Java基礎教程

Java是一個跨平台的編程語言，擁有廣泛的應用場景，包括Web開發、桌面應用、移動應用和遊戲開發等等。Java語言的特點包括面向對象、健壯性、平台無關性、可移植性和安全性等等。本文將從多個方面對Java基礎進行詳細的闡述，旨在幫助初學者快速掌握Java基礎知識。

一、Java語言基礎

Java語言基礎是學習Java編程的基礎，包括變量、數據類型、運算符、流程控制語句、方法和異常處理等內容。

1、變量和數據類型：

public class Main {
    public static void main(String[] args) {
        int a = 10;
        float b = 3.14f;
        String c = "Hello world!";
        System.out.println(a);
        System.out.println(b);
        System.out.println(c);
    }
}

2、運算符：

public class Main {
    public static void main(String[] args) {
        int a = 10;
        int b = 5;
        int c = a + b;
        int d = a - b;
        int e = a * b;
        int f = a / b;
        int g = a % b;
        System.out.println(c);
        System.out.println(d);
        System.out.println(e);
        System.out.println(f);
        System.out.println(g);
    }
}

3、流程控制語句：

public class Main {
    public static void main(String[] args) {
        int a = 10;
        if (a > 5) {
            System.out.println("a大於5");
        } else {
            System.out.println("a小於等於5");
        }
        for (int i = 0; i  0) {
            System.out.println(a);
            a--;
        }
    }
}

4、方法：

public class Main {
    public static void main(String[] args) {
        int a = 10;
        int b = 5;
        int c = add(a, b);
        System.out.println(c);
    }
    
    public static int add(int a, int b) {
        int c = a + b;
        return c;
    }
}

5、異常處理：

public class Main {
    public static void main(String[] args) {
        try {
            int[] arr = new int[10];
            System.out.println(arr[11]);
        } catch (Exception e) {
            System.out.println("數組越界異常");
        }
    }
}

二、面向對象編程

Java是一種面向對象的語言，面向對象編程是Java開發中非常重要的一部分。

1、類和對象：

public class Dog {
    String name;
    int age;

    public void bark() {
        System.out.println("汪汪汪");
    }

    public static void main(String[] args) {
        Dog dog1 = new Dog();
        dog1.name = "旺財";
        dog1.age = 2;
        System.out.println("名字：" + dog1.name);
        System.out.println("年齡：" + dog1.age);
        dog1.bark();
        
        Dog dog2 = new Dog();
        dog2.name = "小白";
        dog2.age = 1;
        System.out.println("名字：" + dog2.name);
        System.out.println("年齡：" + dog2.age);
        dog2.bark();
    }
}

2、繼承和多態：

public class Animal {
    String name;
    int age;

    public void eat() {
        System.out.println("動物進食");
    }
}

public class Dog extends Animal {
    public void bark() {
        System.out.println("汪汪汪");
    }

    public void eat() {
        System.out.println("狗吃骨頭");
    }
}

public class Main {
    public static void main(String[] args) {
        Dog dog = new Dog();
        dog.name = "旺財";
        dog.age = 2;
        System.out.println("名字：" + dog.name);
        System.out.println("年齡：" + dog.age);
        dog.bark();
        dog.eat();
        Animal animal = new Dog();
        animal.eat();
    }
}

3、接口和抽象類：

public interface Animal {
    void eat();
}

public abstract class Fish implements Animal {
    public void swim() {
        System.out.println("游泳");
    }
}

public class Salmon extends Fish {
    public void eat() {
        System.out.println("鮭魚吃蝦");
    }
}

public class Main {
    public static void main(String[] args) {
        Salmon salmon = new Salmon();
        salmon.eat();
        salmon.swim();
    }
}

三、常用類庫

Java常用類庫為Java編程提供了更加便捷的開發方式，其中包括String、集合框架、IO流、日期時間類和正則表達式等等。

1、String類：

public class Main {
    public static void main(String[] args) {
        String str1 = "Hello";
        String str2 = "World";
        String str3 = str1 + " " + str2;
        String str4 = new String("Hello");
        System.out.println(str1.equals(str2));
        System.out.println(str1.equals(str4));
        System.out.println(str1.length());
        System.out.println(str3);
        System.out.println(str1.substring(2));
    }
}

2、集合框架：

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List list = new ArrayList();
        list.add("Hello");
        list.add("World");
        list.add("Java");
        System.out.println(list.get(1));
        System.out.println(list.size());
        for (String str : list) {
            System.out.println(str);
        }
    }
}

3、IO流：

import java.io.*;

public class Main {
    public static void main(String[] args) throws Exception {
        File file = new File("test.txt");
        FileWriter writer = new FileWriter(file);
        writer.write("Hello World!\n");
        writer.write("Nice to meet you!");
        writer.close();

        FileReader reader = new FileReader(file);
        BufferedReader bufferedReader = new BufferedReader(reader);
        String line;
        while ((line = bufferedReader.readLine()) != null) {
            System.out.println(line);
        }
        reader.close();
    }
}

4、日期時間類：

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

public class Main {
    public static void main(String[] args) {
        LocalDateTime now = LocalDateTime.now();
        System.out.println(now);
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
        String formattedDateTime = now.format(formatter);
        System.out.println("Formatted date and time: " + formattedDateTime);
    }
}

5、正則表達式：

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Main {
    public static void main(String[] args) {
        String content = "I am a java programmer!";
        String patternString = ".*java.*";
        Pattern pattern = Pattern.compile(patternString);
        Matcher matcher = pattern.matcher(content);
        boolean isMatch = matcher.matches();
        System.out.println("The text contains 'java'? " + isMatch);
    }
}

以上僅是Java基礎的冰山一角，但它卻是Java編程的基石。希望本文能對初學Java的讀者有所幫助，讓大家能夠快速入門Java編程。