diff --git a/JSONObjectTest.java b/JSONObjectTest.java index de36c01..c02ebd1 100644 --- a/JSONObjectTest.java +++ b/JSONObjectTest.java @@ -493,7 +493,7 @@ public class JSONObjectTest { "\"negativeHexFloat\":-0x1.fffp1,"+ "\"hexFloat\":0x1.0P-1074,"+ "\"floatIdentifier\":0.1f,"+ - "\"doubleIdentifier\":0.1d,"+ + "\"doubleIdentifier\":0.1d"+ "}"; JSONObject jsonObject = new JSONObject(str); Object obj; @@ -694,6 +694,17 @@ public class JSONObjectTest { "\"keyLong\":9999999993,"+ "\"keyDouble\":3.1,"+ // TODO: not sure if this will work on other platforms + + // Should work the same way on any platform, this the effect of a float to double conversion and happens because + // java type-casts float to double. A 32 bit float is type-casted to 64 bit double by simply appending zero-bits to the + // mantissa (and extended the signed exponent by 3 bits.) + + // Like 1/3 cannot be represented as base10 number because it is periodically, 1/5 cannot be represented as base2 number + // since it is periodically in base2 (take a look at http://www.h-schmidt.net/FloatConverter/) + // The same happens to 3.1, that decimal number (base10 representation) is periodic in base2 representation, therefore + // appending zero-bits is inaccurate only repeating the periodically repeating bits (0110) would be a proper conversion. + // However one cannot detect from a 32 bit IEE754 representation which bits would "repeat infinitely", since the missing + // bits would not fit into the 32 bit float, i.e. the information needed is not there! ;) "\"keyFloat\":3.0999999046325684,"+ "}"; JSONObject jsonObject = new JSONObject(str); @@ -709,6 +720,53 @@ public class JSONObjectTest { jsonObject.increment("keyFloat"); JSONObject expectedJsonObject = new JSONObject(expectedStr); Util.compareActualVsExpectedJsonObjects(jsonObject, expectedJsonObject); + + /* + float f = 3.1f; + double df = (double) f; + double d = 3.1d; + System.out.println(Integer.toBinaryString(Float.floatToRawIntBits(f))); + System.out.println(Long.toBinaryString(Double.doubleToRawLongBits(df))); + System.out.println(Long.toBinaryString(Double.doubleToRawLongBits(d))); + + - Float: + seeeeeeeemmmmmmmmmmmmmmmmmmmmmmm + 1000000010001100110011001100110 + - Double + seeeeeeeeeeemmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm + 10000000 10001100110011001100110 + 100000000001000110011001100110011000000000000000000000000000000 + 100000000001000110011001100110011001100110011001100110011001101 + */ + assertTrue( "Correctly converting float to double via base10 (string) representation!", new Double( 3.1d ).equals( new Double( new Float( 3.1f ).toString() ) ) ); + + // Pinpointing the not so obvious "buggy" conversion from float to double in JSONObject + JSONObject jo = new JSONObject(); + jo.put( "bug", 3.1f ); // will call put( String key, double value ) with implicit and "buggy" type-cast from float to double + assertFalse( "The java-compiler did add some zero bits for you (probably unexpected, but well documented)", jo.get( "bug" ).equals( new Double( 3.1d ) ) ); + + JSONObject inc = new JSONObject(); + inc.put( "bug", new Float( 3.1f ) ); // This will put in instance of Float into JSONObject, i.e. call put( String key, Object value ) + assertTrue( "Everything is ok here!", inc.get( "bug" ) instanceof Float ); + inc.increment( "bug" ); // after adding 1, increment will call put( String key, double value ) with implicit and "buggy" type-cast from float to double! + // this.put(key, (Float) value + 1); + // 1. The (Object)value will be typecasted to (Float)value since it is an instanceof Float actually nothing is done. + // 2. Float instance will be autoboxed into float because the + operator will work on primitives not Objects! + // 3. A float+float operation will be performed and results into a float primitive. + // 4. There is no method that matches the signature put( String key, float value), java-compiler will choose the method + // put( String key, double value) and does an implicit type-cast(!) by appending zero-bits to the mantissa + assertTrue( "JSONObject increment unexpected behaviour, Float will not stay Float!", jo.get( "bug" ) instanceof Float ); + // correct implementation (with change of behaviour) would be: + // this.put(key, new Float((Float) value + 1)); + // Probably it would be better to deprecate the method and remove some day, while convenient processing the "payload" is not + // really in the the scope of a JSON-library (IMHO.) + + // Some more examples of well documented unexpeced "numbercrunching" ;) + assertTrue("Stumbled over explicitly type-casting float as double!", (double)0.2f == 0.2d ); + assertTrue("Stumbled over comparing float with double any implicit type-cast!", 0.2f == 0.2d ); + Double d1 = new Double( 1.1f ); + Double d2 = new Double( "1.1f" ); + assertTrue( "Stumbled again over implicit type cast from float to double before calling Double(double d) constructor", d1.equals( d2 ) ); } @Test