Q:

TreeMap foreach doesn't change value object

So I have a TreeMap<Integer, Transmitter> and through a foreach I am trying to modify an inner attribute of the transmitter, but it feels like it is making a copy of the object in the TreeMap, since it does not change the value in the TreeMap.
My foreach code:
        for (TreeMap<Integer, Transmitter> current : transmitterDiagnosticMap.values()) {
            for (Transmitter t : current.values()) {
                String transmitterError = t.printErrorReport(date, appContext);
                if (transmitterError != null)
                    stringsErrorsAndWarnings.add(transmitterError);
            }
        }

My printErrorReport code:
     public String printErrorReport(String data, Context context) {
        String output = null;
        if (this.writeOnReport()) { // This is the function that will modify the object
            output = data + " - " + this.getTension();
        }
        return output;
    }
    // This is the method that tells whether or not the report will be written, and changes the variable lastStatus if necessary
    private boolean writeOnReport() {
        if (this.status > 0) {
            if (this.lastStatus == 0 || this.lastStatus != this.status) {
                this.lastStatus = this.status;
                return true;
            }
            return false;
        } else {
            this.lastStatus = 0;
            return false;
        }
    }

What I could notice is that the Transmitter t actually changes the value from lastStatus = 0 to lastStatus = 1, but nothing is changed in the TreeMap. 

A:

You have to use the iterator to mutate the values in the TreeMap. Using current.values() will create a copy instead of mutating the object. 
You need to iterate over the keys of the TreeMap and update the values.
for (TreeMap<Integer, Transmitter> current : transmitterDiagnosticMap.values()) {
    for (Map.Entry<Integer, Transmitter> entry : current.entrySet()) {
        Transmitter t = entry.getValue();
        String transmitterError = t.printErrorReport(date, appContext);
        if (transmitterError != null)
            stringsErrorsAndWarnings.add(transmitterError);
        entry.setValue(t);
    }
}

