实体类和封装的困惑 – 如何在main()中调用非静态方法?

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英文:

Entity class and encapsulation confusion - how do you call a non-static method from main()?

问题

我正在创建一个名为"Crate"的类,该类将读取一个包含以英寸表示的物体尺寸的整数的输入文件,并输出一个报告,其中包含每个物体所需的包装箱尺寸。一切都看起来很好,直到我尝试测试我的方法时,因为我无法从main()中调用它们。尝试时,我收到了一个错误消息,上面写着:"无法从静态上下文引用非静态方法createReport()。"

我是一个初学者,所以如果有什么基本错误,我将乐意接受批评。我只是想知道是否有一个简单的修复方法,以及关于静态/非静态方法的知识差距是什么。

package crate;

import java.io.File;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Scanner;

public class Crate {
    private Crate[] crateList;
    private int crateAmount;
    private int height, width, depth, panels;
    private int smallestCratePosition, smallestCrateSize;
    private int largestCratePosition, largestCrateSize;
    private double averageCrateSize;

    public static void main(String[] args) throws IOException {
        Crate crate = new Crate(); // Create an instance of the Crate class
        crate.createReport(); // Call the createReport method on the instance
    }

    public Crate() {
        height = 1;
        width = 1;
        depth = 1;
        panels = 6;
    }

    public void createReport() throws IOException {
        Scanner infile = new Scanner(new File("ShippingSizes.txt"));
        PrintWriter outfile = new PrintWriter(new File("CrateReport.txt"));

        createCrateList(infile);

        outfile.println("There are " + crateAmount + " crates.");
        for (int i = 0; i < crateList.length; i++) {
            outfile.println(i + ": height=" + crateList[i].getHeight() + ", width=" +
                    crateList[i].getWidth() + ", depth=" + crateList[i].getDepth() +
                    ", panels=" + crateList[i].getPanels());
        }
        outfile.println("\n" + "Smallest crate is at " + smallestCratePosition +
                ": height=" + crateList[smallestCratePosition].getHeight() + ", width=" +
                crateList[smallestCratePosition].getWidth() + ", depth=" +
                crateList[smallestCratePosition].getDepth() + ", panels=" +
                crateList[smallestCratePosition].getPanels());
        outfile.println("Largest crate is at " + largestCratePosition + ": height," +
                crateList[largestCratePosition].getHeight() + ", width=" +
                crateList[largestCratePosition].getWidth() + ", depth=" +
                crateList[largestCratePosition].getDepth() + ", panels=" +
                crateList[largestCratePosition].getPanels());
        outfile.println("Average crate size is: " + averageCrateSize);

        outfile.close();
        infile.close();
    }

    public void setCrateDimensions(Scanner scnr) {
        int tempValue = scnr.nextInt();
        height = tempValue / 12;
        if (tempValue % 12 != 0)
            height++;

        tempValue = scnr.nextInt();
        width = tempValue / 12;
        if (tempValue % 12 != 0)
            width++;

        tempValue = scnr.nextInt();
        depth = tempValue / 12;
        if (tempValue % 12 != 0)
            depth++;

        panels = 2 * ((height * width) + (height * depth) + (width * depth));
    }

    public void createCrateList(Scanner scnr) {
        crateAmount = scnr.nextInt();
        crateList = new Crate[crateAmount]; // Initialize the array
        for (int i = 0; i < crateAmount; i++) {
            crateList[i] = new Crate(); // Create instances of Crate for each element
            crateList[i].setCrateDimensions(scnr);
        }
    }

    public void getSmallestCrate() {
        smallestCrateSize = crateList[0].getPanels();

        for (int i = 0; i < crateList.length; i++) {
            if (crateList[i].getPanels() < smallestCrateSize) {
                smallestCratePosition = i;
                smallestCrateSize = crateList[i].getPanels();
            }
        }
    }

    public void getLargestCrate() {
        largestCrateSize = crateList[0].getPanels();

        for (int i = 0; i < crateList.length; i++) {
            if (crateList[i].getPanels() > largestCrateSize) {
                largestCratePosition = i;
                largestCrateSize = crateList[i].getPanels();
            }
        }
    }

    public void getAverageCrateSize() {
        double totalPanels = 0;
        for (int i = 0; i < crateList.length; i++)
            totalPanels += crateList[i].getPanels();
        averageCrateSize = totalPanels / (double) (crateList.length);
    }

    public int getHeight() {
        return height;
    }

    public int getWidth() {
        return width;
    }

    public int getDepth() {
        return depth;
    }

    public int getPanels() {
        return panels;
    }
}

我已经在main()方法中创建了Crate类的一个实例,并且通过该实例调用了createReport()方法,这应该解决了你遇到的问题。希望这能帮助你。

英文:

I am creating a class called "Crate" that will read an input file of integers representing objects dimensions in inches and output a report containing the crate sizes required for each respective item. Everything seemed fine until I tried to test my methods, as I am unable to call them from main(). When trying, I get an error message saying, "non-static method createReport() cannot be referenced from a static context."

