Demo Three-Valued-Logic (TVL) for PostgreSQL boolean to/from Java conversion and logical operators.

TVL.java

// Demo of PostgreSQL to Java Three-Valued-Logic and conversion functions
// Paul Brebner, April 2021

public enum TVL {
		    TRUE,
		    FALSE,
		    UNKNOWN;
		
		// Note this function won't work with PreparedStatements only Statements
		public String toPGBoolString()
		{
			if (this == TVL.TRUE)
				return "t";
			else if (this == TVL.FALSE)
				return "f";
			else return null;
		}
		
		// given a String return TVL enum
		// note that String is null if SQL returns NULL!
		public TVL fromString(String x)
		{
			if (x == null)
				return TVL.UNKNOWN;
			else
				if (x.contentEquals("t"))
				return TVL.TRUE;
			else // if (x.contentEquals("f"))
				return TVL.FALSE;
		}
		
		// TVL Logical Operators
		
		/*
		 * T = TRUE
		 * F = FALSE
		 * U = UNKNOWN
		 * 
		 * AND truth table:
		 * x	y	x AND y
		 * ---------------
		 * T	T	T
		 * T	F	F
		 * T	U	U
		 * F	T	F
		 * F	F	F
		 * F	U	F
		 * U	T	U
		 * U	F	F
		 * U	U	U
		 * 
		 * X = any value
		 * simplifies to:
		 * 
		 * AND truth table:
		 * x	y	x AND y
		 * ---------------
		 * T	T	T
		 * T	F	F
		 * T	U	U
		 * F	X	F
		 * U	F	F
		 * U	X	U
		 * 
		 */
		 
		public TVL and(TVL y)
		{
			if (this == TVL.TRUE)
				return y;
			else if (this == TVL.FALSE)
				return TVL.FALSE;
			else // this == TVL.UNKNOWN
				return (y == TVL.FALSE) ? TVL.FALSE : TVL.UNKNOWN;
		}
		
		/*
		 * T = TRUE
		 * F = FALSE
		 * U = UNKNOWN
		 * 
		 * OR truth table:
		 * x	y	x OR y
		 * --------------
		 * T	T	T
		 * T	F	T
		 * T	U	T
		 * F	T	T
		 * F	F	F
		 * F	U	U
		 * U	T	T
		 * U	F	U
		 * U	U	U
		 * 
		 * simplifies to:
		 * OR truth table:
		 * x	y	x OR y
		 * --------------
		 * T	X	T
		 * F	T	T
		 * F	F	F
		 * F	U	U
		 * U	T	T
		 * U	X	U
		 * 
		 */
		
		public TVL or(TVL y)
		{
			if (this == TVL.TRUE)
				return TRUE;
			else if (this == TVL.FALSE)
				return y;
			else // if this == UNKNOWN
				return (y == TRUE) ? TRUE : UNKNOWN;
		}
		
		/*
		 * T = TRUE
		 * F = FALSE
		 * U = UNKNOWN
		 * 
		 * NOT truth table:
		 * x	NOT x
		 * ---------
		 * T	F
		 * F	T
		 * U	U
		 */
		
		public TVL not()
		{
			if (this == TVL.TRUE)
				return FALSE;
			else if (this == TVL.FALSE)
				return TRUE;
			else // if this == UNKNOWN
				return UNKNOWN;
		}
		
		
		public static void main(String[] args) {
			
			System.out.println("Three Valued Logic AND Truth Table");
			for(TVL x:TVL.values())
				for(TVL y:TVL.values())
					System.out.println(x + " AND " + y + " = " + x.and(y));
				
			System.out.println("Three Valued Logic OR Truth Table");

			for(TVL x:TVL.values())
				for(TVL y:TVL.values())
					System.out.println(x + " OR " + y + " = " + x.or(y));
				
			System.out.println("Three Valued Logic NOT Truth Table");

			for(TVL x:TVL.values())
				System.out.println("NOT " + x + " = " + x.not());
			
			for(TVL x:TVL.values())
				System.out.println("to PG Bool " + x + " = " + x.toPGBoolString());
			
			for(TVL x:TVL.values())
				System.out.println("from PG Bool " + x.toPGBoolString() + " = " + x.fromString(x.toPGBoolString()));
			
		}
}