diff --git a/java/Makefile b/java/Makefile index bffca4b27..4f0b2cbfc 100644 --- a/java/Makefile +++ b/java/Makefile @@ -64,6 +64,7 @@ JAVA_TESTS = org.rocksdb.BackupableDBOptionsTest\ org.rocksdb.BackupEngineTest\ org.rocksdb.BackupableDBTest\ org.rocksdb.BlockBasedTableConfigTest\ + org.rocksdb.util.BytewiseComparatorTest\ org.rocksdb.CheckPointTest\ org.rocksdb.ColumnFamilyOptionsTest\ org.rocksdb.ColumnFamilyTest\ diff --git a/java/benchmark/src/main/java/org/rocksdb/benchmark/DbBenchmark.java b/java/benchmark/src/main/java/org/rocksdb/benchmark/DbBenchmark.java index fde2824b7..32146bcf9 100644 --- a/java/benchmark/src/main/java/org/rocksdb/benchmark/DbBenchmark.java +++ b/java/benchmark/src/main/java/org/rocksdb/benchmark/DbBenchmark.java @@ -21,10 +21,14 @@ */ package org.rocksdb.benchmark; +import java.io.IOException; import java.lang.Runnable; import java.lang.Math; import java.io.File; +import java.lang.reflect.Constructor; +import java.lang.reflect.InvocationTargetException; import java.nio.ByteBuffer; +import java.nio.file.Files; import java.util.Collection; import java.util.Date; import java.util.EnumMap; @@ -601,6 +605,11 @@ public class DbBenchmark { (Integer)flags_.get(Flag.max_successive_merges)); options.setWalTtlSeconds((Long)flags_.get(Flag.wal_ttl_seconds)); options.setWalSizeLimitMB((Long)flags_.get(Flag.wal_size_limit_MB)); + if(flags_.get(Flag.java_comparator) != null) { + options.setComparator( + (AbstractComparator)flags_.get(Flag.java_comparator)); + } + /* TODO(yhchiang): enable the following parameters options.setCompressionType((String)flags_.get(Flag.compression_type)); options.setCompressionLevel((Integer)flags_.get(Flag.compression_level)); @@ -774,6 +783,7 @@ public class DbBenchmark { } private void open(Options options) throws RocksDBException { + System.out.println("Using database directory: " + databaseDir_); db_ = RocksDB.open(options, databaseDir_); } @@ -1475,7 +1485,7 @@ public class DbBenchmark { return Integer.parseInt(value); } }, - db("/tmp/rocksdbjni-bench", + db(getTempDir("rocksdb-jni"), "Use the db with the following name.") { @Override public Object parseValue(String value) { return value; @@ -1486,6 +1496,31 @@ public class DbBenchmark { @Override public Object parseValue(String value) { return parseBoolean(value); } + }, + java_comparator(null, "Class name of a Java Comparator to use instead\n" + + "\tof the default C++ ByteWiseComparatorImpl. Must be available on\n" + + "\tthe classpath") { + @Override + protected Object parseValue(final String value) { + try { + final ComparatorOptions copt = new ComparatorOptions(); + final Class clsComparator = + (Class)Class.forName(value); + final Constructor cstr = + clsComparator.getConstructor(ComparatorOptions.class); + return cstr.newInstance(copt); + } catch(final ClassNotFoundException cnfe) { + throw new IllegalArgumentException("Java Comparator '" + value + "'" + + " not found on the classpath", cnfe); + } catch(final NoSuchMethodException nsme) { + throw new IllegalArgumentException("Java Comparator '" + value + "'" + + " does not have a public ComparatorOptions constructor", nsme); + } catch(final IllegalAccessException | InstantiationException + | InvocationTargetException ie) { + throw new IllegalArgumentException("Unable to construct Java" + + " Comparator '" + value + "'", ie); + } + } }; private Flag(Object defaultValue, String desc) { @@ -1516,6 +1551,18 @@ public class DbBenchmark { private final String desc_; } + private final static String DEFAULT_TEMP_DIR = "/tmp"; + + private static String getTempDir(final String dirName) { + try { + return Files.createTempDirectory(dirName).toAbsolutePath().toString(); + } catch(final IOException ioe) { + System.err.println("Unable to create temp directory, defaulting to: " + + DEFAULT_TEMP_DIR); + return DEFAULT_TEMP_DIR + File.pathSeparator + dirName; + } + } + private static class RandomGenerator { private final byte[] data_; private int dataLength_; diff --git a/java/src/main/java/org/rocksdb/AbstractSlice.java b/java/src/main/java/org/rocksdb/AbstractSlice.java index c5fd2f58c..b6335a5f4 100644 --- a/java/src/main/java/org/rocksdb/AbstractSlice.java +++ b/java/src/main/java/org/rocksdb/AbstractSlice.java @@ -24,7 +24,7 @@ package org.rocksdb; * C++ BaseComparatorJniCallback subclass, which in turn destroys the * Java @see org.rocksdb.AbstractSlice subclass Objects. */ -abstract class AbstractSlice extends RocksMutableObject { +public abstract class AbstractSlice extends