Java8 - Get list of instance variable values from obj matching search criteria(list of field names)

Question

I am having an object with 30 instance variables, depending on one instance variable eg:

if obj.var1 == 'A' : Get 5 variable values
else if obj.var1 == 'B' : Get another 5 variable values

I have a mapping of condition to required fields from obj eg:

A -> {FieldName1, FieldName2, FieldName3}
B -> {FieldName1, FieldName3, FieldName5, FieldName7}

Reflection would give me the values of the instance variables, how to achieve this with java8 in an efficient way.

Few of my references: Get the class instance variables and print their values using reflection , https://codereview.stackexchange.com/questions/108222/getting-a-list-of-all-fields-in-an-object

eg:test mock code

public class Test {

private String var1;
private String var2;
private String var3;
private String var4;
private String var5;
private String var6;
private String var7;
private String var8;
private String var9;
private String var10;

// getters and setters

public Test() {
}


public static void main(String[] args) {
    Test obj = new Test();
    // Assume all ins variables are set with some value
    List resp = new ArrayList();
    if(obj.var1 == "A") {
        resp.add(obj.var2 + DescriptionEnum.Var2); // Also Here I need to get the variable description from enum)
        resp.add(obj.var3 + DescriptionEnum.);
        resp.add(obj.var7 + DescriptionEnum.);
        resp.add(obj.var8 + DescriptionEnum.);
    }

    else if(obj.var1 == "B") {
        resp.add(obj.var1 + DescriptionEnum.);
        resp.add(obj.var2 + DescriptionEnum.);
        resp.add(obj.var9  + DescriptionEnum.);
        resp.add(obj.var10 + DescriptionEnum.);
    }
}

Instead of above if-else, I thought of put the condition and list of fields in hashmap and do the operations eg:

map.put("A1", new String[]{'fieldName1', 'fieldName2'})
map.put("B1", new String[]{'fieldName1', 'fieldName2'})

So if the obj.var1 == 'A', I can get the list of required fields from map, now how to get the values from object and from enum to form the response?

IMHO your question is too broad. If you're trying to instantiate an Object depending on 30 (or less/more) variables, you should consider a different approach. Enum, or else.... — zlakad, Feb 18 '18 at 14:46
I already have an object :( now i need to work on that to get the details — Rinsen S, Feb 18 '18 at 14:55

jrtapsell · Accepted Answer · 2018-02-18T15:13:01.337

3

Using functions

import java.util.List;
import java.util.Map;
import java.util.function.Function;

class Big {
    String director;
    String a;
    String b;
    String c;
    String d;
    String e;
    String f;
    String g;
}

public class Ex2 {
    Map<String, Function<Big, List<String>>> mappers = Map.of(
            "A", a -> List.of(a.a, a.b),
            "B", a -> List.of(a.c, a.d)
    );

    public List<String> parse(Big input) {
        return mappers.get(input.director).apply(input);
    }
}

The code above uses Map.of and List.of from Java 9 to improve readability, if you want to do this using Java 8 you can swap these out like this:

Map<String, Function<Big, List<String>>> mappers = new HashMap<String, Function<Big, List<String>>>() {
    {
        put("A", a -> Arrays.asList(a.a, a.b));
        put("B", a -> Arrays.asList(a.c, a.d));
    }
};

public List<String> parse(Big input) {
    return mappers.get(input.director).apply(input);
}

If you can avoid having such a large object you probably should, as it will simplify your code massively.

edited Feb 18 '18 at 15:13

answered Feb 18 '18 at 15:03

jrtapsell

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Thanks for the solution, but we are using only java8 in production, so I cannot use java9. I will try to understand the above code :) Thanks for it, but can i know what changes to do for java8 instead of java9 – Rinsen S Feb 18 '18 at 15:13
1

Added it 10s before the comment :), I am using double brace though, which you might want to swap for initialising the object in the constructor. – jrtapsell Feb 18 '18 at 15:13
sure, thanks bro !!! i will read function and try above solution, once again thanks !! – Rinsen S Feb 18 '18 at 15:17
2

The double brace is evil, so definitely avoid that. Otherwise, a very nice solution that dodges reflection but retains the brevity. – kaqqao Feb 19 '18 at 07:50
1

@kaqqao and a clean `… mappers = new HashMap<>(); mappers.put("A", a -> Arrays.asList(a.a, a.b)); mappers.put("B", a -> Arrays.asList(a.c, a.d));` is even shorter… – Holger Feb 19 '18 at 10:40
@jrtapsell, thanks for the above solution, can I also know how to efficiently check if the ins variables are not null eg: whether a.a or a.b is not null and then add a.a + displaytext for that field from enum to get the result list.. – Rinsen S Feb 20 '18 at 14:34
So you are looking to try items and get the first non-null answer? – jrtapsell Feb 20 '18 at 15:13
@jrtapsell, no the above answer works well, but I want to ensure that the value is not null and add enum value to it and add to final arraylist. eg: instead of put("A", a -> Arrays.asList(a.a, a.b)) , I want to check for not null too eg: put("A", a-> { List – Rinsen S Feb 21 '18 at 08:04
At that level of complexity you may be better off making a factory that makes display classes (you could have the enum members extract the relevant fields, which may decrease the complexity slightly) – jrtapsell Feb 21 '18 at 15:07
@jrtapsell, sorry for replying late, as I haven't seen the comment, oh but I am not sure of how to implement that, so I am implementing like put("A", a-> { List – Rinsen S Feb 26 '18 at 12:01

Java8 - Get list of instance variable values from obj matching search criteria(list of field names)

1 Answers1

Using functions