Java语言。工厂创建应该返回泛型类

Question

有人能帮忙吗。

我想创建一个工厂，它将返回特定的类。

有问题的行- filterFactory.getFilter(myColumn.getType()).setMin(5);有一个错误:无法解决'setMin‘中的'Object’方法

public enum Columns {
  Name(ColumnType.String),
  Age(ColumnType.Numeric);

  private final ColumnType type;

  Columns(ColumnType type) {
    this.type = type;
  }

  public ColumnType getType() {
    return type;
  }
}

public enum ColumnType {
  String,
  Numeric
}


public class NumericFilter extends ColumnFilter {
  public void setMin(int min) {
    System.out.println("min is set" + min);
  }
}

public class StringFilter extends ColumnFilter {
  public void setFilter(String filter) {
    System.out.println("filter is set to:" + filter);
  }
}

public class ColumnFilterFactory {
  public <T> T getFilter(ColumnType type) {
    if (type == null) {
      return null;
    }
    if (type == ColumnType.String) {
      return (T) new StringFilter();

    } else if (type == ColumnType.Numeric) {
      return (T) new NumericFilter();
    }

    return null;
  }
}

public class BasicTest {
  public static void main(String[] args) {
    Columns myColumn = Columns.Age;
    ColumnFilterFactory filterFactory = new ColumnFilterFactory();
    filterFactory.getFilter(myColumn.getType()).setMin(5);
  }
}

Answer 1

编译器不可能知道工厂将返回哪种类型，所以您需要给它一点帮助。例如，通过使用中间变量：

NumericFilter nf = filterFactory.getFilter(myColumn.getType());
nf.setMin(5);