java - 抽象类出现 "cannot find symbol"错误

Question

这是我的大学类(class)。

我有这个顶级抽象类，Expression.java(由教授提供):

public abstract class Expression {
    public abstract int eval();
    public abstract String toString();
}

BinaryExpression.java(也是由教授提供的)，它扩展了Expression.java:

public abstract class BinaryExpression extends Expression {

    protected Expression left;
    protected Expression right;

    public BinaryExpression (Expression l, Expression r) {
        left = l;
        right = r;
    }

    public abstract int eval();
    public abstract String toString();
}

最后，Add.java(我写的)扩展了 BinaryExpression.java:

public class Add extends BinaryExpression {

    //Fields
    protected Expression left;
    protected Expression right;

    //Constructor
    public Add(Expression l, Expression r) {
        left = l;
        right = r;
    }

    //Methods
    public int eval() {
        return left.eval()+right.eval();
    }

    public String toString() {
        return left + "+" + right;
    }
}

还提供了测试器 Test.java。还有其他三个类扩展了 BinaryExpression:Subtract、Multiply 和 Divide，但它们实际上与 Add 相同。加号和减号仅存储一个值，可以是负值，也可以是正值。

public class Test {
    public static void main (String [] args) {
        Expression s1 = new SimpleExpression(10);
        Expression s2 = new SimpleExpression(-3);
        System.out.println("s1: " +  s1 + " = " +  s1.eval());        
        System.out.println("s2: " +  s2 + " = " +  s2.eval());        
        Expression u1 = new Plus(s2);      
        Expression u2 = new Minus(s1);      
        System.out.println("u1: " +  u1 + " = " +  u1.eval());        
        System.out.println("u2: " +  u2 + " = " +  u2.eval());   
        Expression b1 = new Add(u1, s2);  
        Expression b2= new Subtract(s1, u2);
        Expression b3= new Multiply(u1, b1);
        Expression b4= new Divide(u2, b3);   
        System.out.println("b1: " +  b1 + " = " +  b1.eval());        
        System.out.println("b2: " +  b2 + " = " +  b2.eval());        
        System.out.println("b3: " +  b3 + " = " +  b3.eval());        
        System.out.println("b4: " +  b4 + " = " +  b4.eval());   
        Expression e = new Minus( new Multiply(b3, b2));     
        System.out.println("e: " +  e + " = " +  e.eval());   
        // example given in the assignment
        e = new Divide(new Add(new Minus(new SimpleExpression (10)), 
                               new SimpleExpression (50)),
                       new Minus(new Multiply(new SimpleExpression(5),
                                              new Minus (new SimpleExpression (3)))));

        System.out.println("Another e: " + e + " = " +  e.eval());
    }
}

当我编译 Test.java 时，出现此错误(减法、乘法和除法也出现类似错误):

./Add.java:8: cannot find symbol
symbol  : constructor BinaryExpression()
location: class BinaryExpression
    public Add(Expression l, Expression r) {

发生了什么事？

Answer 1

根本原因就在这里

public Add(Expression l, Expression r) {
    left = l;
    right = r;
}

您的子类正在尝试隐式调用无参数 super 构造函数。你的父类没有这样的无参数构造函数。因此，请调用其现有的构造函数

public Add(Expression l, Expression r) {
    super(l, r);
    left = l;
    right = r;
}

此外，您还需要在 Add 类中重新声明 left 和 right 字段。这样做没有意义。您的子类已经可以访问其父类 BinaryExpression 的 left 和 right 字段。