Kathy Sierra - Head First Java, 3rd Edition Early Release

by Kathy Sierra and Bert Bates

Copyright 2021 Kathy Sierra and Bert Bates. All rights reserved.

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• December 2021: Third Edition

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Java takes you to new places. From its humble release to the public as the (wimpy) version 1.02, Java seduced programmers with its friendly syntax, object-oriented features, memory management, and best of all the promise of portability. The lure of write-once/run-anywhere is just too strong. A devoted following exploded, as programmers fought against bugs, limitations, and, oh yeah, the fact that it was dog slow. But that was ages ago. If youre just starting in Java, youre lucky. Some of us had to walk five miles in the snow, uphill both ways (barefoot), to get even the most trivial applet to work. But you, why, you get to ride the sleeker, faster, much more powerful Java of today.

The Way Java Works

The goal is to write one application (in this example, an interactive party invitation) and have it work on whatever device your friends have.

What youll do in Java

Youll type a source code file, compile it using the javac compiler, then run the compiled bytecode on a Java virtual machine.

Note

this is NOT meant to be a tutorial... youll be writing real code in a moment, but for now, we just want you to get a feel for how it all fits together.

In other words, the code on this page isnt quite real, dont try to compile it..)

A Very Brief History of Java

Java was initially released (some would say escaped), on January 23, 1996. Its over 25 years old! In the first 25 years, Java as a language evolved, and the Java API grew enormously. The best estimate we have is that over 17 gazillion lines of Java code have been written in the last 25 years. As you spend time being a Java programmer, you will most certainly come across Java code thats quite old, and some thats much newer.

In this book well generally start off by using older coding styles (remember, youre likely to encounter such code in the real world), and then well introduce newer style code.

In a similar fashion, we will sometimes show you older classes in the Java API, and then show you newer alternatives.

Speed and Memory Usage

When Java was first released, it was slow. But soon after, the HotSpot VM was created, as were other performance enhancers, and while its true that Java isnt the fastest language out there, its considered to be a very fast language. Almost as fast as languages like C and Rust, and MUCH, MUCH faster than languages like Python or C# or even the popular new JVM language Kotlin.

But - full disclosure - compared to C and Rust, Java uses a lot of memory.

Sharpen your pencil

Look how easy it is to write Java.

Try to guess what each line of code is doing... (answers are on the next page).

int size = 27;String name = Fido;Dog myDog = new Dog(name, size);x = size - 5;if (x < 15) myDog.bark(8);while (x > 3) { myDog.play();}int[] numList = {2,4,6,8};System.out.print(Hello);System.out.print(Dog: + name);String num = 8;int z = Integer.parseInt(num);try { readTheFile(myFile.txt);}catch(FileNotFoundException ex) { System.out.print(File not found.);}

Q: The naming conventions for Javas various versions are confusing. There was JDK 1.0, and j2SE 1.2, 1.3, 1.4 then a jump to J2SE 5.0, then it changed to Java SE6, Java SE7, and last time I checked, Java was up to Java SE 15. Can you explain whats going on?

A: Its true that for the first several years, Javas version numbers were hard to predict. But since the release of Java SE 9 in 2017, Java has been on a rapid release model, and it looks like from 2017 through 2020, the version numbers have become more stable and predictable.

Sharpen your pencil Answers

Look how easy it is to write Java.

int size = 27;String name = Fido;Dog myDog = new Dog(name, size);x = size - 5;if (x < 15) myDog.bark(8);while (x > 3){ myDog.play();}int[] numList = {2,4,6,8};System.out.print(Hello);System.out.print(Dog: + name);String num = 8;int z = Integer.parseInt(num);try { readTheFile(myFile.txt);}catch(FileNotFoundException ex) { System.out.print(File not found.);}