If else with java 8 Lambda

Question

I want to replace conventional if else with lambda. Consider following highlighted code, is there some simple way to have this represented with Lambda ?

public class IfElseLambda {

    public static void main(String[] args) {

        String value = null;
        DataObj data = new DataObj();

        List<DataObj> dataObjs = data.getDataObjs();

        ***if (dataObjs != null) {

            value = dataObjs.stream().map(dataObject -> getValue(dataObject)).filter(Objects::nonNull).findFirst().orElse(null);
        } else {
            value = getValue(data);
        }***

    }


    public static String getValue(DataObj dataObj) {
        return "Get value from dataObj";
    }
}

class DataObj {
    List<DataObj> dataObjs;

    public List<DataObj> getDataObjs() {
        return dataObjs;
    }

    public void setDataObjs(List<DataObj> dataObjs) {
        this.dataObjs = dataObjs;
    }
}

lambda is a function and if-else is.. if-else how do you want to replace a condition with a function? — Omri Attiya
– Omri Attiya, Commented Apr 7, 2020 at 7:00
I see no good way to replace if-else with lambda. If it's because you don't like if-else, use a ? : ternary operator. — Andreas
– Andreas, Commented Apr 7, 2020 at 7:01
@Andreas Even with ternary operation, you'd be replacing if-else with that ternary operation... lambda simply cannot do that. Even if you have a function which accepts two lambdas and a condition, you would still have to have a branching pattern somewhere. In some lower-level language you could theoretically put two pointers to lambdas in an array, then use pointer arithmetic to figure out which one to call. But ... WHY??? — v010dya
– v010dya, Commented Apr 7, 2020 at 7:07
Not only how would you want to do that, but why? if-else is a basic, well-known construct. Why would you want to overengineer it into a lambda? — PaianganuOm
– PaianganuOm, Commented Apr 7, 2020 at 7:07
@v010dya Seems you totally misunderstood my comment. I said to use ternary operator instead of if-else, which means e.g. value = (dataObjs == null ? getValue(data) : dataObjs.stream()...findFirst().orElse(null)); --- See, no if-else. — Andreas
– Andreas, Commented Apr 7, 2020 at 7:11

Andy Turner · Accepted Answer · 2020-04-07 07:24:32Z

2

One thing you can do is to change the null list to something which results in the same output:

List<DataObj> dataObjs = Optional.ofNullable(data.getDataObjs()).orElse(Collections.singletonList(data));

dataObjs will now be a list with a single element in the case that data.getDataObjs() is null.

Now you don't need the if/else:

value = dataObjs.stream().map(dataObject -> getValue(dataObject)).filter(Objects::nonNull).findFirst().orElse(null);

answered Apr 7, 2020 at 7:24

Andy Turner

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1 Comment

Holger Over a year ago

Starting with Java 9, you’d preferably use List<DataObj> dataObjs = Objects.requireNonNullElse(data.getDataObjs(), Collections.singletonList(data)); instead of the Optional

amanin · Accepted Answer · 2020-04-07 07:23:56Z

I your aim is to isolate the logic of your if-else, and potentially allowing it to be replaced, maybe you could do the following : Your lambda take as input your data list, and gives you back a String value. Therefore, you can use a java.util.Function interface, like this:

Function<List<DataObj>, String> extractor = dataList 
    -> dataList == null? Stream.of(DEFAULT_DATA_OBJ) : dataList.stream()
       .map(dataObject -> getValue(dataObject))
       .filter(Objects::nonNull)
       .findFirst()
       .orElse(null)

Note, you still have a ternary operator (Do not see how you could do without it, because if your list can be null, you cannot even use Stream.concat to protect from empty-list). However, with that construct, the logic of your ternary operator is replaceable if you make the extractor function replaceable in your code.

Exemple:

public static void main(String... args) {

    final List<DataObj> dataList = ...;
    final DataObj defaultValue = ...;

    Function<List<DataObj>, String> extractor = dataList 
        -> dataList == null? Stream.of(defaultValue) : dataList.stream()
           .map(dataObject -> getValue(dataObject))
           .filter(Objects::nonNull)
           .findFirst()
           .orElse(null);

    doStuff(dataList, extractor);

    // Now, if you want to change your extraction logic, do
    doStuff(dataList, whatever -> "Return a constant title");
}

public static void doStuff(final List<DataObj> dataList, final Function<List<DataObj, String> titleExtractor) {
   // Do stuff here
}

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