Added the subtract method to the AssetAmount class

graphenej/src/main/java/de/bitsharesmunich/graphenej/AssetAmount.java

@@ -54,6 +54,25 @@ public class AssetAmount implements ByteSerializable, JsonSerializable {
         return new AssetAmount(combined, asset);
     }
 
+    /**
+     * Subtracts another instance of AssetAmount from this one. This method will always
+     * return absolute values.
+     * @param other: The other asset amount to subtract from this.
+     * @return: The absolute value of the subtraction of the other minus this asset amount.
+     */
+    public AssetAmount subtract(AssetAmount other){
+        if(!this.getAsset().getObjectId().equals(other.getAsset().getObjectId())){
+            throw new IncompatibleOperation("Cannot subtract two AssetAmount instances that refer to different assets");
+        }
+        UnsignedLong result = null;
+        if(this.amount.compareTo(other.getAmount()) < 0){
+            result = other.getAmount().minus(this.amount);
+        }else{
+            result = this.amount.minus(other.getAmount());
+        }
+        return new AssetAmount(result, asset);
+    }
+
     public void setAmount(UnsignedLong amount){
         this.amount = amount;
     }

graphenej/src/test/java/de/bitsharesmunich/graphenej/AssetAmountTest.java

@@ -0,0 +1,30 @@
+package de.bitsharesmunich.graphenej;
+
+import com.google.common.primitives.UnsignedLong;
+import org.junit.Before;
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+/**
+ * Created by nelson on 2/23/17.
+ */
+public class AssetAmountTest {
+    private final int LARGE_VALUE = 1000;
+    private final int SMALL_VALUE = 500;
+    private AssetAmount large;
+    private AssetAmount small;
+    private Asset testAsset = new Asset("1.3.0");
+
+    @Before
+    public void setUp(){
+        large = new AssetAmount(UnsignedLong.valueOf(LARGE_VALUE), testAsset);
+        small = new AssetAmount(UnsignedLong.valueOf(SMALL_VALUE), testAsset);
+    }
+
+    @Test
+    public void testSubtraction(){
+        assertEquals(large.subtract(small).getAmount(), new AssetAmount(UnsignedLong.valueOf(LARGE_VALUE - SMALL_VALUE), testAsset).getAmount());
+        assertEquals(small.subtract(large).getAmount(), new AssetAmount(UnsignedLong.valueOf(Math.abs(SMALL_VALUE - LARGE_VALUE)), testAsset).getAmount());
+    }
+}