Java: Class

Creating Classes

打开左侧的 Project 面板
右击 main class 的商机文件夹，选择 New -> Java Class
命名时每个单词的首字母大写

public class TextBox {
    public String text = "";

    public void setText(String text) {
        this.text = text;
    }

    public void clear() {
        text = "";
    }
}

Creating Objects

public class Main {

    public static void main(String[] args) {
//        TextBox textBox1 = new TextBox();
        var textBox1 = new TextBox();
        textBox1.setText("Box 1");
        System.out.println(textBox1.text);
    }
}

Memory Allocation

Java 管理着两个不同的内存区域：堆和栈。

堆用于存放 Objects。

栈用于存放短期变量，包括对 Objects 的引用。

堆和栈的关系

Encapsulation 封装

定义：将数据和与数据相关的操作绑定在一个单位中。

创建 setter 和 getter

control + enter
创建 setter 和 getter

Abstraction 抽象

定义：通过隐藏不必要的细节来减少复杂性

Coupling 耦合

定义：Classes 之间的依赖程度。

编程时应尽力减少不同 class 之间的耦合。

减少耦合

public class Browser {
    public void navigate(String address) {
        String ip = findIpAddress(address);
        String html = sendHttpRequest(ip);
        System.out.println(html);
    }

    public String sendHttpRequest(String ip) {
        return "<html></html>";
    }

    public String findIpAddress(String address) {
        return "127.0.0.1";
    }
}

在本例中，sendHttpRequest 和 findIpAddress 两个方法属于不必要的细节，应当改为 private 型方法。

Constructors

public class Employee {
    private int baseSalary;
    private int hourlyRate;

    public static int numberOfEmployees;

    public Employee(int baseSalary) {
        this(baseSalary, 0);
    }

    public Employee(int baseSalary, int hourlyRate) {
        setBaseSalary(baseSalary);
        setHourlyRate(hourlyRate);
        numberOfEmployees++;
    }

    private int getHourlyRate() {
        return hourlyRate;
    }

    private void setHourlyRate(int hourlyRate) {
        if (hourlyRate < 0)
            throw new IllegalArgumentException("Hourly rate can't be less than 0.");
        this.hourlyRate = hourlyRate;
    }

    private int getBaseSalary() {
        return baseSalary;
    }

    private void setBaseSalary(int baseSalary) {
        if (baseSalary <= 0)
            throw new IllegalArgumentException("Salary can't be 0 or less.");
        this.baseSalary = baseSalary;
    }

    public int calculateWage(int extraHours) {
        return baseSalary + (hourlyRate * extraHours);
    }

    public int calculateWage() {
        return calculateWage(0);
    }
  
  	public void printNumberOfEmployees() {
        System.out.println(numberOfEmployees);
    }
}

Method Overloading

定义：通过不同的方式创建 class。

注意：不要过度使用重载！

public int calculateWage(int extraHours) {
		return baseSalary + (hourlyRate * extraHours);
}

public int calculateWage() {
		return calculateWage(0);
}

Constructor Overloading

public Employee(int baseSalary) {
		this(baseSalary, 0);
}

public Employee(int baseSalary, int hourlyRate) {
		setBaseSalary(baseSalary);
		setHourlyRate(hourlyRate);
  	numberOfEmployees++;
}