Question

我尝试在Java中重写以下scala代码（取自https://github.com/DataSystemsLab/GeoSpark/blob/master/sql/src/main/scala/org/apache/spark/sql/geosparksql/expressions/Functions.scala），但不知道如何为此行执行此操作：

val geometry = GeometrySerializer.deserialize(inputExpressions(0).eval(input).asInstanceOf[ArrayData]).

有人能帮帮我吗？包括我到目前为止所做的Java代码。

     /**
      * Return the convex hull of a Geometry.
      *
      * @param inputExpressions
      */
    case class ST_ConvexHull(inputExpressions: Seq[Expression])
      extends Expression with CodegenFallback {
      override def nullable: Boolean = false

      override def eval(input: InternalRow): Any = {
        assert(inputExpressions.length == 1)
        val geometry = GeometrySerializer.deserialize(inputExpressions(0).eval(input).asInstanceOf[ArrayData])
        new GenericArrayData(GeometrySerializer.serialize(geometry.convexHull()))
      }

      override def dataType: DataType = new GeometryUDT()

      override def children: Seq[Expression] = inputExpressions
    }

这是我到目前为止所做的Java代码：

import com.vividsolutions.jts.geom.Geometry;
import org.apache.spark.sql.catalyst.InternalRow;
import org.apache.spark.sql.catalyst.expressions.Expression;
import org.apache.spark.sql.catalyst.expressions.codegen.CodegenContext;
import org.apache.spark.sql.catalyst.expressions.codegen.CodegenFallback;
import org.apache.spark.sql.catalyst.expressions.codegen.ExprCode;
import org.datasyslab.geosparksql.utils.GeometrySerializer;
import org.apache.spark.sql.geosparksql.UDT.GeometryUDT;
import org.apache.spark.sql.catalyst.util.ArrayData;
import org.apache.spark.sql.catalyst.util.GenericArrayData;
import org.apache.spark.sql.types.DataType;
import scala.collection.Seq;

/**
 *
 * @author Paul Z. Wu Apr 16, 2018
 */
public class ST_ConcaveHull extends Expression implements CodegenFallback {

    Seq<Expression> inputExpressions;

    public ST_ConcaveHull(Seq<Expression> inputExpressions) {
        this.inputExpressions = inputExpressions;
    }

    @Override
    public boolean nullable() {
        return false;
        //throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
    }

    @Override
    public Object eval(InternalRow input) {
        assert (inputExpressions.length() == 1);
        //****** Have trouble here!!!!
        ArrayData ad = inputExpressions.take(0).eval(input.);
        Geometry g = GeometrySerializer.deserialize(ad);
        return new GenericArrayData(GeometrySerializer.serialize(g.convexHull()));
    }

    @Override
    public ExprCode doGenCode(CodegenContext cc, ExprCode ec) {
        throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
    }

    @Override
    public DataType dataType() {
        return new GeometryUDT();
        //throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
    }

    @Override
    public Seq<Expression> children() {
        return this.inputExpressions;
    }

    @Override
    public Object productElement(int i) {
        throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
    }

    @Override
    public int productArity() {
        throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
    }

        @Override
        public boolean canEqual(Object o) {
            throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
        }

    }

Answer 1

该行可以“翻译”为：

// val geometry = GeometrySerializer.deserialize(
//  inputExpressions(0)         // get value with index 0 in Seq
//    .eval(input)              // call "eval" method on that value
//    .asInstanceOf[ArrayData]  // cast to ArrayData
//)
ArrayData ad = (ArrayData) inputExpressions.apply(0).eval(input);
Geometry g = GeometrySerializer.deserialize(ad);

这里发生了什么：

inputExpressions是scala.collection.Seq。要从i的索引xs: Seq获取值，在Scala中我们只需编写xs(i)，但这只是xs.apply(i)的“语法糖”（即速记） - “apply”方法在Scala中是特殊的，因为方法名称可以简单地省略。因此，在迁移到Java时，我们会明确地调用apply。
Scala的x.asInstanceOf[T]相当于Java的投射 (T) x，在本例中 - 为了分配eval的结果Object进入ArrayData我们必须使用强制转换

如何将此scala代码翻译成java？

1 个答案: