在java中创建类的基础[关闭]

Question

这是一个简单的问题，但我的 AP Comp Sci 书解释得不够好，你们总是很有用的。

在 Java 中创建自定义类并在该类中创建方法，然后调用这些方法的基本方法是什么。我知道它很简单，但我在任何地方都找不到好的解释

Answer 1

看来您需要更好地了解 OOP。因此，让我们创建一个类和一个测试客户端来帮助您。我们将创建一个类和一个测试客户端（或驱动程序）来计算电势。请注意，此示例来自 Robert Sedgewick 和 Kevin Wayne 的 Introduction to Programming in Java（我强烈推荐这本书）。

首先我们创建一个Charge 类：

/*
Separate classes must be in the same directory as main method or must invoke 
classpath
*/
public class Charge {
    // first declare instance variables which are usually private
    private final double rx;
    private final double ry;
    private final double q;

    /* A class contains constructors that are invoked to create objects from a 
    class blueprint. Constructor declarations look like method declarations 
    -except that they use the name of the class and have no return type.  
    Constructors must use the exact name of the class, case sensitive.
    Classes and Constructors are capitalized - methods are camelCase.
    */

    // Constructor
    public Charge(double x0, double y0, double q0) {
        rx = x0;
        ry = y0;
        q = q0;
    }

    /*
    The method to compute electrical potential which is defined by the equation
    V = kq/r
    */
    public double potentialAt(double x, double y) {
        double k = 8.99e09;  // Electrostatic Constant that k=8.99 X 10^9 Nm^2/C^2 (N = Newtons, m = meters, C = Coloumbs)
        // r = delta x - delta y
        double dx = x - rx; // delta x for distance
        double dy = y - ry; // delta y for distance
        return k*q/Math.sqrt(dx*dx + dy*dy); // Computation using distance formula
    }
}

这将是使用此类的 API。 Java编程中的一个重要概念 是你不需要知道数据类型是如何实现的就可以使用它。

public class Charge

这是构造函数：

Charge(double x0, double y0, double q0)

这些是实例方法。变量之间最重要的区别 引用类型与原始类型的区别在于您可以使用引用类型变量 调用实现数据类型操作的方法。

double potentialAt(double x, double y)
String toString()

使用这个类的两个部分是：

1. Create an object

    ClassName   object   =  new   ClassName (invoke Constructor)
    ---------   ------      ---   ---------  -----------------
    Charge      c        =  new    Charge     (2.2, 3.4, 7.2)

2. Use instance methods on object

    c.potentialAt(2.3, 4.2)

这将是可以与此类一起使用的客户端（或驱动程序）：

import java.util.*;

public class ChargeClient {
    public static void main(String[] args) {
        // Using a scanner object to get values
        System.out.println("Please enter an X Value");
        Scanner in = new Scanner(System.in);
        double x = in.nextDouble();
        System.out.println("Please enter a Y Value");
        double y = in.nextDouble();
        /*
        1. Instantiate objects c1 and c2

            ClassName   object   =  new   ClassName (invoke Constructor)
            ---------   ------      ---   ---------  -----------------
            Charge      c        =  new    Charge     (2.2, 3.4, 7.2)

        2. We are invoking constructor from API

            Charge(double x0, double y0, double q0)
        */
        Charge c1 = new Charge(.51, .63, 21.3);
        Charge c2 = new Charge(.13, .94, 81.9);
        // print out charge so we know what we are dealing with
        System.out.println(c1);
        System.out.println(c2);
        /*
        Here we create variables to hold the return from our potential method
        which is enacted on our c1 and c2 objects.  

        1. We call a method on an object by:

            objectName.methodName(appropriate parameters)

        */
        double v1 = c1.potentialAt(x, y);
        double v2 = c2.potentialAt(x, y);
        // Concatenate results and print them out.
        System.out.println(v1 + v2);
        System.out.println("This is the printf statement:");
        System.out.printf("%.2E\n", v1 + v2);
    }
}