accidental linear access times instead of constant

Methods for finding keys in the map would iterate over all keys
when the key did not exist.
Fixed by introducing a new sentinel value (-1) that is used to
mark slots that were previously occupied.

build.gradle

@@ -20,8 +20,16 @@ plugins {
 apply plugin: 'java'
 apply plugin: 'eclipse'
 
+
+ext {
+  javaVersion=11
+  version_junit = '5.7.1'
+  version_junit_platform = '1.7.1'
+}
+
+
 // java compatibility version
-sourceCompatibility = 11
+sourceCompatibility = javaVersion
 
 /*
  * The shared configuration for all sub-projects:
@@ -76,9 +84,10 @@ subprojects {
 		
 	// dependencies that all sub-projects have 
 	dependencies {
-		testImplementation 'org.junit.jupiter:junit-jupiter-api:5.7.0'
-    	testRuntime 'org.junit.jupiter:junit-jupiter-engine:5.7.0'
-    	testRuntime 'org.junit.platform:junit-platform-launcher:1.7.0' // needed by eclipse
+		testImplementation "org.junit.jupiter:junit-jupiter-api:${version_junit}"
+		testImplementation "org.junit.jupiter:junit-jupiter-params:${version_junit}"  // for @ParameterizedTest
+    	testRuntime "org.junit.jupiter:junit-jupiter-engine:${version_junit}"
+    	testRuntime "org.junit.platform:junit-platform-launcher:${version_junit_platform}" // needed by eclipse
 	}
 	
 	test {