Replace null checks with Objects.requireNonNull

Closes #45
diff --git a/indexer-reader/src/main/java/org/apache/maven/index/reader/IndexReader.java b/indexer-reader/src/main/java/org/apache/maven/index/reader/IndexReader.java
index 54a5c92..b2c11f9 100644
--- a/indexer-reader/src/main/java/org/apache/maven/index/reader/IndexReader.java
+++ b/indexer-reader/src/main/java/org/apache/maven/index/reader/IndexReader.java
@@ -29,6 +29,7 @@
 import java.util.Date;
 import java.util.Iterator;
 import java.util.List;
+import java.util.Objects;
 import java.util.Properties;
 
 import static org.apache.maven.index.reader.Utils.loadProperties;
@@ -62,10 +63,7 @@
     public IndexReader( final WritableResourceHandler local, final ResourceHandler remote )
         throws IOException
     {
-        if ( remote == null )
-        {
-            throw new NullPointerException( "remote resource handler null" );
-        }
+        Objects.requireNonNull( remote, "remote resource handler null" );
         this.local = local;
         this.remote = remote;
         remoteIndexProperties = loadProperties( remote.locate( Utils.INDEX_FILE_PREFIX + ".properties" ) );
diff --git a/indexer-reader/src/main/java/org/apache/maven/index/reader/IndexWriter.java b/indexer-reader/src/main/java/org/apache/maven/index/reader/IndexWriter.java
index dd91510..6dbae48 100644
--- a/indexer-reader/src/main/java/org/apache/maven/index/reader/IndexWriter.java
+++ b/indexer-reader/src/main/java/org/apache/maven/index/reader/IndexWriter.java
@@ -27,6 +27,7 @@
 import java.util.Date;
 import java.util.Iterator;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Properties;
 import java.util.UUID;
 
@@ -59,14 +60,8 @@
     public IndexWriter( final WritableResourceHandler local, final String indexId, final boolean incrementalSupported )
         throws IOException
     {
-        if ( local == null )
-        {
-            throw new NullPointerException( "local resource handler null" );
-        }
-        if ( indexId == null )
-        {
-            throw new NullPointerException( "indexId null" );
-        }
+        Objects.requireNonNull( local, "local resource handler null" );
+        Objects.requireNonNull( indexId, "indexId null" );
         this.local = local;
         Properties indexProperties = loadProperties( local.locate( Utils.INDEX_FILE_PREFIX + ".properties" ) );
         if ( incrementalSupported && indexProperties != null )
diff --git a/indexer-reader/src/main/java/org/apache/maven/index/reader/Record.java b/indexer-reader/src/main/java/org/apache/maven/index/reader/Record.java
index 6b502b5..046ff9b 100644
--- a/indexer-reader/src/main/java/org/apache/maven/index/reader/Record.java
+++ b/indexer-reader/src/main/java/org/apache/maven/index/reader/Record.java
@@ -20,6 +20,7 @@
  */
 
 import java.util.Map;
+import java.util.Objects;
 
 /**
  * Maven 2 Index record.
@@ -41,14 +42,8 @@
 
         public EntryKey( final String name, final Class<T> proto )
         {
-            if ( name == null )
-            {
-                throw new NullPointerException( "name is null" );
-            }
-            if ( proto == null )
-            {
-                throw new NullPointerException( "proto is null" );
-            }
+            Objects.requireNonNull( name, "name is null" );
+            Objects.requireNonNull( proto, "proto is null" );
             this.name = name;
             this.proto = proto;
         }