Can anyone show a sample program of KUKA LBR iiwa sunrise OS and your experience?

you absolutely must learn Java first, there is no way around it.

if you take KUKA training, that is two weeks in the classroom. entire first week is only about java basics.

and i would say that for most people, that first week is barely enough to follow what comes next.

i would strongly suggest to learn Java before even getting into classroom.

one of the things that scares most people is how verbose java code is compared to KRL.

instructions in KRL are usually very short and simple, comparable code in Java tends to be much longer

KRL example to move robot to a (transport) position with 25% speed and single PTP motion:

DEF Transport()
  BAS(#initmov,0)
  BAS(#VEL_PTP,25)
  MsgNotify("Moving to transport position")
  PTP {A1 0, A2 -120, A3 150, A4 0, A5 70, A6 0}
END

comparable JAVA code is something like this:

package lbrExampleApplications;

import com.kuka.roboticsAPI.applicationModel.RoboticsAPIApplication;
import static com.kuka.roboticsAPI.motionModel.BasicMotions.*;
import com.kuka.roboticsAPI.deviceModel.LBR;
import com.kuka.roboticsAPI.motionModel.PTP;
import com.kuka.roboticsAPI.uiModel.ApplicationDialogType;

public class TransportPosition extends RoboticsAPIApplication {
    private LBR lbr;

    public void initialize() {
        lbr = getContext().getDeviceFromType(LBR.class);
    }

    public void run() {
        getLogger().info("Moving to transport position");
        PTP ptpToTransportPosition = ptp(0, Math.toRadians(25), 0, Math.toRadians(90), 0, 0, 0);
        ptpToTransportPosition.setJointVelocityRel(0.25);
        lbr.move(ptpToTransportPosition);
    }

}

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Can you please tell me your experience with KUKA LBR iiwa.

I've worked on them for 5 years now, they're pretty awesome in many aspects: Safety, Torque sensing, Hand-guiding, ease of integration in non-industrial setups, you don't need to buy packages for every protocol, etc.

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how deep we must know the java programming?

To start you need to know some programming (Variables and Types, Conditionals, Loops, functions, etc) and Object Oriented programming (Classes, Objects, etc.). If you ever programmer before, it's not very hard to learn.

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When compared to normal KRC4 controller programming. how is the programming experience with KUKA LBR iiwa.

Each one has pros and cons, or let's say conveniences and inconveniences. From the top of my head, and as aforementioned, KRL is less verbose and you can change your code on the fly directly from the smartPad. You can't do that on the LBR iiwa, you need to change your code on the sunrise Workbench editor each time then synchronize. You can get around some of that by using process data or file exchange, to update some parameters, but not code (I think).

On Java you have tons of libraries available online (communication protocols, XML/JSON parsers, etc.), you have much less in KRL.

The bigger a program/project on KRL the clunkier it becomes. In java with some good package management, you can keep your project tidy.