How to convert multiple list into single list using Java streams?

Question

I have an class A with multiple List members.

class A {     List<X> xList;     List<Y> yList;     List<Z> zList;          // getters and setters }  class X {      String desc;      String xtype;           // getters and setters     }  class Y {        String name;     String ytype;          //getters and setters }  class Z {     String description;     String ztype;          // getters and setters     } 

And a class B with just 2 attributes:

class B {     String name;     String type;      } 

I need to iterate through the various lists in class A and create class B object and add to a list like this:

public void convertList(A a) {      List<B> b = new ArrayList<>();          if (!a.getXList().isEmpty()) {       for (final X x : a.getXList()) {            b.add(new B(x.getDesc(), x.getXType()));       }     }          if (!a.getYList().isEmpty()) {       for (final Y y : a.getYList()) {            b.add(new B(y.getName(), y.getYType()));       }     }          if (!a.getZList().isEmpty()) {       for (final Z z : a.getZList()) {            b.add(new B(z.getDescription(), z.getZType()));       }     }     } 

As the if and for loops are repeated here.

How can I achieve this using Java streams?

Note: There is no relation between the classes X, Y and Z and there is no common interface.

Answer 1

Since your X, Y and Z types don't have a common super-type, you have to convert them into some common type, such as Map.Entry<String,String>.

You can create a Stream of all pairs of names and types, and then map it to instances of B:

List<B> b =     Stream.of(         a.getXList().stream().map(x -> new SimpleEntry<>(x.getDesc(),x.getXType())),         a.getYList().stream().map(y -> new SimpleEntry<>(y.getName(),y.getYType())),         a.getZList().stream().map(z -> new SimpleEntry<>(z.getDescription(),z.getZType())))           .flatMap(Function.identity())           .map(e -> new B(e.getKey(), e.getValue()))           .collect(Collectors.toList()); 

Or directly generate B instances:

List<B> b =     Stream.of(         a.getXList().stream().map(x -> new B(x.getDesc(),x.getXType())),         a.getYList().stream().map(y -> new B(y.getName(),y.getYType())),         a.getZList().stream().map(z -> new B(z.getDescription(),z.getZType())))           .flatMap(Function.identity())           .collect(Collectors.toList()); 

Answer 2

You can use Stream.concat() like following

public List<B> convertList (A a) {     return Stream.concat(Stream.concat(a.getXList().stream().map(x -> new B(x.getDesc(), x.getXType()))             , a.getYList().stream().map(y -> new B(y.getName(), y.getYType())))             , a.getZList().stream().map(z -> new B(z.getDescription(), z.getZType()))).collect(Collectors.toList()); }