RocksMutableObject { protected AbstractSlice() { super(); diff --git a/java/src/main/java/org/rocksdb/util/BytewiseComparator.java b/java/src/main/java/org/rocksdb/util/BytewiseComparator.java new file mode 100644 index 000000000..17337bfc8 --- /dev/null +++ b/java/src/main/java/org/rocksdb/util/BytewiseComparator.java @@ -0,0 +1,91 @@ +// Copyright (c) 2011-present, Facebook, Inc. All rights reserved. +// This source code is licensed under the BSD-style license found in the +// LICENSE file in the root directory of this source tree. An additional grant +// of patent rights can be found in the PATENTS file in the same directory. + +package org.rocksdb.util; + +import org.rocksdb.*; + +import java.nio.ByteBuffer; + +/** + * This is a Java Native implementation of the C++ + * equivalent BytewiseComparatorImpl using {@link Slice} + * + * The performance of Comparators implemented in Java is always + * less than their C++ counterparts due to the bridging overhead, + * as such you likely don't want to use this apart from benchmarking + * and you most likely instead wanted + * {@link org.rocksdb.BuiltinComparator#BYTEWISE_COMPARATOR} + */ +public class BytewiseComparator extends Comparator { + + public BytewiseComparator(final ComparatorOptions copt) { + super(copt); + } + + @Override + public String name() { + return "rocksdb.java.BytewiseComparator"; + } + + @Override + public int compare(final Slice a, final Slice b) { + return compare(a.data(), b.data()); + } + + @Override + public String findShortestSeparator(final String start, + final Slice limit) { + final byte[] startBytes = start.getBytes(); + final byte[] limitBytes = limit.data(); + + // Find length of common prefix + final int min_length = Math.min(startBytes.length, limit.size()); + int diff_index = 0; + while ((diff_index < min_length) && + (startBytes[diff_index] == limitBytes[diff_index])) { + diff_index++; + } + + if (diff_index >= min_length) { + // Do not shorten if one string is a prefix of the other + } else { + final byte diff_byte = startBytes[diff_index]; + if(diff_byte < 0xff && diff_byte + 1 < limitBytes[diff_index]) { + final byte shortest[] = new byte[diff_index + 1]; + System.arraycopy(startBytes, 0, shortest, 0, diff_index + 1); + shortest[diff_index]++; + assert(compare(shortest, limitBytes) < 0); + return new String(shortest); + } + } + + return null; + } + + private static int compare(final byte[] a, final byte[] b) { + return ByteBuffer.wrap(a).compareTo(ByteBuffer.wrap(b)); + } + + @Override + public String findShortSuccessor(final String key) { + final byte[] keyBytes = key.getBytes(); + + // Find first character that can be incremented + final int n = keyBytes.length; + for (int i = 0; i < n; i++) { + final byte byt = keyBytes[i]; + if (byt != 0xff) { + final byte shortSuccessor[] = new byte[i + 1]; + System.arraycopy(keyBytes, 0, shortSuccessor, 0, i + 1); + shortSuccessor[i]++; + return new String(shortSuccessor); + } + } + // *key is a run of 0xffs. Leave it alone. + + return null; + } +} diff --git a/java/src/main/java/org/rocksdb/util/DirectBytewiseComparator.java b/java/src/main/java/org/rocksdb/util/DirectBytewiseComparator.java new file mode 100644 index 000000000..170f0f42e --- /dev/null +++ b/java/src/main/java/org/rocksdb/util/DirectBytewiseComparator.java @@ -0,0 +1,88 @@ +// Copyright (c) 2011-present, Facebook, Inc. All rights reserved. +// This source code is licensed under the BSD-style license found in the +// LICENSE file in the root directory of this source tree. An additional grant +// of patent rights can be found in the PATENTS file in the same directory. + +package org.rocksdb.util; + +import org.rocksdb.ComparatorOptions; +import org.rocksdb.DirectComparator; +import org.rocksdb.DirectSlice; + +import java.nio.ByteBuffer; + +/** + * This is a Java Native implementation of the C++ + * equivalent BytewiseComparatorImpl using {@link DirectSlice} + * + * The performance of Comparators implemented in Java is always + * less than their C++ counterparts due to the bridging overhead, + * as such you likely don't want to use this apart from benchmarking + * and you most likely instead wanted + * {@link org.rocksdb.BuiltinComparator#BYTEWISE_COMPARATOR} + */ +public class DirectBytewiseComparator extends DirectComparator { + + public DirectBytewiseComparator(final ComparatorOptions copt) { + super(copt); + } + + @Override + public String name() { + return "rocksdb.java.DirectBytewiseComparator"; + } + + @Override + public int