java lambda expression with multiple generic parameters not compiled

Question

I'm coding a Excel import logic:

public class ExcelImporter {
    //...ignore constructor and other methods ...
    public <R> Builder<R> sheet(String sheetName, RowConsumer<R> rowConsumer) {
        return new Builder<R>(sheetName, rowConsumer);
    }
    public class Builder<R> {
        //...ignore other method
        public <F> Builder header(String name, CellConsumer<R, F> cellConsumer) {
            sheetReader.header(new DefaultHeader<>(name, cellConsumer));
            return this;
        }
        public <F> Builder header(String name, Class<F> fieldType, CellConsumer<R, String> cellConsumer) {
            return header(name,cellConsumer);
        }
    }
}

And on my test code I got compile error:

@Test
public void processSmallExcelWithConsumer() throws Exception {
    try (InputStream is = getClass().getClassLoader().getResourceAsStream("工作簿1.xls")) {
        ExcelImporter excelImporter = new ExcelImporter(is, "application/vnd.ms-excel")
                .sheet("Sheet1", () -> new RowBean())
                .header("姓名",String.class, (cell, row) -> row.setName(cell)) // no error
                .header("性别",String.class, (cell, row) -> row.setSex(cell)) // "setSex" got error and `row` evaluate to `Object` ? why !?
                .build();

setSex() can not compile,and that row evaluate to Object, I'm so confusion that it works well at first time but fails next time?

This is CellConsumer:

@FunctionalInterface
public interface CellConsumer<R,F> {
    void read(F cell,R row);
}

And RowConsumer:

@FunctionalInterface
public interface RowConsumer<R> {
    R newRow();
}

And error:

[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 1.797 s
[INFO] Finished at: 2017-07-28T23:56:26+08:00
[INFO] Final Memory: 17M/166M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.2:testCompile (default-testCompile) on project graceful-excel: Compilation failure
[ERROR] /home/terrason/workspace/maven/cnx/graceful-excel/src/test/java/cn/lenyar/excel/ExcelImporterTest.java:[81,66] 找不到符号
[ERROR] 符号:   方法 setSex(java.lang.Object)
[ERROR] 位置: 类型为java.lang.Object的变量 row
[ERROR] -> [Help 1]

Help me please!

Can you paste exact complete error you are getting and CellConsumer interface. — hagrawal7777
– hagrawal7777, Commented Jul 28, 2017 at 15:49
This needs more context, what is the row class that you're calling setName and setSex on? And what is RowBean? — Sean Van Gorder
– Sean Van Gorder, Commented Jul 28, 2017 at 16:21

Andreas · Accepted Answer · 2017-07-28 16:21:22Z

The problem is that you aren't specifying the generic type of the returned builder in the header methods:

public <F> Builder header(...)
//this returns Builder which is the same as Builder<?>
//All java can infer from Builder<?> is that the generic type is an Object.
//which makes row in the second call an Object

Try returning Builder<R> in your header methods:

public class ExcelImporter {
  //...ignore constructor and other methods ...
  public <R> Builder<R> sheet(String sheetName, RowConsumer<R> rowConsumer) {
    return new Builder<R>(sheetName, rowConsumer);
  }

  private static class RowBean {
    private String name;
    private String sex;

    public void setName(String name) {
      this.name = name;
    }

    public void setSex(String sex) {
      this.sex = sex;
    }
  }

  public class Builder<R> {
    public Builder(String sheetName, RowConsumer<R> rowConsumer) {

    }

    //...ignore other method
    public <F> Builder<R> header(String name, CellConsumer<R, F> cellConsumer) {
      return this;
    }

    public <F> Builder<R> header(String name, Class<F> fieldType, CellConsumer<R, String> cellConsumer) {
      return header(name, cellConsumer);
    }

    public ExcelImporter build() {
      return null;
    }
  }

  @FunctionalInterface
  public interface CellConsumer<R, F> {
    void read(F cell, R row);
  }

  @FunctionalInterface
  public interface RowConsumer<R> {
    R newRow();
  }

  public static void main(String[] args) {
    ExcelImporter excelImporter = new ExcelImporter()
        .sheet("Sheet1", () -> new RowBean())
        .header("姓名", String.class, (cell, row) -> row.setName(cell)) // no error
        .header("性别", String.class, (cell, row) -> row.setSex(cell)) // no error!
        .build();
  }
}

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