Added JavaStreamUtilities with a predicate to test if a value is th…

…e first value the predicate ever saw.

Marking older issues as fixed.

Fixes #2,#3,#4,#6

pom.xml

@@ -3,7 +3,7 @@
     <modelVersion>4.0.0</modelVersion>
     <artifactId>julielab-java-utilities</artifactId>
     <packaging>jar</packaging>
-    <version>1.3.2</version>
+    <version>1.4.0-SNAPSHOT</version>
     <name>julielab-java-utilities</name>
     <url>http://maven.apache.org</url>
     <dependencies>

src/main/java/de/julielab/java/utilities/JavaStreamUtilities.java

@@ -0,0 +1,30 @@
+package de.julielab.java.utilities;
+
+import java.util.function.Function;
+import java.util.function.Predicate;
+
+public class JavaStreamUtilities {
+    /**
+     * A predicate that tests true for all values that equal the first value it saw.
+     *
+     * @param valueFunction A function to retrieve the value.
+     * @param <T>           The input type.
+     * @param <E>           The value type for comparison.
+     * @return The predicate.
+     */
+    public static <T, E> Predicate equalsFirstSeenValue(Function<T, E> valueFunction) {
+        return new Predicate<T>() {
+            private E firstValue = null;
+
+            @Override
+            public boolean test(T entry) {
+                E value = valueFunction.apply(entry);
+                if (firstValue == null) {
+                    firstValue = value;
+                    return true;
+                }
+                return value.equals(firstValue);
+            }
+        };
+    }
+}