Java编程从入门到放弃

1.配置开发环境

安装JDK

官网下载地址: https://www.oracle.com/java/technologies/downloads/

配置环境变量

最新版本JDK22无需手动配置环境变量。

老版本: 此电脑-右键属性-高级系统设置-环境变量-系统变量-Path-编辑 C:\Java\jdk1.8.0_65\bin

检查结果 java -version

Hello World

//HelloWorld.java
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}

# 编译
javac HelloWorld.java
# 执行
java HelloWorld

安装 IntelliJ IDEA

下载地址: https://www.jetbrains.com/idea/download/?section=windows

2. 基础语法

2.1 数据类型

基本数据类型

类型转换

1. 自动类型转换
   byte -> short -> char -> int -> long -> float -> double

2. 手动类型转换
   double -> float -> long -> int -> char -> short -> byte

数组

String[] cars = {"Volvo", "BMW", "Ford"};

2.2 流程控制

If ... Else

if (condition) {
  // block of code to be executed if the condition is true
} else {
  // block of code to be executed if the condition is false
}

// 三元运算符
variable = (condition) ? expressionTrue :  expressionFalse;

Switch

switch(expression) {
  case x:
    // code block
    break;
  case y:
    // code block
    break;
  default:
    // code block
}

While Loop

while (condition) {
  // code block to be executed
}

do {
  // code block to be executed
}
while (condition);

For Loop

for (statement 1; statement 2; statement 3) {
  // code block to be executed
}

// For-Each
for (type variableName : arrayName) {
  // code block to be executed
}

2.3 函数方法

public class Main {
  static void myMethod() {
    System.out.println("I just got executed!");
  }

  public static void main(String[] args) {
    myMethod();
  }
}

3. 面向对象

3.1 类与对象

属性与方法

public class Main {
  int x = 5; // 属性
  // 方法
  static void myMethod() {
    System.out.println("Hello World!");
  }

  public static void main(String[] args) {
    Main myObj1 = new Main();
    myObj1.x = 25;
    System.out.println(myObj1.x);
    myMethod();
  }
}

构造函数

public class Main {
  int x;

  public Main(int y) {
    x = y;
  }

  public static void main(String[] args) {
    Main myObj = new Main(5);
    System.out.println(myObj.x);
  }
}

修饰符

访问修饰符

For classes, you can use either public or default:

For attributes, methods and constructors, you can use the one of the following:

非访问修饰符

For classes, you can use either final or abstract:

For attributes and methods, you can use the one of the following:

3.2 封装

public class Person {
  private String name; // private = restricted access

  // Getter
  public String getName() {
    return name;
  }

  // Setter
  public void setName(String newName) {
    this.name = newName;
  }
}

3.2 继承

class Vehicle {
  protected String brand = "Ford";        // Vehicle attribute
  public void honk() {                    // Vehicle method
    System.out.println("Tuut, tuut!");
  }
}

class Car extends Vehicle {
  private String modelName = "Mustang";    // Car attribute
  public static void main(String[] args) {

    // Create a myCar object
    Car myCar = new Car();

    // Call the honk() method (from the Vehicle class) on the myCar object
    myCar.honk();

    // Display the value of the brand attribute (from the Vehicle class) and the value of the modelName from the Car class
    System.out.println(myCar.brand + " " + myCar.modelName);
  }
}

3.3 多态

class Animal {
  public void animalSound() {
    System.out.println("The animal makes a sound");
  }
}

class Pig extends Animal {
  public void animalSound() {
    System.out.println("The pig says: wee wee");
  }
}

class Dog extends Animal {
  public void animalSound() {
    System.out.println("The dog says: wow wow");
  }
}

class Main {
  public static void main(String[] args) {
    Animal myAnimal = new Animal();  // Create a Animal object
    Animal myPig = new Pig();  // Create a Pig object
    Animal myDog = new Dog();  // Create a Dog object
    myAnimal.animalSound();
    myPig.animalSound();
    myDog.animalSound();
  }
}

3.4 抽象

抽象类和方法

// Abstract class
abstract class Animal {
  // Abstract method (does not have a body)
  public abstract void animalSound();
  // Regular method
  public void sleep() {
    System.out.println("Zzz");
  }
}

// Subclass (inherit from Animal)
class Pig extends Animal {
  public void animalSound() {
    // The body of animalSound() is provided here
    System.out.println("The pig says: wee wee");
  }
}

class Main {
  public static void main(String[] args) {
    Pig myPig = new Pig(); // Create a Pig object
    myPig.animalSound();
    myPig.sleep();
  }
}

4. 进阶

多线程

class ThreadDemo extends Thread {
   private Thread t;
   private String threadName;
   
   ThreadDemo( String name) {
      threadName = name;
      System.out.println("Creating " +  threadName );
   }
   
   public void run() {
      System.out.println("Running " +  threadName );
      try {
         for(int i = 4; i > 0; i--) {
            System.out.println("Thread: " + threadName + ", " + i);
            // 让线程睡眠一会
            Thread.sleep(50);
         }
      }catch (InterruptedException e) {
         System.out.println("Thread " +  threadName + " interrupted.");
      }
      System.out.println("Thread " +  threadName + " exiting.");
   }
   
   public void start () {
      System.out.println("Starting " +  threadName );
      if (t == null) {
         t = new Thread (this, threadName);
         t.start ();
      }
   }
}
 
public class TestThread {
 
   public static void main(String args[]) {
      ThreadDemo T1 = new ThreadDemo( "Thread-1");
      T1.start();
      
      ThreadDemo T2 = new ThreadDemo( "Thread-2");
      T2.start();
   }   
}