add a check of java version string >=1.7 to startup class

stopping start with error msg on version < 1.7

source/net/yacy/yacy.java

@@ -135,6 +135,21 @@ public final class yacy {
             // check java version
             try {
                 "a".isEmpty(); // needs at least Java 1.6
+
+                // check java version string (required min 1.7)
+                final String javaVersion = System.getProperty("java.version");
+                if (javaVersion != null) { // unknown property !?
+                    int pos = javaVersion.indexOf('.');
+                    int count = 1;
+                    for (; pos < javaVersion.length() && count < 2; pos++) {
+                        if (javaVersion.charAt(pos + 1) == '.') count++;
+                    }
+                    Double dVersion = Double.parseDouble(javaVersion.substring(0, pos));
+                    if (dVersion < 1.7) { // required min java 1.7
+                        System.err.println("STARTUP: Java Version too low. You need at least Java 1.7 to run YaCy");
+                        System.exit(-1);
+                    }
+                }
             } catch (final NoSuchMethodError e) {
                 System.err.println("STARTUP: Java Version too low. You need at least Java 1.7 to run YaCy");
                 System.exit(-1);