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javalistgenericsarraylistconverters

Java Convert List<TypeA> to List<TypeB>

For example

class TypeA {
    private String a;
    private String b;
    private String c;
}

class TypeB {
    private String a;
}

Now I have a list of TypeA, and I only need the information a from TypeA. What's the most efficient way to convert List<TypeA> to List<TypeB>

Solution

  • Answer depends on what you mean by "efficient".

    // Option 1
List<TypeB> bList = new ArrayList<>();
for (TypeA a : aList) {
    bList = new TypeB(s.getA());
}

// Option 2
List<TypeB> bList = aList.stream()
        .map(TypeA::getA)
        .map(TypeB::new)
        .collect(Collectors.toList());

// Option 3
List<TypeB> bList = aList.stream().map(a -> new TypeB(s.getA())).collect(toList());

    The first performs best. That is one type of efficiency.

    The second and third are single statements. That is another type of efficiency.