I have findout one mistake in my combobox binding statement i.e
var oItemTemplate1 = new sap.ui.core.ListItem();
oItemTemplate1.bindProperty("key", "Matnr");
oItemTemplate1.bindProperty("text", "Maktx");
oComboBox3.bindItems("/GITEMSSet", oItemTemplate1);
I was using the literal "text" instead of "key" while declaring it now the first problem is resolved and i am able to assigne the table columns to the combobox and can read the selectedkey and selectedvalue using following methods this alert is defined with in the change: Function(oEvent) method of Combobox properties.
alert("Key"+oEvent.oSource.getSelectedKey() +"\nValue:"+oEvent.oSource.getLiveValue());
Now i want to Use the Enter Key and Function Keys Like F1, F2, F3, F4 etc. how can i do it ???