import java.util.Date;
import java.util.Properties;

public class MailDebugDemo {
    public static void main(String[] args) {
        MailDebugDemo.sendMail("[email protected]", "[email protected]",
                "Debug Demo", "Mail Debug Demo");

    private static void sendMail(String mailFrom, String mailTo, String mailSubject,
                                 String mailText) {
        Properties props = new Properties() {{
            put("mail.smtp.auth", "true");
            put("mail.smtp.host", "smtp.gmail.com");
            put("mail.transport.protocol", "smtp");
            put("mail.smtp.port", "587");
            put("mail.smtp.starttls.enable", "true");
            put("mail.smtp.ssl.protocols", "TLSv1.2");
            put("mail.debug", "true");

        // Creates a mail session. We need to supply username and
        // password for GMail authentication.
        Session session = Session.getInstance(props, new Authenticator() {
            protected PasswordAuthentication getPasswordAuthentication() {
                return new PasswordAuthentication("[email protected]", "password");

        try {
            // Creates email message
            Message message = new MimeMessage(session);
            message.setSentDate(new Date());
            message.setFrom(new InternetAddress(mailFrom));
                    new InternetAddress(mailTo));

            // Send a message
        } catch (MessagingException e) {

Here is an example of debug information produce by the JavaMail API when sending an email using Gmail SMTP server.

DEBUG: JavaMail version 1.5.6
DEBUG: successfully loaded resource: /META-INF/javamail.default.providers
DEBUG: Tables of loaded providers
DEBUG: Providers Listed By Class Name: {com.sun.mail.smtp.SMTPSSLTransport=javax.mail.Provider[TRANSPORT,smtps,com.sun.mail.smtp.SMTPSSLTransport,Oracle], com.sun.mail.smtp.SMTPTransport=javax.mail.Provider[TRANSPORT,smtp,com.sun.mail.smtp.SMTPTransport,Oracle], com.sun.mail.imap.IMAPSSLStore=javax.mail.Provider[STORE,imaps,com.sun.mail.imap.IMAPSSLStore,Oracle], com.sun.mail.pop3.POP3SSLStore=javax.mail.Provider[STORE,pop3s,com.sun.mail.pop3.POP3SSLStore,Oracle], com.sun.mail.imap.IMAPStore=javax.mail.Provider[STORE,imap,com.sun.mail.imap.IMAPStore,Oracle], com.sun.mail.pop3.POP3Store=javax.mail.Provider[STORE,pop3,com.sun.mail.pop3.POP3Store,Oracle]}
DEBUG: Providers Listed By Protocol: {imaps=javax.mail.Provider[STORE,imaps,com.sun.mail.imap.IMAPSSLStore,Oracle], imap=javax.mail.Provider[STORE,imap,com.sun.mail.imap.IMAPStore,Oracle], smtps=javax.mail.Provider[TRANSPORT,smtps,com.sun.mail.smtp.SMTPSSLTransport,Oracle], pop3=javax.mail.Provider[STORE,pop3,com.sun.mail.pop3.POP3Store,Oracle], pop3s=javax.mail.Provider[STORE,pop3s,com.sun.mail.pop3.POP3SSLStore,Oracle], smtp=javax.mail.Provider[TRANSPORT,smtp,com.sun.mail.smtp.SMTPTransport,Oracle]}
DEBUG: successfully loaded resource: /META-INF/javamail.default.address.map
DEBUG: getProvider() returning javax.mail.Provider[TRANSPORT,smtp,com.sun.mail.smtp.SMTPTransport,Oracle]
DEBUG SMTP: useEhlo true, useAuth true
DEBUG SMTP: useEhlo true, useAuth true
DEBUG SMTP: trying to connect to host "smtp.gmail.com", port 587, isSSL false
220 smtp.gmail.com ESMTP b2sm4488826pgh.33 - gsmtp
DEBUG SMTP: connected to host "smtp.gmail.com", port: 587

Maven Dependencies


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