How to transform List to another List

Question

I have two lists of objects; List<X> and List<Y>. X and Y are ojects that look like:

public class X {
    String a;
    String b;
    String v;
    String w;
    String m;
    String n;
}

public class Y {
    String a;
    String b;
    List<A> aList;
}
public class A {
    String v;
    String w;
    List<B> bList;
}
public class B {
    String m;
    String n;
}

How transform List<X> into List<Y> based on a rule:
Some fields' values must be equal.
For example:
In List<Y>, for one object Y, field a's value must equal.
In Y's field List<A>, for one object A, field w's value must equal.
In A's field List<B>, for one object B, field m's value must equal and so on.

Guava has this method, Lists#transform, but I don't know how to transform.

Or any other way?

Answer 1

public static <F,T> List<T> transform(List<F> fromList,
                                      Function<? super F,? extends T> function

You might want to read up the API docs for Lists.transform() and Function, but basically the caller of the transform provides a Function object that converts an F to a T.

For example if you have a List<Integer> intList and you want to create a List<String> such that each element of the latter contains the english representation of that number (1 becomes "one" etc) and you have a access to a class such as IntToEnglish then

Function<Integer, String> intToEnglish = 
    new Function<Integer,String>() { 
        public String apply(Integer i) { return new IntToEnglish().english_number(i); }
    };

List<String> wordsList = Lists.transform(intList, intToEnglish);

Does that conversion.

You can apply the same pattern to transform your List<X> to List<Y>

Answer 2

With java lambda:

public static <K,V,Q extends K> List<V> transform( final List<Q> input, final java.util.function.Function<K,V> tfunc ) {
    if( null == input ) {
        return null;
    }
    return input.stream().map(tfunc).collect( Collectors.toList() );
}

You just need to implement: java.util.function.Function

Answer 3

How about this?

import java.util.ArrayList;
import java.util.List;
import com.google.common.base.Function;
import com.google.common.collect.Lists;

public class GuavaTransform {
    public static void main(String[] args) {
        List<X> xList = new ArrayList<X>();
        xList.add(new X("a", "b", "v", "w", "m", "n"));
        xList.add(new X("a1", "b1", "v1", "w1", "m1", "n1"));
        for(X elem: xList) {
            System.out.println("An instance of X:"+ elem);
        }
        System.out.println();
        List<Y> yList = Lists.transform(xList, new TransformXY());
        for(Y elem: yList) {
            System.out.println("The corresponding instance of Y: \n"+elem);
        }
    }
}

class TransformXY implements Function<X, Y> {

    @Override
    public Y apply(X x) {
        List<B> bList = new ArrayList<B>();
        bList.add(new B(x.m, x.n));
        List<A> aList = new ArrayList<A>();
        aList.add(new A(x.v, x.w, bList));
        return new Y(x.a, x.b, aList);
    }
}

class X {
    String a;
    String b;
    String v;
    String w;
    String m;
    String n;
    X(String a, String b, String v, String w, String m, String n) {
        super();
        this.a = a;
        this.b = b;
        this.v = v;
        this.w = w;
        this.m = m;
        this.n = n;
    }
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append("(");
        sb.append(a+",");
        sb.append(b+",");
        sb.append(v+",");
        sb.append(w+",");
        sb.append(m+",");
        sb.append(n);
        sb.append(")");
        return sb.toString();
    }
}
class Y {
    String a;
    String b;
    List<A> aList;
    Y(String a, String b, List<A> aList) {
        super();
        this.a = a;
        this.b = b;
        this.aList = aList;
    }
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append(a+"\n");
        sb.append(b+"\n");
        for(A elem: aList) {
            sb.append(elem+"\n");
        }
        return sb.toString();
    } 
}
class A {
    String v;
    String w;
    List<B> bList;
    A(String v, String w, List<B> bList) {
        super();
        this.v = v;
        this.w = w;
        this.bList = bList;
    }
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append("--------"+v+"\n");
        sb.append("--------"+w+"\n");
        for(B elem: bList) {
            sb.append(elem+"\n");
        }
        return sb.toString();
    }

}
class B {
    String m;
    String n;
    B(String m, String n) {
        super();
        this.m = m;
        this.n = n;
    }
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append("----------------"+m+"\n");
        sb.append("----------------"+n+"\n");
        return sb.toString();
    }
}

Console output:

An instance of X:(a,b,v,w,m,n)
An instance of X:(a1,b1,v1,w1,m1,n1)

The corresponding instance of Y: 
a
b
--------v
--------w
----------------m
----------------n



The corresponding instance of Y: 
a1
b1
--------v1
--------w1
----------------m1
----------------n1

Answer 4

Same as @Isaace but with the lambda syntax (got it from this example):

List<X> xList = new ArrayList<>();
List<Y> yList = xList
        .stream()
        .map(n -> someTransformFunc(n))
        .collect(Collectors.toList());

Answer 5

Java 8 style, IntelliJ IDEA‎ helped me out:

List<X> xList = new ArrayList<>();
List<Y> yList = xList
        .stream()
        .map(X::getY)
        .collect(Collectors.toList());

Answer 6

assume have two object can interconversion, Coach and EntityBase

1.declare generic method

   public static <TSourse, TResult> void ToList(List<TSourse> list, List<TResult> results) {
    if (list.size() > 0) {
        for (TSourse obj : list) {
            TResult tResult = (TResult) obj;
            if (tResult == null) {
                throw new AppException("error....");
            }
            results.add(tResult);
        }
    }
}

2.call this method

  List<EntityBase> entityBaseList = new ArrayList<>();
    Coach coach = new Coach();
    coach.setId("123");
    entityBaseList.add(coach);

    List<Coach> coachList = new ArrayList<>();
    ToList(entityBaseList, coachList);
    //will complete List<AObj> to another List<BObj>