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Task/Semiprime/Java/semiprime.java

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+import java.math.BigInteger;
+import java.util.ArrayList;
+import java.util.List;
+
+public class SemiPrime{
+	private static final BigInteger TWO = BigInteger.valueOf(2);
+	
+	public static List<BigInteger> primeDecomp(BigInteger a){
+	    // impossible for values lower than 2
+	    if(a.compareTo(TWO) < 0){
+	        return null;
+	    }
+	
+	    //quickly handle even values
+	    List<BigInteger> result = new ArrayList<BigInteger>();
+	    while(a.and(BigInteger.ONE).equals(BigInteger.ZERO)){
+	        a = a.shiftRight(1);
+	        result.add(TWO);
+	    }
+	
+	    //left with odd values
+	    if(!a.equals(BigInteger.ONE)){
+	        BigInteger b = BigInteger.valueOf(3);
+	        while(b.compareTo(a) < 0){
+	            if(b.isProbablePrime(10)){
+	                BigInteger[] dr = a.divideAndRemainder(b);
+	                if(dr[1].equals(BigInteger.ZERO)){
+	                    result.add(b);
+	                    a = dr[0];
+	                }
+	            }
+	            b = b.add(TWO);
+	        }
+	        result.add(b); //b will always be prime here...
+	    }
+	    return result;
+	}
+	
+	public static boolean isSemi(BigInteger x){
+		List<BigInteger> decomp = primeDecomp(x);
+		return decomp != null && decomp.size() == 2;
+	}
+	
+	public static void main(String[] args){
+		for(int i = 2; i <= 100; i++){
+			if(isSemi(BigInteger.valueOf(i))){
+				System.out.print(i + " ");
+			}
+		}
+		System.out.println();
+		for(int i = 1675; i <= 1680; i++){
+			if(isSemi(BigInteger.valueOf(i))){
+				System.out.print(i + " ");
+			}
+		}
+	}
+}