Strange Java cast exception. Why can't I cast Long to a Float?

Question

Why can't I cast Long to a Float?

I get this error message:

java.lang.ClassCastException: java.lang.Long cannot be cast to java.lang.Float

Why is this a problem? The numbers that I'm trying to cast are decimals in the domain [-10.0, 10.0]. They start out as Object instances returned using JFormattedTextField.getValue(). But they must be converted to floats.

Stack trace:

Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: java.lang.Long cannot be cast to java.lang.Float
    at submodeler.animation.Timeline.setKeyedAttribute(Timeline.java:59)
    at submodeler.ui.attributes.TransformationAttributePanel.actionPerformed(TransformationAttributePanel.java:247)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2028)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2351)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
    at java.awt.Component.processMouseEvent(Component.java:6348)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
    at java.awt.Component.processEvent(Component.java:6113)
    at java.awt.Container.processEvent(Container.java:2085)
    at java.awt.Component.dispatchEventImpl(Component.java:4714)
    at java.awt.Container.dispatchEventImpl(Container.java:2143)
    at java.awt.Component.dispatchEvent(Component.java:4544)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4618)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4282)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4212)
    at java.awt.Container.dispatchEventImpl(Container.java:2129)
    at java.awt.Window.dispatchEventImpl(Window.java:2475)
    at java.awt.Component.dispatchEvent(Component.java:4544)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:635)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:296)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:211)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:196)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:188)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

Answer 1

It would help if you provide a stacktrace.

Otherwise, the standard solution is to replace (Float) someLongValue with someLongValue.floatValue()

If you are dealing with primitive types, you can just cast from long to float, although it is a 5.1.2 Widening Primitive Conversion, but one that may lose precision, so be careful! Obviously you have the wrapper type Long, which cannot implicitly be converted, thus you get the classcastexception. This may be because of autoboxing, or explicit Long object creation.

Some more uninvited advice: if your valid values are decimals in the range of -10 to +10 the standard data type is int (primitive). avoid float if you mean exact numbers. long is also not optimal, because it is not fully atomic like int and it takes 2x the memory. If you allow a different state "not assigned" then Integer, which may take null, is also OK.

Answer 2

The exception happens because the compiler recognizes that Long and Float do not have a parent-child dependency. Unlike C++, which will try to find an overloaded cast operator.

You can call doubleValue() on any of the primitive wrapper types that are derived from Number, so this is the recommended way to do what you want:

double val = ((Number)textField.getValue()).doubleValue()

A couple of things to note here: first, I casted the output of getValue() to Number. That way, you can change your mind and actually return Double values in the future, and you won't break.

Also, I'm using double values, not float. The JVM works with double internally, so there's no reason to use float except to save space in an array.