Package org.apache.flink.table.api.dataview

Class ListView<T>

java.lang.Object
org.apache.flink.table.api.dataview.ListView<T>
Type Parameters:
T - element type
All Implemented Interfaces:
DataView
@PublicEvolving public class ListView<T> extends Object implements DataView
A DataView that provides List-like functionality in the accumulator of an AggregateFunction or TableAggregateFunction when large amounts of data are expected.

A ListView can be backed by a Java ArrayList or can leverage Flink's state backends depending on the context in which the aggregate function is used. In many unbounded data scenarios, the ListView delegates all calls to a ListState instead of the ArrayList.

Note: Elements of a ListView must not be null. For heap-based state backends, hashCode/equals of the original (i.e. external) class are used. However, the serialization format will use internal data structures.

The DataType of the view's elements is reflectively extracted from the accumulator definition. This includes the generic argument T of this class. If reflective extraction is not successful, it is possible to use a DataTypeHint on top the accumulator field. It will be mapped to the underlying collection.

The following examples show how to specify an AggregateFunction with a ListView: 


  public class MyAccumulator {

    public ListView<String> list = new ListView<>();

    // or explicit:
    // {@literal @}DataTypeHint("ARRAY<STRING>")
    // public ListView<String> list = new ListView<>();

    public long count = 0L;
  }

  public class MyAggregateFunction extends AggregateFunction<String, MyAccumulator> {

   {@literal @}Override
   public MyAccumulator createAccumulator() {
     return new MyAccumulator();
   }

   public void accumulate(MyAccumulator accumulator, String id) {
     accumulator.list.add(id);
     accumulator.count++;
   }

   {@literal @}Override
   public String getValue(MyAccumulator accumulator) {
     // return the count and the joined elements
     return count + ": " + String.join("|", acc.list.get());
   }
 }

ListView(TypeInformation<?> elementType) method was deprecated and then removed. Please use a DataTypeHint instead.

  • Constructor Details

    • ListView

      public ListView()
      Creates a list view.

      The DataType of the contained elements is reflectively extracted.

  • Method Details

    • getList

      public List<T> getList()
      Returns the entire view's content as an instance of List.

    • setList

      public void setList(List<T> list)
      Replaces the entire view's content with the content of the given List.

    • get

      public Iterable<T> get() throws Exception
      Returns an iterable of the list view.
      Returns:
      The iterable of the list.
      Throws:
      Exception - Thrown if the system cannot get data.

    • add

      public void add(T value) throws Exception
      Adds the given value to the list.
      Parameters:
      value - The element to be appended to this list view.
      Throws:
      Exception - Thrown if the system cannot add data.

    • addAll

      public void addAll(List<T> list) throws Exception
      Adds all of the elements of the specified list to this list view.
      Parameters:
      list - The list with the elements that will be stored in this list view.
      Throws:
      Exception - Thrown if the system cannot add all data.

    • remove

      public boolean remove(T value) throws Exception
      Removes the given value from the list.
      Parameters:
      value - The element to be removed from this list view.
      Throws:
      Exception

    • clear

      public void clear()
      Removes all of the elements from this list view.
      Specified by:
      clear in interface DataView

    • equals

      public boolean equals(Object o)
      Overrides:
      equals in class Object

    • hashCode

      public int hashCode()
      Overrides:
      hashCode in class Object

    • newListViewDataType

      public static DataType newListViewDataType(DataType elementDataType)
      Utility method for creating a DataType of ListView explicitly.