sap.ui.Table multiple selection mode

Hi I am using the sap.ui.table having the multiple row selction mode. how to read all the rows which are selected by click on some button from table.                 

aContexts =sap.ui.getCore().byId("tableName").getSelectedIndices();                 

var oTable =sap.ui.getCore().byId("tableName");   

for ( var i=0; i<aContexts.length; i++){       

            var oRow =  oTable.getContextByIndex(aContexts[i]);        

               console.log ("In the if loop-->"+oRow.getObject().columnName);                   

}

I want to read the two clumn values in the table which are selected. but it is coming null. Can you share the code. thanks