Converting lists of one element type to a list of another type

Question

I'm writing an adapter framework where I need to convert a list of objects from one class to another. I can iterate through the source list to do this as in

Java: Best way of converting List<Integer> to List<String>

However, I'm wondering if there is a way to do this on the fly when the target list is being iterated, so I don't have to iterate through the list twice.

Answer 1

Java 8 way:

List<String> original = ...;
List<Wrapper> converted = original.stream().map(Wrapper::new).collect(Collectors.toList());

assuming Wrapper class has a constructor accepting a String .

Answer 2

My answer to that question applies to your case:

import com.google.common.collect.Lists;
import com.google.common.base.Functions

List<Integer> integers = Arrays.asList(1, 2, 3, 4);

List<String> strings = Lists.transform(integers, Functions.toStringFunction());

The transformed list is a view on the original collection, so the transformation happens when the destination List is accessed.

Answer 3

As an alternative to the iterator pattern, you can use a abstract generic mapper class, and only override the transform method:

1. create a generic collection mapper for any data type
2. [optional] create a library of methods that transform between different data types (and override the method)
3. use that library

the implementation:

// Generic class to transform collections
public abstract class CollectionTransformer<E, F> {

    abstract F transform(E e);

    public List<F> transform(List<E> list) {
        List<F> newList = new ArrayList<F>();
        for (E e : list) {
            newList.add(transform(e));
        }
        return newList;
    }
}

// Method that transform Integer to String
// this override the transform method to specify the transformation
public static List<String> mapIntegerToStringCollection(List<Integer> list) {

    CollectionTransformer transformer = new CollectionTransformer<Integer, String>() {
        @Override  
        String transform(Integer e) {
            return e.toString();
        }
    };
    return transformer.transform(list);
}

// Example Usage
List<Integer> integers = Arrays.asList(1,2);
List<String> strings = mapIntegerToStringCollection(integers);

This would be useful is you have to use transformations every time, encapsulating the process. So you can make a library of collection mappers very easy.

Answer 4

You can write a mapping iterator that decorates an existing iterator and applies a function on it. In this case, the function transforms the objects from one type to another "on-the-fly".

Something like this:

import java.util.*;

abstract class Transformer<T, U> implements Iterable<U>, Iterator<U> {
    public abstract U apply(T object);  

    final Iterator<T> source;       
    Transformer(Iterable<T> source)    { this.source = source.iterator(); }

    @Override public boolean hasNext() { return source.hasNext(); }
    @Override public U next()          { return apply(source.next()); }
    @Override public void remove()     { source.remove(); } 

    public Iterator<U> iterator()      { return this; }
}

public class TransformingIterator { 
    public static void main(String args[]) {
        List<String> list = Arrays.asList("1", "2", "3");
        Iterable<Integer> it = new Transformer<String, Integer>(list) {
            @Override public Integer apply(String s) {
                return Integer.parseInt(s);
            }
        };
        for (int i : it) {
            System.out.println(i);
        }
    }
}

Answer 5

Lambdaj allows to do that in a very simple and readable way. For example, supposing you have a list of Integer and you want to convert them in the corresponding String representation you could write something like that;

List<Integer> ints = asList(1, 2, 3, 4);
Iterator<String> stringIterator = convertIterator(ints, new Converter<Integer, String> {
    public String convert(Integer i) { return Integer.toString(i); }
});

Lambdaj applies the conversion function only while you're iterating on the result. There is also a more concise way to use the same feature. The next example works supposing that you have a list of persons with a name property and you want to convert that list in an iterator of person's names.

Iterator<String> namesIterator = convertIterator(persons, on(Person.class).getName());

Pretty easy. Isn't it?

Answer 6

This Could be a solutions --> by using map

List<Employee> employee = Arrays.asList(new Emp(1, 100), new Emp(2, 200), new Emp(3, 300));
List<Employee> employeS = employee.stream()
                                             .map(emp -> new Emp(emp.getId(), emp.getSalary * 100))
                                             .collect(Collectors.toList());
employeS .stream() .forEach(s -> System.out.println("Id :" + s.getId() + " Salary :" + s.getSalary()));

Answer 7

如果您尝试获取列表中的元素列表，请使用以下代码。这里的对象列表包含属性名称，下面为您提供该列表中的名称列表

inputList.stream().map(p -> p.getName()).collect(Collectors.toList());

Answer 8

That question does not iterate through the list twice. It just iterates once and by far is the only known method.

Also you could use some transformer classes in commons-collections of google-collections but they all do the same thing under the hood :) the following being one way

CollectionUtils.collect(collectionOfIntegers, new org.apache.commons.collections.functors.StringValueTransformer());

Answer 9

Well, you could create your own iterator wrapper class to do this. But I doubt that you would save much by doing this.

Here's a simple example that wraps any iterator to a String iterator, using Object.toString() to do the mapping.

public MyIterator implements Iterator<String> {

    private Iterator<? extends Object> it;

    public MyIterator(Iterator<? extends Object> it) {
        this.it = it;
    }

    public boolean hasNext() {
        return it.hasNext();
    }

    public String next() {
        return it.next().toString();
    }

    public void remove() {
        it.remove();
    }
}

Answer 10

I think you would either have to create a custom List (implementing the List interface) or a custom Iterator. For example:

ArrayList<String> targetList = new ArrayList<String>();
ConvertingIterator<String> iterator = new ConvertingIterator<String>(targetList);
// and here you would have to use a custom List implementation as a source List
// using the Iterator created above

But I doubt that this approach would save you much.

Answer 11

Here's an on-the-fly approach. (There must be something already like this in the jdk; I just can't find it.)

  package com.gnahraf.util;

  import java.util.AbstractList;
  import java.util.List;
  import java.util.Objects;
  import java.util.function.Function;

  /**
   * 
   */
  public class Lists {

    private Lists() { }


    public static <U,V> List<V> transform(List<U> source, Function<U, V> mapper) {
      return new ListView<U, V>(source, mapper);
    }

    protected static class ListView<U, V> extends AbstractList<V> {

      private final List<U> source;
      private final Function<U, V> mapper;

      protected ListView(List<U> source, Function<U, V> mapper) {
        this.source = Objects.requireNonNull(source, "source");
        this.mapper = Objects.requireNonNull(mapper, "mapper");
      }

      @Override
      public V get(int index) {
        return mapper.apply(source.get(index));
      }

      @Override
      public int size() {
        return source.size();
      }

    }

  }