前言

从今日起，陆续分享《HeadFirstJava》的读书笔记，希望能够帮助大家更好的理解Java，提高自己的基础编码能力。

Java是一门面向对象的高级编程语言，常年霸占编程语言排行榜前三。

Java是目前国内的主流开发语言，基本每家大型公司都对Java开发工程师有巨大需求，因此打好Java基本功对我们的职业生涯有很好的帮助。

今天要分享的是【基本概念-进入Java的世界】，希望对大家有所帮助。

01-进入Java的世界-前言

书籍精华

Java的工作方式

01-进入Java的世界-Java的工作方式-1
01-进入Java的世界-Java的工作方式-2

几个核心概念：源代码，编译器，虚拟机

public class Party {
    public void buildInvite() {
        Frame f = new Frame();
        Label l = new Label("Party at Tim's");
        Button b = new Button("You bet");
        Button c = new Button("Shoot me");
        Panel p = new Panel();
        p.add(l);
    }
}

Java简史

01-进入Java的世界-Java简史-1

几个重大版本：Java 1.02，Java 1.1，Java2（1.2到1.4），Java5.0（1.5及以上）

见过Java 2和Java 5.0，有Java3和Java4么

Java就是这么酷

01-进入Java的世界-Java就是这么酷-1
01-进入Java的世界-Java就是这么酷-2

源文件、类、方法

Java的类

01-进入Java的世界-Java的类-1
01-进入Java的世界-Java的类-2

public class MyFirstApp {
    public static void main(String[] args) {
        System.out.println("I Rule!");
        System.out.println("The World");
    }
}

public，static，main方法

语句、循环、条件分支

01-进入Java的世界-语句、循环、条件分支-1
01-进入Java的世界-语句、循环、条件分支-2

语句：做某件事

循环：反复做某件事

条件分支：在适当的条件下做某件事

Java基本概念

01-进入Java的世界-Java基本概念-1
01-进入Java的世界-Java基本概念-2

public class IfTest {
    public static void main(String[] args) {
        int x = 3;
        if (x == 3) {
            System.out.println("x must be 3");
        }
        System.out.println("This runs no matter what");
    }
}

public class IfTest2 {
    public static void main(String[] args) {
        int x = 2;
        if (x == 3) {
            System.out.println("x must be 3");
        } else {
            System.out.println("x is NOT 3");
        }
        System.out.println("This runs no matter what");
    }
}

万物皆对象

真正的应用

01-进入Java的世界-真正的应用-1
01-进入Java的世界-真正的应用-2

活生生的物联网（智能家居）

public class BeerSong {
    public static void main(String[] args) {
        int beerNum = 99;
        String word = "bottles";
        while (beerNum > 0) {
            if (beerNum == 1) {
                word = "bottle";
            }
            System.out.println(beerNum + " " + word + " of beer on the wall");
            System.out.println(beerNum + " " + word + "of beer.");
            System.out.println("Take one down");
            System.out.println("Pass it around.");
            beerNum = beerNum - 1;
            if (beerNum > 0) {
                System.out.println(beerNum + " " + word + " of beer on the wall");
            } else {
                System.out.println("No more bottles of beer on the wall");
            }
        }
    }
}

开始编写程序

01-进入Java的世界-开始编写程序-1
01-进入Java的世界-开始编写程序-2

public class PhraseOMatic {
    public static void main(String[] args) {
        //你可以随意的加上其他术语
        String[] wordListOne = {"24/7", "multi-Tier", "30,000 foot", "B-to-B", "win-win", "front-end", "web-based", "pervasive", "smart", "six-sigma", "critical-path", "dynamic"};
        String[] wordListTwo = {"empowered", "sticky",
                "value-added", "oriented", "centric", "distributed",
                "clustered", "branded", "outside-the-box", "positioned",
                "networked", "focused", "leveraged", "aligned",
                "targeted", "shared", "cooperative", "accelerated"};
        String[] wordListThree = {"process", "tipping-point", " solution", " architecture", " core competency",
                "strategy", "mindshare", "portal", "space", "vision",
                "paradigm", "mission"};
        //计算每一组有多少个名词术语
        int oneLength = wordListOne.length;
        int twoLength = wordListTwo.length;
        int threeLength = wordListThree.length;
        //产生随机数字
        int rand1 = (int) (Math.random() * oneLength);
        int rand2 = (int) (Math.random() * twoLength);
        int rand3 = (int) (Math.random() * threeLength);
        //组合出专家术语
        String phrase = wordListOne[rand1] + " " +
                wordListTwo[rand2] + " " + wordListThree[rand3];
        //输出
        System.out.println("What we need is a " + phrase);
    }

}