Java - 在类外部调用但从内部调用时返回正确值的方法？

Question

我正在做 Java 的基础知识，所以在做一个更大的程序时，我遇到了这个问题，我想我会尝试在一个单独的程序中修复它。但这仍然让我感到困惑。 我已将问题分解为我无法解释的一件事 这是代码。

package Basics;

import java.util.Scanner;
import java.util.Arrays;

public class Polygon {

    Scanner input = new Scanner(System.in);
    int num;

    Polygon(int n) {
        this.num = n;
    }

    int[] getSides() {
        int[] sides = new int[this.num];
        for (int i = 0; i < this.num; i++) {
            System.out.println("Enter length of side " + (i + 1));
            sides[i] = input.nextInt();
        }
        //System.out.println(Arrays.toString(sides));
        return sides;
    }

    int[] sides = this.getSides();

}

class runner {

    public static void main(String[] args) {
        Polygon rectangle = new Polygon(4);
        int[] sides = rectangle.getSides();
        System.out.println(Arrays.toString(sides));
        System.out.println(Arrays.toString(rectangle.sides));
        
    }

}

这是输出

run:
Enter length of side 1
5
Enter length of side 2
4
Enter length of side 3
3
Enter length of side 4
2
[5, 4, 3, 2]
[]
[]
BUILD SUCCESSFUL (total time: 3 seconds)

正如您所见，当从类外调用时，getSides() 方法返回正确的值，即用户输入的任何值。但从里面它没有？ 我很确定这在某种程度上是一个愚蠢或菜鸟的问题，而且我缺少一些巨大的东西，但我真的需要帮助 这是什么原因造成的，我该如何解决？

Answer 1

在函数getSides() 中，您定义了一个新数组sides，您没有将其称为您定义为int[] sides = this.getSides(); 的类实例变量

int[] getSides() {
    sides = new int[this.num];
    for (int i = 0; i < this.num; i++) {
        System.out.println("Enter length of side " + (i + 1));
        sides[i] = input.nextInt();
    }
    // System.out.println(Arrays.toString(sides));
    return sides;
}

完整代码：

import java.util.Arrays;
import java.util.Scanner;

public class Polygon {
    Scanner input = new Scanner(System.in);
    int num;

    Polygon(int n) {
        this.num = n;
    }

    int[] getSides() {
        this.sides = new int[this.num];
        for (int i = 0; i < this.num; i++) {
            System.out.println("Enter length of side " + (i + 1));
            sides[i] = input.nextInt();
        }
        // System.out.println(Arrays.toString(sides));
        return sides;
    }

    int[] sides = this.getSides();
}

跑步者类：

public class Runner {
    public static void main(String[] args) {
        Polygon rectangle = new Polygon(4);
        int[] sides = rectangle.getSides();
        System.out.println(Arrays.toString(sides));
        System.out.println(Arrays.toString(rectangle.sides));
    }
}

要获得更好的代码，您可以编写Polygon 类，如下所示：

public class Polygon {
    Scanner input = new Scanner(System.in);
    int num;
    int[] sides;

    Polygon(int n) {
        this.num = n;
        this.sides = new int[this.num];
    }

    int[] getSides() {
        for (int i = 0; i < this.num; i++) {
            System.out.println("Enter length of side " + (i + 1));
            sides[i] = input.nextInt();
        }
        System.out.println(Arrays.toString(sides));
        return sides;
    }
}

Answer 2

public class Polygon {
    Scanner input = new Scanner(System.in);
    int num;    // Java default value for integers is zero
 
    Polygon(int n) {
        this.num = n; // The value is changed when an object is created;
    }

    int[] getSides() {
        int[] sides = new int[this.num]; 
        for (int i = 0; i < this.num; i++) {
            System.out.println("Enter length of side " + (i + 1));
            sides[i] = input.nextInt();
        }
        return sides;
    }

    int[] sides = this.getSides(); // This method call to getSides will be executed before the Constructor, using all the instance variables(so, the variable num is 0)
}

你的问题，是你分配变量边调用方法getSides()，因为这个分配将在构造函数之前运行，变量num是0（默认value)，所以没有迭代。

示例在构造函数之前执行的方法

public class Polygon {

    Scanner input = new Scanner(System.in);
    int num;
    int[] sides = this.getSides();  // Assign a variable with  the return of a method, this is executed before the constructor

    Polygon(int n) {
        System.out.println("I am the constructor");
        this.num = n;
    }

    int[] getSides() {
        System.out.println("I will be executed before the constructor");
        int[] sides = new int[this.num];
        for (int i = 0; i < this.num; i++) {
            System.out.println("Enter length of side " + (i + 1));
            sides[i] = input.nextInt();
        }
        //System.out.println(Arrays.toString(sides));
        return sides;
    }



    public static void main(String[] args) {
        new Polygon(5);
    }