Javac type inference bug

Scenario: A generics based configuration reader that worked perfectly fine under Eclipse, suddenly started failing inside a Jenkins build. The cause was a compilation error: "type parameters of <T>T cannot be determined; no unique maximal instance exists for type variable T with upper bounds int,java.lang.Object".

According to this bug report, this is an almost 6 years old javac bug. Apprently Java 1.7 fixes it, but I haven't had a chance to verify it. Here's some sample code to reproduce it:

package com.lucidelectricdreams;

public class GenericsTest
{
	@SuppressWarnings("unchecked")
	public <T> T genericReturnTest(int typeToReturn)
	{
		
		switch(typeToReturn)
		{
			case 1: // integer
				return (T)(new Integer(12));
				
			case 2: // double
				return (T)(new Double(56.2D));
				
			default: // String
				return (T)"test";			
		}
	}
	
	
	public static void main(String[] args)
	{
		GenericsTest gt = new GenericsTest();
		
		int intRetVal = gt.genericReturnTest(1);
		double doubleRetVal = gt.genericReturnTest(2);
		String stringRetVal = gt.genericReturnTest(3);
		
		System.out.println(intRetVal);
		System.out.println(doubleRetVal);
		System.out.println(stringRetVal);		
	}
}

 

Changing the main method as follows, gets rid of the compiler error:

        public static void main(String[] args)
	{
		GenericsTest gt = new GenericsTest();
		
		int intRetVal = gt.<Integer>genericReturnTest(1);
		double doubleRetVal = gt.<Double>genericReturnTest(2);
		String stringRetVal = gt.genericReturnTest(3);
		
		System.out.println(intRetVal);
		System.out.println(doubleRetVal);
		System.out.println(stringRetVal);		
	}