[ARROW-101] Fix java compiler rawtypes warnings

Java compiler emits several warnings regarding the use of raw types.
This fixes the warnings by using generic types instead.

Author: laurentgo

java/vector/src/main/java/org/apache/arrow/vector/ZeroVector.java

@@ -19,6 +19,7 @@
 
 import io.netty.buffer.ArrowBuf;
 
+import java.util.Collections;
 import java.util.Iterator;
 
 import org.apache.arrow.memory.BufferAllocator;
@@ -109,8 +110,8 @@ public TransferPair getTransferPair(BufferAllocator allocator) {
 //  }
 
   @Override
-  public Iterator iterator() {
-    return Iterators.emptyIterator();
+  public Iterator<ValueVector> iterator() {
+    return Collections.emptyIterator();
   }
 
   @Override

java/vector/src/main/java/org/apache/arrow/vector/complex/impl/PromotableWriter.java

@@ -85,16 +85,16 @@ private void setWriter(ValueVector v) {
     state = State.SINGLE;
     vector = v;
     type = v.getField().getType().getMinorType();
-    Class writerClass = BasicTypeHelper
+    Class<?> writerClass = BasicTypeHelper
         .getWriterImpl(v.getField().getType().getMinorType(), v.getField().getDataMode());
     if (writerClass.equals(SingleListWriter.class)) {
       writerClass = UnionListWriter.class;
     }
-    Class vectorClass = BasicTypeHelper.getValueVectorClass(v.getField().getType().getMinorType(), v.getField()
+    Class<?> vectorClass = BasicTypeHelper.getValueVectorClass(v.getField().getType().getMinorType(), v.getField()
         .getDataMode());
     try {
-      Constructor constructor = null;
-      for (Constructor c : writerClass.getConstructors()) {
+      Constructor<?> constructor = null;
+      for (Constructor<?> c : writerClass.getConstructors()) {
         if (c.getParameterTypes().length == 3) {
           constructor = c;
         }

java/vector/src/main/java/org/apache/arrow/vector/util/JsonStringArrayList.java

@@ -42,7 +42,7 @@ public boolean equals(Object obj) {
     if (!(obj instanceof List)) {
       return false;
     }
-    List other = (List) obj;
+    List<?> other = (List<?>) obj;
     return this.size() == other.size() && this.containsAll(other);
   }
 

java/vector/src/main/java/org/apache/arrow/vector/util/JsonStringHashMap.java

@@ -46,7 +46,7 @@ public boolean equals(Object obj) {
     if (!(obj instanceof Map)) {
       return false;
     }
-    Map other = (Map) obj;
+    Map<?, ?> other = (Map<?, ?>) obj;
     if (this.size() != other.size()) {
       return false;
     }

java/vector/src/test/java/org/apache/arrow/vector/TestValueVector.java

@@ -535,7 +535,7 @@ public void testListVectorShouldNotThrowOversizedAllocationException() throws Ex
       vector.clear();
     }
 
-    vectorFrom.clear();
-    vector.clear();
+    vectorFrom.close();
+    vector.close();
   }
 }