用于计算类的实例数量的字段 [英] A field to count the amount instances of a class

问题描述

我需要创建一个字段来计算类的实例数。

I need to create a field to count the number of instances made by a class

public class Circle
{
private int diameter;
private int xPosition;
private int yPosition;
private String color;
private boolean isVisible;
private static int count = 0;

/**
 * Create a new circle at default position with default color.
 */
public Circle()
{
    diameter = 30;
    xPosition = 20;
    yPosition = 60;
    color = "blue";
    isVisible = false;
    count = count++;


}

public void returnCount(){
    System.out.println(count);
}

这是我一直在玩的。我希望计数将增加1每次创建一个变量。

This is what Ive been playing with. I was hoping the count would increment by 1 each time a variable is created. However it just stays at 0.

推荐答案

感谢任何帮助，Ciaran。 >

这是因为 ++ 运算符的无效使用。

This is because of the invalid use of ++ operator.

// count = count++; incorrect
count++; // correct
// count = count + 1; // correct

当使用 count ++ count 变量增加1;但从操作符返回的值是 count 变量的先前值。

When you use count++, the count variable is incremented by 1; but the value returned from the operator is the previous value of count variable.

您可以通过尝试

public class Test {
    public static void main(String[] args) {
        int count = 0;
        System.out.println(count++); // line 1
        System.out.println(count++);
    }
}

当您运行以上是结果。 p>

When you run above below is the results.

0
1

这是行1只打印 0 ，因为 count ++ 的返回值总是上一个值。

That is "line 1" prints only 0 since the return value of count++ is always the previous value.

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