add another totally useful test

core/java/test/net/i2p/data/SimpleDataStructureTest.java

+package net.i2p.data;
+/*
+ * free (adj.): unencumbered; not under the control of others
+ * Written by jrandom in 2003 and released into the public domain 
+ * with no warranty of any kind, either expressed or implied.  
+ * It probably won't make your computer catch on fire, or eat 
+ * your children, but it might.  Use at your own risk.
+ *
+ */
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+
+import junit.framework.TestCase;
+
+/**
+ * Test harness for the simple data structure
+ *
+ * @author welterde
+ */
+public class SimpleDataStructureTest extends TestCase {
+    
+    public void testSetDataImmutable() throws Exception {
+        // create new test subject
+        TestStruct struct = new TestStruct();
+        
+        // try to set null object.. should not fail..
+        struct.setData(null);
+        
+        // set data to something
+        struct.setData(new byte[3]);
+        
+        // now setting it to null should fail
+        try {
+            struct.setData(null);
+            fail("Should not have allowed us to change this..");
+        } catch(RuntimeException exc) {
+            // all good
+        }
+        
+        // setting it to something non-null should fail as well.
+        try {
+            struct.setData(new byte[3]);
+            fail("Should not have allowed us to change this..");
+        } catch(RuntimeException exc) {
+            // all good
+        }
+    }
+    
+    public void testReadBytesImmutable() throws Exception {
+        // create new test subject
+        TestStruct struct = new TestStruct();
+        
+        // load some data using setData
+        struct.setData(new byte[3]);
+        
+        // now try to load via readBytes
+        try {
+            struct.readBytes(null);
+            fail("blah blah blah..");
+        } catch(RuntimeException exc) {
+            // all good
+        }
+    }
+    
+    public void testToBase64Safe() throws Exception {
+        // create new test subject
+        TestStruct struct = new TestStruct();
+        
+        // now try to get the Base64.. should not throw an exception, but should not be an empty string either
+        assertNull(struct.toBase64());
+    }
+
+    public void testCalculateHashSafe() throws Exception {
+        // create new test subject
+        TestStruct struct = new TestStruct();
+        
+        // now try to get the hash.. should not throw an exception
+        assertNull(struct.calculateHash());
+    }
+    
+    public void testHashCodeSafe() throws Exception {
+        // create new test subject
+        TestStruct struct = new TestStruct();
+        
+        // just make sure it doesn't explode in our face
+        struct.hashCode();
+    }
+    
+    public class TestStruct extends SimpleDataStructure {
+        public int length() {
+            return 3;
+        }
+    }
+    
+}
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