[FLINK-1040] Make type() call in projections optional

[Chen Xu]

Hi Stephan,

I am working on the issue [FLINK-1040].

At first, I would like to remove type() call in projections totally.

However, considering the case in JoinProjection and CrossProjection,
"chaining projection" like

ds1.join(ds2).where(0).equalTo(0)
.projectSecond(2)
.projectFirst(1)
                .types(String.class);

usually happens.

Of course, String.class would be extracted by compiler.

However, as the final transformation are activated by types(), it seems we
can turn the sharp to be

ds1.join(ds2).where(0).equalTo(0)
.projectSecond(2)
.projectFirst(1)
                .types();

or rename types() here like doJoinProject()..

Does it make sense or anything wrong here?

Cheers!
-Chen
​ Xu​