Java 8 Lambda with Function<String, Object> as argument

Question

Interesting compilation error in Lambda Java 8 (Oracle JDK)

java -version
java version "1.8.0_45"
Java(TM) SE Runtime Environment (build 1.8.0_45-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.45-b02, mixed mode)

I have a method call:

new CSVFile()
.of(new FileInputStream("MyFile.csv"))
.withColumnMapping("name", "fullName", s -> s.toUpperCase())
.withColumnMapping("gender", "gender", s -> s.toUpperCase());

This is the method I'm trying to call:

 public CSVFile withColumnMapping(final String columnName, final String   beanPropertyName, final Function<String, Object> columnTransformFunction) {
    columnMappings.add(new ColumnMapping(columnName, beanPropertyName, Optional.of(columnTransformFunction)));
    return this;
}

Compilation Error I get is:

[ERROR] /Users/sai/fun/reactivecsv/src/test/java/reactivecsv/CSVFileTest.java:[26,50] cannot find symbol
[ERROR] symbol:   method toUpperCase()
[ERROR] location: variable s of type java.lang.Object

Weirdly, This compiles

Function<String, Object> upperCaseConversion = String::toUpperCase;
new CSVFile()
.of(new FileInputStream("MyFile.csv"))
.withColumnMapping("name", "fullName", upperCaseConversion)
.withColumnMapping("gender", "gender", upperCaseConversion);

Why is the compiler not able to synthesise the lambda into a Function ?

By any change how your CSVFile class is declared? Is it generic? — Tagir Valeev
– Tagir Valeev, Commented Jul 29, 2015 at 10:53

Tagir Valeev · Accepted Answer · 2015-07-29 10:56:53Z

8

When you create new CSVFile() for generic type, it becomes a raw type. You must not use raw types. Creating it as raw type changes all its methods to raw as well, thus withColumnMapping(String, String, Function<String, Object>) becomes withColumnMapping(String, String, Function) and type of your lambda argument cannot be inferred. To fix your problem specify the proper generic arguments when calling the constructor.

answered Jul 29, 2015 at 10:56

Tagir Valeev

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1 Comment

user1189332 Over a year ago

That's indeed the issue.

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