代码的布尔部分在 java 中不起作用

Question

嗨，我必须编写这个程序：

public class Book {

    String title;
    boolean borrowed;

    // Creates a new Book
    public Book(String bookTitle) {
        // Implement this method
    }

    // Marks the book as rented
    public void borrowed() {
        // Implement this method
    }

    // Marks the book as not rented
    public void returned() {
        // Implement this method
    }

    // Returns true if the book is rented, false otherwise
    public boolean isBorrowed() {
        // Implement this method
    }

    // Returns the title of the book
    public String getTitle() {
        // Implement this method
    }

    public static void main(String[] arguments) {
        // Small test of the Book class
        Book example = new Book("The Da Vinci Code");
        System.out.println("Title (should be The Da Vinci Code): " + example.getTitle());
        System.out.println("Borrowed? (should be false): " + example.isBorrowed());
        example.rented();
        System.out.println("Borrowed? (should be true): " + example.isBorrowed());
        example.returned();
        System.out.println("Borrowed? (should be false): " + example.isBorrowed());
    }
}

输出应该是这样的：

Title (should be The Da Vinci Code): The Da Vinci Code
Rented? (should be false): false
Rented? (should be true): true
Rented? (should be false): false

我喜欢这个:

public class Book {
String title;
boolean borrowed;
// Creates a new Book
public Book(String bookTitle) {
// Implement this method
    String book=new String();
}
// Marks the book as rented
public void borrowed() {
// Implement this method
    borrowed=true;
}
// Marks the book as not rented
public void returned() {
// Implement this method
    borrowed=false;
}
// Returns true if the book is rented, false otherwise
public boolean isBorrowed() {
// Implement this method

    if (book=borrowed()){
        return true;
    }else if(book==returned()){
        return false;

    }
}

// Returns the title of the book
public String getTitle() {
// Implement this method
    return "The Da Vinci Code";

} 
public static void main(String[] arguments) {
// Small test of the Book class
Book example = new Book("The Da Vinci Code");
System.out.println("Title (should be The Da Vinci Code): " + example.getTitle());
System.out.println("Borrowed? (should be false): " + example.isBorrowed());
System.out.println("Borrowed? (should be true): " + example.isBorrowed());
System.out.println("Borrowed? (should be false): " + example.isBorrowed());
}
}

我认为我的代码总体上不错，但 eclipse 说：book cannot be resolved to a variable.. in the following part :

public boolean isBorrowed() {
// Implement this method

    if (book=borrowed()){
        return true;
    }else if(book==returned()){
        return false;

    }
}

如何更正代码前面部分的问题？我对其余代码是否正确？谢谢

Answer 1

让我们放大这段代码：

if (book=borrowed()) {
        return true;
} else if(book==returned()) {
        return false;
}

首先要注意的是，在使用 == 检查相等性时，= 运算符用于赋值。例如，borrowed == true 检查 borrowed 是否为真，borrowed = true 将 borrowed 赋值给真。

第二点要注意的是 book 甚至不是 isBorrowed 中可用的变量，因为您没有像 borrowed 和 [=22= 那样将其保存为实例变量].

第三点要注意的是你的检查 book == borrowed() 和 book == returned() 没有意义。 borrowed() 和 returned() 是 void 方法，所以它们不会 return 任何东西，而 book 是一个字符串。您真正想要检查的是变量 borrowed 是真还是假：

if (borrowed == true) {
    return true;
} else if (borrowed == false) {
    return false;
}

可以简化为：

return borrowed;

您还需要更改 main 方法以在打印语句之间调用适当的方法：

System.out.println("Title (should be The Da Vinci Code): " + example.getTitle());
System.out.println("Borrowed? (should be false): " + example.isBorrowed());
example.borrowed();
System.out.println("Borrowed? (should be true): " + example.isBorrowed());
example.returned();
System.out.println("Borrowed? (should be false): " + example.isBorrowed());

Answer 2

您没有在 isBorrowed() 中定义 book 变量，所以它是未定义的，您应该收到一个编译器错误。

即使您有一个 book 变量，isBorrowed() 方法也太复杂了。就 return borrowed.

您还需要在调用第二个 isBorrowed() 之前和之后分别调用 borrowed() 和 returned() 以进行适当的测试。

此外，在构造函数中设置 title 变量。

this.title = bookTitle;

和 return title 在 getTitle() 中，而不是对其进行硬编码。

代码的布尔部分在 java 中不起作用

boolean part of the code does not work in java

java

string

boolean