我如何創建具有與類相同的參數的泛型構造函數

Question

我有一個類Expression：

public class Expression < E extends Number, V extends Number >
{
   public Expression(E lV, OPERATION operation, V rV) {
   }

   public Expression(Expression< E, V > lE, OPERATION operation, Expression< E, V > rE) {
   }
}

Expression.java編譯沒有錯誤。

這是我的主要課程代碼。

public static void main(String[] args)
{
        // Line 1. 
    refactored.Expression< ?, ? > ex1 = new refactored.Expression< Double, Float >(10d, OPERATION.PLUS, 10f);

        // Line 2.
    refactored.Expression< ?, ? > ex2 = new refactored.Expression< Double, Float >(-3d, OPERATION.MUL, 1f);

        // Line 3.
    refactored.Expression< ?, ? > ex3 = new refactored.Expression< refactored.Expression< Double, Float >, refactored.Expression< Double, Float > >(ex1, OPERATION.MINUS, ex2);
}

3號線無法編譯，它說：

Exception in thread "main" java.lang.Error: Unresolved compilation problems: 
The constructor Expression<Expression<Double,Float>,Expression<Double,Float>>(Expression<capture#1-of ?,capture#2-of ?>, OPERATION, Expression<capture#3-of ?,capture#4-of ?>) is undefined
Bound mismatch: The type Expression<Double,Float> is not a valid substitute for the bounded parameter <E extends Number> of the type Expression<E,V>
Bound mismatch: The type Expression<Double,Float> is not a valid substitute for the bounded parameter <V extends Number> of the type Expression<E,V>

它出什么問題了？

Answer 1

表達式不擴展數字，因此不是合法的類型參數。

編輯-解決評論
我不確定為什么您需要通用的解決方案...
你可以有類似的東西

interface Expression {
  Number compute();
}

class AtomicExpression implements Expression {
  private final Number number;
  AtomicExpression(Number number) {
    this.number = number;
  }
  public Number compute() {
    return number;
  }
}

class BinaryExpression implements Expression {
  private final Expression expr1;
  private final Expression expr2;
  private final Operator op;
  AtomicExpression(Expression expr1, Expression expr1. Operator op) {
    this.expr1 = expr1;
    this.expr2 = expr2;
    this.op = op;
  }
  public Number compute() {
    return op(expr1.compute(), expr2.compute());
  }
}

Answer 2

重構.Expression <？，？ > ex3 =新重構的.Expression <Double，Float>（ex1，OPERATION.MINUS，ex2）;

這應該工作。

Answer 3

從錯誤中可以明顯看出， Expression<?, ?>不是Number的子類。 如果您確實想在構造函數中接受Expression，則需要從泛型中刪除上限的綁定參數 （數字）。

這樣的事情應該為您工作：

public class Expression <E, V> {
    public Expression(E lV, OPERATION operation, V rV) {}
    public Expression(Expression< E, V > lE, OPERATION operation, Expression< E, V > rE) {}
}

更新：這應該工作然后：

Expression< Double, Float > ex1 = new Expression< Double, Float >(10d, OPERATION.PLUS, 10f);

Expression< Double, Float > ex2 = new Expression< Double, Float >(-3d, OPERATION.MUL, 1f);

Expression< ?, ? > ex3 = new Expression
 < Expression< Double, Float >, Expression< Double, Float > >(ex1, OPERATION.MINUS, ex2);