Map creation sample code

Map<Date, List<Task>> tasksbyweek = new Map<Date, List<Task>>();
for(Task t : tasks) {
     Date weekstart = new Date();
     if(tasksbyweek.get(weekstart)==null)
          tasksbyweek.put(weekstart, new List<Task>());
     tasksbyweek.get(weekstart).add(t);
}

Map<Id, LIST<Case>> caseMap = new Map<Id, LIST<Case>>();
 
 for(Case cc : childCases) {
 
     if(caseMap.containsKey(cc.parentId)) {
 
         caseMap.get(cc.parentId).add(cc);
     }
     else {
 
         caseMap.put(cc.parentId, new List<Case>{cc});
     }
 }