Java 手记

Java 手记

// 单行注释
/*
多行注释
*/
/**
JavaDoc 文档格式
*/

// 导入 ArrayList 类
import java.util.ArrayList;
// 导入 java.security 包中所有类
import java.security.*;

// 每一个 java 源文件包含一个名字与文件名一致的公共类
public class LearnJava {

    // 一个程序必须要有一个 main 方法作为程序的入口
    public static void main (String[] args) {

        // 使用 System.out.println 打印并换行
        System.out.println("Hello World!");
        System.out.println(
            "Integer: " + 10 +
            " Double: " + 3.14 +
            " Boolean: " + true);

        // 打印且不换行使用 System.out.print
        System.out.print("Hello ");
        System.out.print("World");


        ///////////////////////////////////////
        // 类型与变量
        ///////////////////////////////////////

        // 使用 <type> <name> 声明变量

        // Byte 型
        // (-128 <= byte <= 127)
        byte fooByte = 100;

        // Short 型
        // (-32,768 <= short <= 32,767)
        short fooShort = 10000;

        // Integer 型
        // (-2,147,483,648 <= int <= 2,147,483,647)
        int fooInt = 1;

        // Long 型
        // (-9,223,372,036,854,775,808 <= long <= 9,223,372,036,854,775,807)
        long fooLong = 100000L;
        // L 是 long 型的后缀
        // 没有后缀则当做 int 型

        // 需要注意的是 Java 没有无符号型整型

        // Float 型
        float fooFloat = 234.5f;
        // f 作为后缀标记一个浮点数为 float 型否则当做 double 型看待

        // Double 型
        double fooDouble = 123.4;

        // Boolean 型
        boolean fooBoolean = true;
        boolean barBoolean = false;

        // Char 型,为 unicode 2 字节
        char fooChar = 'A';

        // 使用 final 定义常量
        final int HOURS_I_WORK_PER_WEEK = 9001;

        // 字符串
        String fooString = "My String Is Here!";

        // \n 换行
        String barString = "Printing on a new line?\nNo Problem!";
        // \t 制表符
        String bazString = "Do you want to add a tab?\tNo Problem!";
        System.out.println(fooString);
        System.out.println(barString);
        System.out.println(bazString);

        // 数组
        // 创建数组格式:
        //<datatype> [] <var name> = new <datatype>[<array size>];
        int [] intArray = new int[10];
        String [] stringArray = new String[1];
        boolean [] booleanArray = new boolean[100];

        // 另外一种方式
        int [] y = {9000, 1000, 1337};

        // 使用索引取得数组中的值
        System.out.println("intArray @ 0: " + intArray[0]);

        // 数组元素是可变的
        intArray[1] = 1;
        System.out.println("intArray @ 1: " + intArray[1]); // => 1

        // 其他需要了解的类型
        // 数组类别 - 与数组类似,但是容量可变。
        // 链表
        // Maps
        // HashMaps

        ///////////////////////////////////////
        // 运算符
        ///////////////////////////////////////
        System.out.println("\n->Operators");

        int i1 = 1, i2 = 2; // 多个变量声明

        // 数学运算可以直接写
        System.out.println("1+2 = " + (i1 + i2)); // => 3
        System.out.println("2-1 = " + (i2 - i1)); // => 1
        System.out.println("2*1 = " + (i2 * i1)); // => 2
        System.out.println("1/2 = " + (i1 / i2)); // => 0 (0.5 取整)

        // 取余
        System.out.println("11%3 = "+(11 % 3)); // => 2

        // 比较运算
        System.out.println("3 == 2? " + (3 == 2)); // => false
        System.out.println("3 != 2? " + (3 != 2)); // => true
        System.out.println("3 > 2? " + (3 > 2)); // => true
        System.out.println("3 < 2? " + (3 < 2)); // => false
        System.out.println("2 <= 2? " + (2 <= 2)); // => true
        System.out.println("2 >= 2? " + (2 >= 2)); // => true

        // 位运算
        /*
        ~       取反
        <<      有符号左移
        >>      无符号右移
        >>>     无符号右移
        &       取且
        ^       取异或
        |       取或
        */

        // 自增自减
        int i = 0;
        System.out.println("\n->Inc/Dec-rementation");
        System.out.println(i++); //i = 1.
        System.out.println(++i); //i = 2.
        System.out.println(i--); //i = 1.
        System.out.println(--i); //i = 0.

