How do I write all the junit with full code coverage ?

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Java

public void metrics( MGroup mgroup, detail Detail)
{
if (object.isNull(registry)){
LOGGER.info("ERROR:null");
return;
}
string code = Detail.getcode();
if(objects.isNull(code){
LOGGER.debug("code is null)
return;
}

stringbuffer ID= new stringbuffer();
if(objects.nonNull(mgroup.getsummary())){
ID.append(mgroup.getsummary().getprocessor());
ID.append(mgroup.getsummary().getinstitute());
}

if(code.startswith(constants.FILE_1)){
acceptcounter(ID.toString());
}
elseif(code.startswith(constants.FILE_2)){
rejecttcounter(ID.toString());
}
elseif(code.startswith(constants.FILE_3)){
failedcounter(ID.toString());
}
}

What I have tried:

Mine are not valid , please do write the answers