泛型对象创建在Eclipse中显示错误，但在Maven中编译

Question

Hi I have some initialization as below.... it is showing error in eclipse as type mismatch. if i compile using maven through command prompt, it compiles without any error.. using eclipse luna, and compiler set to java 1.7.

  ReportData<Object> rdata = null;

  rdata =  new ReportData<>(reportCtx.get()); // error shown for this line in //eclipse.

  ReportData<T>{

      private T val;
      public ReportData(T val){
          this.val=val;
      }
  }

I am not understanding, why eclipse show error for this and how it compiles in maven with the same java 1.7. whats wrong with rdata = new ReportData<>(reportCtx.get()); initialization.

Answer 1

ReportData<?> is the supertype of all kinds of report data.

ReportData<Object> is not the supertype of all kinds of report data.

Hence you cannot assign:

ReportData<Object> ← ReportData<of an unknown kind>

See The Java™ Tutorials , Generics, Wildcards .

And BTW, in addition to ...

ReportData<Object> rdata = new ReportData<Object>(reportCtx.get());

... the different declaration in ...

ReportData<?> rdata = new ReportData<>(reportCtx.get());

... does the trick, as well.