Master the JDK: Build a Solid Java Foundation

1 Origin

The article originates from a conversation in a developer group where the author (LZ) was asked about hiring requirements and the meaning of a "solid Java foundation".

2 Introduction

Many candidates are confused by job postings that require a solid Java foundation. The author explains that mastering the JDK is the key.

3 Mastering the JDK

The JDK (Java SE Development Kit) consists of three parts: the Java Runtime Environment (JVM), the core class libraries, and development tools. To master the JDK you need to study each part.

Part 1: Java Runtime Environment

The JRE is essentially the JVM. Understanding the JVM is essential for most Java interview questions and advanced work.

Part 2: Core Class Libraries

The core libraries are vast; the author lists the packages from JDK 6 (shown in the blockquote). Not all classes are equally important; focus on the most frequently used ones.

java.applet java.awt java.awt.color java.awt.datatransfer java.awt.dnd java.awt.event java.awt.font java.awt.geom java.awt.im java.awt.im.spi java.awt.image java.awt.image.renderable java.awt.print java.beans java.beans.beancontext java.io java.lang java.lang.annotation java.lang.instrument java.lang.management java.lang.ref java.lang.reflect java.math java.net java.nio java.nio.channels java.nio.channels.spi java.nio.charset java.nio.charset.spi java.rmi java.rmi.activation java.rmi.dgc java.rmi.registry java.rmi.server java.security java.security.acl java.security.cert java.security.interfaces java.security.spec java.sql java.text java.text.spi java.util java.util.concurrent java.util.concurrent.atomic java.util.concurrent.locks java.util.jar java.util.logging java.util.prefs java.util.regex java.util.spi java.util.zip javax.accessibility javax.activation javax.activity javax.annotation javax.annotation.processing javax.crypto javax.crypto.interfaces javax.crypto.spec javax.imageio javax.imageio.event javax.imageio.metadata javax.imageio.plugins.bmp javax.imageio.plugins.jpeg javax.imageio.spi javax.imageio.stream javax.jws javax.jws.soap javax.lang.model javax.lang.model.element javax.lang.model.type javax.lang.model.util javax.management javax.management.loading javax.management.modelmbean javax.management.monitor javax.management.openmbean javax.management.relation javax.management.remote javax.management.remote.rmi javax.management.timer javax.naming javax.naming.directory javax.naming.event javax.naming.ldap javax.naming.spi javax.net javax.net.ssl javax.print javax.print.attribute javax.print.attribute.standard javax.print.event javax.rmi javax.rmi.CORBA javax.rmi.ssl javax.script javax.security.auth javax.security.auth.callback javax.security.auth.kerberos javax.security.auth.login javax.security.auth.spi javax.security.auth.x500 javax.security.cert javax.security.sasl javax.sound.midi javax.sound.midi.spi javax.sound.sampled javax.sound.sampled.spi javax.sql javax.sql.rowset javax.sql.rowset.serial javax.sql.rowset.spi javax.swing javax.swing.border javax.swing.colorchooser javax.swing.event javax.swing.filechooser javax.swing.plaf javax.swing.plaf.basic javax.swing.plaf.metal javax.swing.plaf.multi javax.swing.plaf.synth javax.swing.table javax.swing.text javax.swing.text.html javax.swing.text.html.parser javax.swing.text.rtf javax.swing.tree javax.swing.undo javax.tools javax.transaction javax.transaction.xa javax.xml javax.xml.bind javax.xml.bind.annotation javax.xml.bind.annotation.adapters javax.xml.bind.attachment javax.xml.bind.helpers javax.xml.bind.util javax.xml.crypto javax.xml.crypto.dom javax.xml.crypto.dsig javax.xml.crypto.dsig.dom javax.xml.crypto.dsig.keyinfo javax.xml.crypto.dsig.spec javax.xml.datatype javax.xml.namespace javax.xml.parsers javax.xml.soap javax.xml.stream javax.xml.stream.events javax.xml.stream.util javax.xml.transform javax.xml.transform.dom javax.xml.transform.sax javax.xml.transform.stax javax.xml.transform.stream javax.xml.validation javax.xml.ws javax.xml.ws.handler javax.xml.ws.handler.soap javax.xml.ws.http javax.xml.ws.soap javax.xml.ws.spi javax.xml.ws.wsaddressing javax.xml.xpath org.ietf.jgss org.omg.CORBA org.omg.CORBA_2_3 org.omg.CORBA_2_3.portable org.omg.CORBA.DynAnyPackage org.omg.CORBA.ORBPackage org.omg.CORBA.portable org.omg.CORBA.TypeCodePackage org.omg.CosNaming org.omg.CosNaming.NamingContextExtPackage org.omg.CosNaming.NamingContextPackage org.omg.Dynamic org.omg.DynamicAny org.omg.DynamicAny.DynAnyFactoryPackage org.omg.DynamicAny.DynAnyPackage org.omg.IOP org.omg.IOP.CodecFactoryPackage org.omg.IOP.CodecPackage org.omg.Messaging org.omg.PortableInterceptor org.omg.PortableInterceptor.ORBInitInfoPackage org.omg.PortableServer org.omg.PortableServer.CurrentPackage org.omg.PortableServer.POAManagerPackage org.omg.PortableServer.POAPackage org.omg.PortableServer.portable org.omg.PortableServer.ServantLocatorPackage org.omg.SendingContext org.omg.stub.java.rmi org.w3c.dom org.w3c.dom.bootstrap org.w3c.dom.events org.w3c.dom.ls org.w3c.dom.ls org.xml.sax org.xml.sax.ext org.xml.sax.helpers

The packages are divided into four learning levels:

Level 1: Source‑code deep reading

Focus on java.lang, java.io, and java.util. Read their source code to improve code‑reading skills.

Level 2: Deep understanding

Includes java.lang.reflect, java.net, java.nio, java.util.concurrent. These are essential for reflection, networking, non‑blocking IO, and concurrency.

Level 3: Use as needed

Covers packages such as java.sql, java.transaction, XML parsers, annotation processing, RMI, and cryptography. Know how to use them but no deep mastery required.

Level 4: Ignore

All remaining packages (e.g., swing, awt) can generally be ignored unless your job specifically demands them.

Part 3: Development Tools

Common tools include javac, jmap, jconsole, jstack, jvisualvm, and other utilities found in the JDK bin directory.

4 Conclusion

The most important levels are 1 and 2. Mastering the JVM is the foundation of any Java skill; without it, even the most sophisticated classes are useless. After mastering the JDK, acquire an additional specialty to become a well‑rounded developer.