Java 作业。 “String 类型的方法 surfaceArea() 未定义”

Question

对于家庭作业，我们只需复制教师代码并阅读笔记即可。我这样做了，但留下了错误：

Exception in thread "main" java.lang.Error: Unresolved compilation problems: 
Syntax error on token "box2", delete this token
The method surfaceArea() is undefined for the type String
Syntax error on token "box3", delete this token
The method surfaceArea() is undefined for the type String

我看不出纸上的代码和我的代码有什么区别。

 package csc120lab3;

 import javax.swing.JOptionPane;

 public class Lab3 {

public static void main(String[] args) {
    final String TITLE_STRING = "Lab 3 by _________"; //edit this and put in your name.

    Box box1; // 3 variables local to main method
    Box box2;
    Box box3;

    String output = ""; // we will output this later with JOptionPane

    box1 = new Box(5, 10, 15); //this is a call to the constructor
    //now call upon some instance methods. note use of of box1 before the method
    //names. In essence, box1 is the actor performing the method.
    //Note: below the "\n" cases text that follows to start on a new line.
    output += "Box 1 says : " + box1.myDescription() + "\n";
    output += "Surface area of box 1 is: " + box1.surfaceArea() + "\n";

    box2 = new Box(4, 11, 13); //this is a call to the constructor
    output += "Box 2 says : " + box2.myDescription() + "\n";
    output += "Surface area of box 2 is: " box2.surfaceArea() + "\n";

    box3 = new Box(4, 14, 19); //this is a call to the constructor
    output += "Box 2 says : " + box3.myDescription() + "\n";
    output += "Surface area of box 3 is: " box3.surfaceArea() + "\n";

    //now we call upon an instance method that takes another box as an argument
    //in the first line, box1 is the actor and box2 is being passed to the method
    //as an argument

    output += "Box2 fits inside of box 1 : " + box1.willHold(box2) + "\n";
    output += "Box1 fits inside of box 2 : " + box2.willHold(box1) + "\n";
    output += "Box3 fits inside of box 1 : " + box1.willHold(box3) + "\n\n";

    //next line call upon the class method. Note use of box before method name
    output += "We now have a total of " + Box.howManyBoxes() + "boxes created.";

    //showMesageDiaglog() is a class method of the JOptionPane class. we call it here
    //INFORMATION_MESSAGE is a class constant that we pass as an argument to method call
    //showMessageDialog() is a void method, hence it does not return anyhing to us
    JOptionPane.showMessageDialog(null, output, TITLE_STRING, JOptionPane.INFORMATION_MESSAGE);

}

}

class Box {
    //This class is actually a hybrid of the two flavors of classes that we discussed in 
    //class. it uses all 5 of the big things that we use into class definitions, namely: class
    //variables, class methods, instance variables, instance methods, and has a constructor method

    //class variable (not use of static)
    public static int boxCounter = 0;

    //class method. header uses static. class itself performs this method
    //this method has no need for any perameters to be passed into it, hence ()
    //it will return an int vaue to the caller, hence "int" as part of the header

    public static int howManyBoxes(){
        //return the value of the class variable.
        return boxCounter;
    }   

    // 3 instance variables [note no use of static]
    //each object of type Box will get its own copy of each of these
    //they are marked as private so that ONLY the methods of this class can access them
    private float width;
    private float length;
    private float height;

    //class costructor (note: no return type and method name is same as class name)
    public Box (float newWidth, float newLength, float newHeight){
        width = newWidth;
        height = newHeight;
        length = newLength;

        //and eachtime we do create a new Box we will also up the boxCounter by one
        boxCounter = boxCounter + 1;
    }

    //an instance method note the lack of static
    public float volume(){
        float answer; //this is a local variable
        answer = width * height * width;
        return answer;
        //note: could replace above three lines with just: return (width * height * length)
    } //end volume

    //another instance method
    public float surfaceArea(){
        //our box has sides and a bottom, but no top
        float bottomArea = length * width;
        float sidesArea = 2 * (length * height) + (width * height);
        return (bottomArea + sidesArea);
    } //end surfaceArea

    //another instance method (the object returns a string that describes its size)
    //note that += causes the right hand string to be appended to the end of the string 
    //on the left side
    public String myDescription(){
        String answer;
        answer = "I am a box with a width of " + width;
        answer += ", a length of " + length;
        answer += " and a height of " + height + ".  ";
        answer += "I also happen to have a volume of " + volume() + ".";
        return answer;
    }//end my description

    //another instance method. this method tells us if the box given in the parameter
    //list below. (i.e. otherBox) will fit fully inside of this box. return true if it does,
    //otherwise return false. note that the other box can be turned on its side or whatever 
    //to make it fit. 
    public boolean willHold(Box otherBox){


        float myMax = Math.max(Math.max(width, length), height);
        float otherBoxMax = Math.max(Math.max(otherBox.width, otherBox.length), otherBox.height);

        float myMin = Math.min(Math.min(width, length), height);
        float otherBoxMin = Math.min(Math.min(otherBox.width, otherBox.length), otherBox.height);

        float myMid = width + length + height - myMin - myMax;
        float otherBoxMid = otherBox.width + otherBox.length + otherBox.height - otherBoxMin -
                            otherBoxMax;

        //below && means and
        if (otherBoxMax < myMax && otherBoxMin < myMin && otherBoxMid < myMid){
            return true; //other box fits in me!
        }
        else{
            return false;
        }
    }//end willHold

}//结束类Box

Answer 1

您忘记了 2 个加号：

output += "Surface area of box 2 is: " box2.surfaceArea() + "\n";

应该是

output += "Surface area of box 2 is: " + box2.surfaceArea() + "\n";

Answer 2

您在这一行的其他几行中缺少连接运算符“+”

output += "Surface area of box 2 is: " box2.surfaceArea() + "\n";