The easiest way, keeping as much of your existing current code as possible, is to just create a second string to hold the text to be displayed. So your insert() would become:
private void insert(int j) {
// TODO Auto-generated method stub
str = str+Integer.toString(j);
num = Integer.valueOf(str).intValue();
displayStr += Integer.toString(j); // Added
showResult.setText(displayStr);
}
Each of your operation methods would need to append to the displayStr. So the one for the minus key would become:
public void btnminusClicked(View v){
perform();
op = '-';
displayStr += "-"; // Added
}
And your reset would need an added line to empty-out the display string:
displayStr = ""; // Added
There may be a couple more tweaks, but that should get you going in the right direction.