Question

这是我的大学课程。

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（也由教授提供），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（我写了这篇文章）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;
    }
}

还提供了测试人员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。 Plus和Minus只存储一个负值或正值。

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，我得到了这个错误（和Subtract，Multiply和Divide类似的错误）：

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

发生了什么？

Answer 1

根本原因在这里

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的超级构造函数。您的父类没有这样的无参数构造函数。因此，请调用其现有构造函数

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

此外，您还要重新声明left课程中的right和Add字段。这样做没有意义。您的子类已有权访问其父类left的{{1}}和right字段。

得到一个＆＃34;找不到符号＆＃34;抽象类的错误

1 个答案: