How come the JRE uses == to compare Strings?

Question

I was looking through the source of javax.swing.plaf.basic.BasicComboBoxUI and came across this:        public void propertyChange(PropertyChangeEvent e) {
            String propertyName = e.getPropertyName();
         JComboBox comboBox = (JComboBox)e.getSource();
            if ( propertyName == "model" ) {
            else if ( propertyName == "editor" && comboBox.isEditable() ) {
            else if ( propertyName == "editable" ) {
            else if ( propertyName == "enabled" ) {
            else if ( propertyName == "focusable" ) {
            else if ( propertyName == "maximumRowCount" ) {
            else if ( propertyName == "font" ) {
            else if ( propertyName == JComponent.TOOL_TIP_TEXT_KEY ) {
            else if ( propertyName == BasicComboBoxUI.IS_TABLE_CELL_EDITOR ) {
         else if (propertyName == "prototypeDisplayValue") {
         else if (propertyName == "renderer") {
        }So
1) Is there a valid reason for the developer to have used == instead of .equals(...) to compare Strings?
2) How does the code work consistently with this anomaly?
Similar use of == and not .equals(...) is seen in many other swing classes.
Doesn't affect me, but I would be interested to know if anyone can throw some light on this.
Thanks, Darryl

Answer 1

Darryl.Burke wrote:
DeltaGeek, thank you for your response.
Paul, I think I understand what you are saying, but the String reference propertyNamein this case comes from e.getPropertyName() from the PropertyChangeEvent class, not from within the BasicComboBoxUI class.
I gather that all loaded classes share the same String pool, and that's how this works correctly. But in that case any comparison of a String variable to a literal String using == should also work.
Then why the brouhaha about never using == to compare Strings?The key thing here is that the String returned by getPropertyName() must have come from a String literal. For example, in javax.swing.JComboBox:
firePropertyChange( "maximumRowCount", oldCount, maximumRowCount );This string literal will be the same instance as the string literal in the code you posted.
If they had instead done something like
String s = "RowCount";
boolean isMaximum = true;
firePropertyChange((isMaximum ? "maximum" : "minimum") + s, oldCount, maximumRowCount );In this case, the String literal "maximumRowCount" is not used, but rather created from a concatenation at runtime, so it's likely that the comparison would fail.
Note that the following would be the same String literal because this concatenation would be done at compile-time, not runtime:
"maximum" + "RowCount"As for why, I agree with DeltaGeek: performance.