Java 學習筆記

本篇筆記為我學習 Java 的一些紀錄，會學 Java 主要是因為 NTU Software Engineering Design by J Lee 所需。本篇撰寫是建立在已有穩固程式語言基礎上，故只會包含 Java 與其他語言的重要差異，以及一些我認為需要特別紀錄的內容。 # 相關連結 ## Java - [Java Wiki](https://zh.wikipedia.org/zh-tw/Java) - [Java 3hr Basic Tutorial](https://www.youtube.com/watch?v=nQ1Z3NxpfIw) - [[推薦] Openhome.cc (Java 17)](https://openhome.cc/zh-tw/java/) ## Design Pattern - [Java设计模式丨图解+框架源码分析23种设计模式](https://www.bilibili.com/video/BV13z4y1D76S/?p=39&share_source=copy_web&vd_source=f95d1e685761ee0ed09f7ba4d03e5484) - [初探設計模式](https://ithelp.ithome.com.tw/users/20112528/ironman/2113?page=1) # 語法基礎 ## 基本架構 - Hello World There should be only one `public class` in one file, also, the filename should be the same as that class. For the following sample the file should be named `HelloWorld.java`. ```java= public class HelloWorld { public static void main(String[] args) { System.out.print("Hello World\n"); System.out.println("Hello World"); System.out.printf("Hello %s%n", "World"); } } ``` - [引用套件與輸出](https://openhome.cc/zh-tw/java/path-package-module/package-import/) The package_name needs to be the same with the file path! See more at [here](https://yubin551.gitbook.io/java-note/basic_java_programming/package_import). ```java= package {package_name} // export as a package import {package_name} // import from existing package ``` - 讀取輸入 ```java= import java.util.Scanner; public class Guess { public static void main(String[] args) { var console = new Scanner(System.in); System.out.print(console.next()); // stops at space System.out.print(console.nextLine()); // stops at newline chat System.out.print(console.nextInt()); System.out.print(console.nextDouble()); } } ``` - [格式化輸出](https://openhome.cc/zh-tw/java/syntax-abc/type/#格式化輸出) - [執行方式](https://openhome.cc/zh-tw/java/path-package-module/hello/) ```cmd= java HelloWorld.java ``` ```cmd= javac HelloWorld.java // Compile to .class file java HelloWorld ``` ## 基本資料型態 - 常見型態：`byte, short, int, long, float, double, char, boolean` - 可使用 `var` 來宣告變數，型態會自動判別，最好僅用在區域變數 - `final` 修飾的變數不可變 - 預設類別：`int` 與 `double`，要放到其他類別的變數要另外轉換 - `String` 不是基本資料型態 ### Casting ```java= double PI = 3.14; float PI = 3.14F; float PI = (float) 3.14; int number = 2147483648; long number = 2147483648L; ``` ### Promote ```java= System.out.println(10 / 3); // 3 System.out.println(10.0 / 3); // 3.333... ``` ```java= short a = 1; short b = 2; short c = (short) (a + b); // need to convert type ``` ### 進階資料型態 - 屬於 `class`，像是`String`、`BigDecimal` - 需要用 `a.equals(b)` 來比較其相等性（不然會比位址） - 關於[字串處理](https://openhome.cc/zh-tw/java/object/string/) - [文字區塊處理](https://openhome.cc/zh-tw/java/object/string/#文字區塊) ### 陣列 - 參考[這裡](https://openhome.cc/zh-tw/java/object/array/) - 陣列複製的時候要注意 shallow copy 的問題！ ```java= // 1d array int[] scores = {88, 81, 74, 68, 78, 76, 77, 85, 95, 93}; System.out.println(scores.length); // 2d array int[][] cords = { {1, 2, 3}, {4, 5, 6} }; for(int[] row : cords) { for(int value : row) { System.out.printf("%2d", value); } System.out.println(); } // uninitialized declaration int[] scores = new int[10]; // copying an array int[] scores1 = {88, 81, 74, 68, 78, 76, 77, 85, 95, 93}; var scores2 = new int[scores1.length]; System.arraycopy(scores1, 0, scores2, 0, scores1.length); // or int[] scores2 = Arrays.copyOf(scores1, scores1.length); // object array class Object { ... } Object [] obj_array = { new Object(...); } ``` ## 其他基礎 ### 運算子 - 常見的都有，參見[這裡](https://openhome.cc/zh-tw/java/syntax-abc/operator/)，特別注意有 `var c = (condition) ? a : b` 的寫法 ### 條件 - 跟 C++ 一樣，記得條件要括號，是用 `else if` - 有 `switch-case` 的語法，也一樣，記得加 `break` ```java= if (cond){ ... } else if (cond2){ ... } else{ ... } switch (var){ case 1: ... break; case 100: ... break; default: ... } var level = switch(quotient) { case 10, 9 -> 'A'; case 8 -> 'B'; case 7 -> 'C'; case 6 -> 'D'; default -> 'E'; }; ``` ### 迴圈 - 跟 C++ 一樣，有 `do-while` ```java= for(var i = 1; i <= N; i++) { ... } for(var item : item_list) { ... } while(cond) { ... } do { ... } while(cond); ``` ### Function ==Important!!!== - [Java: "Pass-by-Value" vs "Pass-by-Reference" - Unraveling the Confusion](https://dev.to/iamcymentho/java-pass-by-value-vs-pass-by-reference-unraveling-the-confusion-3gmh) - When you pass a primitive type, a copy of its value is passed to the method, leaving the original variable unchanged. - When you pass an object reference, the reference's value (memory address) is copied, not the object itself. This means you can modify the object's properties inside the method, but you can't change the reference itself. ### Commonly Used Packages - **`java.lang`**: Provides fundamental classes automatically imported into every Java program. - Key classes: `String`, `Object`, `Math`, `Integer`, `Boolean`. - **`java.util`**: Offers utility classes for collections, date/time manipulation, and other common functions. - Key classes: `ArrayList`, `HashMap`, `HashSet`, `Date`, `Collections`, `Random`, `Scanner`. - **`java.io`**: Facilitates input and output operations, including file handling and stream manipulation. - Key classes: `BufferedReader`, `InputStream`, `OutputStream`, `File`, `FileReader`, `PrintWriter`. - **`java.nio`**: Supports high-performance, non-blocking I/O operations, useful for large-scale applications. - Key classes: `ByteBuffer`, `FileChannel`, `Path`, `Files`. - **`java.time`**: Handles date and time operations in a modern, thread-safe manner. - Key classes: `LocalDate`, `LocalTime`, `LocalDateTime`, `Duration`, `Period`. - **`java.math`**: Supports precise mathematical computations, especially for large integers and decimals. - Key classes: `BigInteger`, `BigDecimal`.  ### Miscellaneous #### Syntactic sugar 可用於修飾方法、參數 - `@Nullable`：可以為空 - `@NonNull`：不可為空 - `@SuppressWarnings("unchecked")`: suppress compiler warnings related to unchecked type casting operations # OOP ## Overall - A class can extend only one class (single inheritance for classes). - An interface can extend multiple interfaces. - A class can implement multiple interfaces. ## Basics - If define a constructor with parameters, then default constructor will NOT be automatically created ```java= package {anything}.{anything} class Clothes { String color; char size; Clothes(String color, char size) { this.color = color; this.size = size; } } // Only one public per file public class Field { public static void main(String[] args) { var sun = new Clothes("red", 'S'); Clothes spring = new Clothes("green", 'M'); System.out.printf("sun (%s, %c)%n", sun.color, sun.size); System.out.printf("spring (%s, %c)%n", spring.color, spring.size); } } ``` ## Record - 如果只是記錄屬性，可以使用 `record`，參見[這裡](https://openhome.cc/zh-tw/java/encapsulation/record/) ```java= record Clothes(String color, char size) {} public class Field { public static void main(String[] args) { var sun = new Clothes("red", 'S'); var spring = new Clothes("green", 'M'); System.out.printf("sun (%s, %c)%n", sun.color(), sun.size()); System.out.printf("spring (%s, %c)%n", spring.color(), spring.size()); } } ``` ## Wrapper - 把基本型態打包成 `class` 使用，細節參考[這裡](https://openhome.cc/zh-tw/java/object/primitive-wrapper/) ## Access Modifier (Public, Private, Protected) - [存取修飾子](https://yubin551.gitbook.io/java-note/basic_java_programming/accessmodifier) - [修飾外部類別](https://yubin551.gitbook.io/java-note/basic_java_programming/accessmodifier/modifyclass) ![image](https://hackmd.io/_uploads/HJgSNJkaA.png) ## Encapsulation - 將重複使用的程式碼打包起來 - 權限控管：僅能透過 `getter` 與 `setter` 操作成員變數 ```java= package cc.openhome; class CashCard { private String number; private int balance; private int bonus; ...略 void store(int money) { if(money > 0) { this.balance += money; if(money >= 1000) { this.bonus++; } } else { System.out.println("儲值是負的？你是來亂的嗎？"); } } int getBalance() { return balance; } int getBonus() { return bonus; } String getNumber() { return number; } } ``` ## Overload ```java= public class Some { private int a = 10; private String text = "n.a."; public Some(int a) { if(a > 0) { this.a = a; } } public Some(int a, String text) { if(a > 0) { this.a = a; } if(text != null) { this.text = text; } } ... } ``` ## Static - `static` 關鍵字代表該成員/方法屬於該類別，不屬於實體本身 (or class variable / method and not a part of any instance) - 因為 `this` 是屬於實體的屬性，因此不可能（也不應該）與 `static` 一起用 ```java= class Ball { double radius; static final double PI = 3.1415926; static double toRadians(double angdeg) { // 角度轉徑度 return angdeg * (Ball.PI / 180); } } // access class variable / method System.out.println(Ball.PI); System.out.println(Ball.toRadians(100)); // System.out.println is also a static method from "System"! ``` ## Inheritance - Use `super` to reference to parent class (like `this`) - Java only allows inheritance from one parent class ```java= public class Role { ... } public class SwordsMan extends Role { ... } public class Magician extends Role { ... } public class RPG { public static void main(String[] args) { ... } // Pull up showBlood() to Role level static void showBlood(Role role) { System.out.printf("%s 血量 %d%n", role.getName(), role.getBlood()); } } ``` ### How to determine compile success or not? - Use is-a method ![image](https://hackmd.io/_uploads/rkwjCtACA.png) - Use casting can pass compile, but will failed at run time ```java= SwordsMan swordsMan = new SwordsMan(); Role role = swordsMan; // SwordsMan 是一種 Role，這行通過編譯 // 你告訴編譯器要讓 Role 扮演 Magician，以下這行通過編譯 Magician magician = (Magician) role; // role 參考 SwordsMan 實例，執行失敗 ``` ### Abstract - Can add at class or at Function - Abstract class cannot be instantiate - Child class of abstract class must implement abstract function, or keep it in abstract for its child class (Thus make itself a abstract class) ```java= public abstract class Role { ... public abstract void fight(); } public class SwordsMan extends Role { @Override // Ask the compiler to validate public void fight() { System.out.println("揮劍攻擊"); } } ``` ### Interface - 定義行為外觀 - 與繼承只能一個相比，可以實作多個介面 ```java= public interface Swimmer { public abstract void swim(); } ``` ![image](https://hackmd.io/_uploads/BJO0iDwMJg.png)