I am a beginner, so if anything is fundamentally wrong I will happily accept criticism. I am just curious if there is a simple fix to this and what my gap in knowledge is regarding static/non-static methods.

package crate;
import java.io.File;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Scanner;
public class Crate {
private Crate[] crateList;
private int crateAmount;
private int height, width, depth, panels;
private int smallestCratePosition, smallestCrateSize;
private int largestCratePosition, largestCrateSize;
private double averageCrateSize;
public static void main(String[] args) throws IOException {
createReport();
}
public Crate() {
height = 1;
width = 1;
depth = 1;
panels = 6;
}
public void createReport() throws IOException {
Scanner infile = new Scanner(new File(&quot;ShippingSizes.txt&quot;));
PrintWriter outfile = new PrintWriter(new File(&quot;CrateReport.txt&quot;));
createCrateList(infile);
outfile.println(&quot;There are &quot; + crateAmount + &quot; crates.&quot;);
for (int i = 0; i &lt; crateList.length; i++) {
outfile.println(i + &quot;: height=&quot; + crateList[i].getHeight() + &quot;, width=&quot; + 
crateList[i].getWidth() + &quot;, depth=&quot; + crateList[i].getDepth() + 
&quot;, panels=&quot; + crateList[i].getPanels());
}
outfile.println(&quot;\n&quot; + &quot;Smallest crate is at &quot; + smallestCratePosition + 
&quot;: height=&quot; + crateList[smallestCratePosition].getHeight() + &quot;, width=&quot; + 
crateList[smallestCratePosition].getWidth() + &quot;, depth=&quot; + 
crateList[smallestCratePosition].getDepth() + &quot;, panels=&quot; + 
crateList[smallestCratePosition].getPanels());
outfile.println(&quot;Largest crate is at &quot; + largestCratePosition + &quot;: height,&quot; + 
crateList[largestCratePosition].getHeight() + &quot;, width=&quot; + 
crateList[largestCratePosition].getWidth() + &quot;, depth=&quot; + 
crateList[largestCratePosition].getDepth() + &quot;, panels=&quot; + 
crateList[largestCratePosition].getPanels());
outfile.println(&quot;Average crate size is: &quot; + averageCrateSize);
outfile.close();
infile.close();
}
public void setCrateDimensions(Scanner scnr) {
int tempValue = scnr.nextInt();
height = tempValue/12;
if (tempValue%12 != 0)
height++;
tempValue = scnr.nextInt();
width = tempValue/12;
if (tempValue%12 != 0)
width++;
tempValue = scnr.nextInt();
depth = tempValue/12;
if (tempValue%12 != 0)
depth++;
panels = 2*((height*width)+(height*depth)+(width*depth));
}
public void createCrateList(Scanner scnr) {
crateAmount = scnr.nextInt();
for (int i = 0; i &lt; crateAmount; i++) {
crateList[i].setCrateDimensions(scnr);
}
}
public void getSmallestCrate() {
smallestCrateSize = crateList[0].getPanels();
for (int i = 0; i &lt; crateList.length; i++) {
if (crateList[i].getPanels() &lt; smallestCrateSize) {
smallestCratePosition = i;
smallestCrateSize = crateList[i].getPanels();
}          
}
}
public void getLargestCrate() {
largestCrateSize = crateList[0].getPanels();
for (int i = 0; i &lt; crateList.length; i++) {
if (crateList[i].getPanels() &gt; largestCrateSize) {
largestCratePosition = i;
largestCrateSize = crateList[i].getPanels();
}
}
}
public void getAverageCrateSize() {
double totalPanels = 0;
for (int i = 0; i &lt; crateList.length; i++)
totalPanels += crateList[i].getPanels();
averageCrateSize = totalPanels / (double)(crateList.length);
}
public int getHeight() {
return height;
}
public int getWidth() {
return width;
}
public int getDepth() {
return depth;
}
public int getPanels() {
return panels;
}
}

I tried declaring the Scanner, PrintWriter, Crate[] object, and crateAmount integer inside of main(), however this only led to the same issue later on as I was unable to use those variables in my methods. Any help would be greatly appreciated!

答案1

得分: 1

因为createReport方法与它所属的Class Crate的实例相关联。
你必须使用new关键字创建一个实例,然后才能调用它的方法createReport

另一种方式是,如果你不想创建类的实例,可以使用关键字static将方法声明为静态方法。这样,在启动JVM时会加载该方法。

建议:你应该通过阅读一本书或另一篇文章来深入理解面向对象编程(OOP)。

英文:

because the createReport method is related to it's instance of Class Crate.
you must create an instance by new keywords, then you can call it's method createReport.

another way, if you don't want create an instance of class, you can declared method to static method by keyword static. then the method will be load when starting the jvm.

suggestion: you should deeply understand the OOP programming by reading an book or another article

答案2

得分: 0

非静态方法属于类的实例,而不属于类本身。这就是为什么您无法从main()函数中访问createReport()方法,因为main()函数位于静态作用域,而createReport()方法不是。尝试实例化类的一个实例,并在该对象上调用createReport()方法。

例如:

Crate crate = new Crate();
crate.createReport();
英文:

Non-static methods belong to an instance of a class, not the class itself. This is why you can't access the createReport() method from main(), because main is in the static scope, while createReport is not. Try instantiating an instance of the class and calling createReport() on the object.

For example:

Crate crate = new Crate();
crate.createReport();

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  • 本文由 发表于 2020年9月18日 10:03:08
  • 转载请务必保留本文链接:https://go.coder-hub.com/63948334.html
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