I want to make it a common fuction to filter a list using a java stream.
but it doesn't work out the way I want it to...
The code below doesn't work, but is it possible to filter using reflection within a lambda expression?
List filterList = commonList.stream()
.filter(x -> x.getClass().getDeclaredField("metadata").get(this)
.getClass().getDeclaredField("name").get(this).matches(keywords))
.collect(Collectors.toList());
You have 2 basic problems:
ObjectYou can however address both problems by employing a typed method that catches and rethrows as RuntimeException:
<T> getField(String fieldName, Object obj) {
try {
return (T)obj.getClass().getDeclaredField(fieldName).get(obj);
} catch (ReflectiveOperationException e) {
throw new RuntimeException(e);
}
}
Then use a typed call to tell the compiler that name is a String, like this:
List filterList = commonList.stream()
.filter(x -> this.<String>getField("name", getField("metadata", x)).matches(keywords))
.collect(Collectors.toList());
The easy way to code the method is using Lombok:
@SneakyThrows
<T> getField(String fieldName, Object obj) {
return (T)obj.getClass().getDeclaredField(fieldName).get(obj);
}
This expression:
x.getClass().getDeclaredField("metadata").get(this)
.getClass().getDeclaredField("name").get(this)
returns an Object. There's no matches method declared in Object class. Imagine rewriting your lambda as:
filter(x -> {
Object y = x.getClass().getDeclaredField("metadata").get(this)
.getClass().getDeclaredField("name").get(this);
return y.matches(keywords);
});
Thus the compilation error.
Also, getDeclaredField method can throw a checked exception, so it should be handled inside the lambda code:
filter(x -> {
try {
return x.getClass().getDeclaredField("metadata").get(this)
.getClass().getDeclaredField("name").get(this).matches(keywords);
} catch (NoSuchFieldException e) {
//handle it here or re throw
throw new RuntimeException("Error filtering the stream", e);
}
});
getDeclaredField would need a try-catch :(