        ///////////////////////////////////////
        // 流程控制
        ///////////////////////////////////////
        System.out.println("\n->Control Structures");

        // if 语句
        int j = 10;
        if (j == 10){
            System.out.println("I get printed");
        } else if (j > 10) {
            System.out.println("I don't");
        } else {
            System.out.println("I also don't");
        }

        // While 循环
        int fooWhile = 0;
        while(fooWhile < 100)
        {
            //System.out.println(fooWhile);
            fooWhile++;
        }
        System.out.println("fooWhile Value: " + fooWhile);

        // Do While 循环
        int fooDoWhile = 0;
        do
        {
            //System.out.println(fooDoWhile);
            //Increment the counter
            //Iterated 99 times, fooDoWhile 0->99
            fooDoWhile++;
        }while(fooDoWhile < 100);
        System.out.println("fooDoWhile Value: " + fooDoWhile);

        // For 循环
        int fooFor;
        //for 循环语句结构 => for(<start_statement>; <conditional>; <step>)
        for(fooFor=0; fooFor<10; fooFor++){
            //System.out.println(fooFor);
            //Iterated 10 times, fooFor 0->9
        }
        System.out.println("fooFor Value: " + fooFor);

        // Switch Case 结构
        // switch 中可以使用的类型 byte, short, char, int,  enumerated types, String class, and a few special classes that wrap
        // primitive types: Character, Byte, Short, and Integer.
        int month = 3;
        String monthString;
        switch (month){
            case 1:
                    monthString = "January";
                    break;
            case 2:
                    monthString = "February";
                    break;
            case 3:
                    monthString = "March";
                    break;
            default:
                    monthString = "Some other month";
                    break;
        }
        System.out.println("Switch Case Result: " + monthString);


        ///////////////////////////////////////
        // 类型转换
        ///////////////////////////////////////

        // Converting data

        // Convert String To Integer
        Integer.parseInt("123");//returns an integer version of "123"

        // Convert Integer To String
        Integer.toString(123);//returns a string version of 123

        // For other conversions check out the following classes:
        // Double
        // Long
        // String

        // Typecasting
        // You can also cast java objects, there's a lot of details and
        // deals with some more intermediate concepts.
        // Feel free to check it out here:
        // http://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html


        ///////////////////////////////////////
        // Classes And Functions
        ///////////////////////////////////////

        System.out.println("\n->Classes & Functions");

        // (definition of the Bicycle class follows)

        // Use new to instantiate a class
        Bicycle trek = new Bicycle();

        // Call object methods
        trek.speedUp(3); // You should always use setter and getter methods
        trek.setCadence(100);

        // toString is a convention to display the value of this Object.
        System.out.println("trek info: " + trek.toString());

    } // End main method
} // End LearnJava class


// You can include other, non-public classes in a .java file


// Class Declaration Syntax:
// <public/private/protected> class <class name>{
//    //data fields, constructors, functions all inside.
//    //functions are called as methods in Java.
// }

class Bicycle {

    // Bicycle's Fields/Variables
    public int cadence; // Public: Can be accessed from anywhere
    private int speed;  // Private: Only accessible from within the class
    protected int gear; // Protected: Accessible from the class and subclasses
    String name; // default: Only accessible from within this package

    // Constructors are a way of creating classes
    // This is a default constructor
    public Bicycle() {
        gear = 1;
        cadence = 50;
        speed = 5;
        name = "Bontrager";
    }

    // This is a specified constructor (it contains arguments)
    public Bicycle(int startCadence, int startSpeed, int startGear, String name) {
        this.gear = startGear;
        this.cadence = startCadence;
        this.speed = startSpeed;
        this.name = name;
    }

    // Function Syntax:
    // <public/private/protected> <return type> <function name>(<args>)

    // Java classes often implement getters and setters for their fields

    // Method declaration syntax:
    // <scope> <return type> <method name>(<args>)
    public int getCadence() {
        return cadence;
    }

    // void methods require no return statement
    public void setCadence(int newValue) {
        cadence = newValue;
    }

    public void setGear(int newValue) {
        gear = newValue;
    }

    public void speedUp(int increment) {
        speed += increment;
    }

    public void slowDown(int decrement) {
        speed -= decrement;
    }

    public void setName(String newName) {
        name = newName;
    }

    public String getName() {
        return name;
    }

    //Method to display the attribute values of this Object.
    @Override
    public String toString() {
        return "gear: " + gear +
                " cadence: " + cadence +
                " speed: " + speed +
                " name: " + name;
    }
} // end class Bicycle

// PennyFarthing is a subclass of Bicycle
class PennyFarthing extends Bicycle {
    // (Penny Farthings are those bicycles with the big front wheel.
    // They have no gears.)

    public PennyFarthing(int startCadence, int startSpeed){
        // Call the parent constructor with super
        super(startCadence, startSpeed, 0, "PennyFarthing");
    }

    // You should mark a method you're overriding with an @annotation
    // To learn more about what annotations are and their purpose
    // check this out: http://docs.oracle.com/javase/tutorial/java/annotations/
    @Override
    public void setGear(int gear) {
        gear = 0;
    }

}

原文: http://learnxinyminutes.com/docs/java/