compare(final DirectSlice a, final DirectSlice b) { + return a.data().compareTo(b.data()); + } + + @Override + public String findShortestSeparator(final String start, + final DirectSlice limit) { + final byte[] startBytes = start.getBytes(); + + // Find length of common prefix + final int min_length = Math.min(startBytes.length, limit.size()); + int diff_index = 0; + while ((diff_index < min_length) && + (startBytes[diff_index] == limit.get(diff_index))) { + diff_index++; + } + + if (diff_index >= min_length) { + // Do not shorten if one string is a prefix of the other + } else { + final byte diff_byte = startBytes[diff_index]; + if(diff_byte < 0xff && diff_byte + 1 < limit.get(diff_index)) { + final byte shortest[] = new byte[diff_index + 1]; + System.arraycopy(startBytes, 0, shortest, 0, diff_index + 1); + shortest[diff_index]++; + assert(ByteBuffer.wrap(shortest).compareTo(limit.data()) < 0); + return new String(shortest); + } + } + + return null; + } + + @Override + public String findShortSuccessor(final String key) { + final byte[] keyBytes = key.getBytes(); + + // Find first character that can be incremented + final int n = keyBytes.length; + for (int i = 0; i < n; i++) { + final byte byt = keyBytes[i]; + if (byt != 0xff) { + final byte shortSuccessor[] = new byte[i + 1]; + System.arraycopy(keyBytes, 0, shortSuccessor, 0, i + 1); + shortSuccessor[i]++; + return new String(shortSuccessor); + } + } + // *key is a run of 0xffs. Leave it alone. + + return null; + } +} diff --git a/java/src/main/java/org/rocksdb/util/ReverseBytewiseComparator.java b/java/src/main/java/org/rocksdb/util/ReverseBytewiseComparator.java new file mode 100644 index 000000000..beedc185d --- /dev/null +++ b/java/src/main/java/org/rocksdb/util/ReverseBytewiseComparator.java @@ -0,0 +1,37 @@ +// Copyright (c) 2011-present, Facebook, Inc. All rights reserved. +// This source code is licensed under the BSD-style license found in the +// LICENSE file in the root directory of this source tree. An additional grant +// of patent rights can be found in the PATENTS file in the same directory. + +package org.rocksdb.util; + +import org.rocksdb.BuiltinComparator; +import org.rocksdb.ComparatorOptions; +import org.rocksdb.Slice; + +/** + * This is a Java Native implementation of the C++ + * equivalent ReverseBytewiseComparatorImpl using {@link Slice} + * + * The performance of Comparators implemented in Java is always + * less than their C++ counterparts due to the bridging overhead, + * as such you likely don't want to use this apart from benchmarking + * and you most likely instead wanted + * {@link BuiltinComparator#REVERSE_BYTEWISE_COMPARATOR} + */ +public class ReverseBytewiseComparator extends BytewiseComparator { + + public ReverseBytewiseComparator(final ComparatorOptions copt) { + super(copt); + } + + @Override + public String name() { + return "rocksdb.java.ReverseBytewiseComparator"; + } + + @Override + public int compare(final Slice a, final Slice b) { + return -super.compare(a, b); + } +} diff --git a/java/src/test/java/org/rocksdb/util/BytewiseComparatorTest.java b/java/src/test/java/org/rocksdb/util/BytewiseComparatorTest.java new file mode 100644 index 000000000..01ea52c49 --- /dev/null +++ b/java/src/test/java/org/rocksdb/util/BytewiseComparatorTest.java @@ -0,0 +1,480 @@ +// Copyright (c) 2011-present, Facebook, Inc. All rights reserved. +// This source code is licensed under the BSD-style license found in the +// LICENSE file in the root directory of this source tree. An additional grant +// of patent rights can be found in the PATENTS file in the same directory. + +package org.rocksdb.util; + +import org.junit.Test; +import org.rocksdb.*; +import org.rocksdb.Comparator; + +import java.io.IOException; +import java.nio.charset.StandardCharsets; +import java.nio.file.FileVisitResult; +import java.nio.file.Files; +import java.nio.file.Path; +import java.nio.file.SimpleFileVisitor; +import java.nio.file.attribute.BasicFileAttributes; +import java.util.*; + +import static org.junit.Assert.*; + +/** + * This is a direct port of various C++ + * tests from db/comparator_db_test.cc + * and some code to adapt it to RocksJava + */ +public class BytewiseComparatorTest { + + /** + * Open the database using the C++ BytewiseComparatorImpl + * and test the results against our Java BytewiseComparator + */ + @Test + public void java_vs_cpp_bytewiseComparator() + throws IOException, RocksDBException { + for(int rand_seed = 301; rand_seed < 306; rand_seed++) { + final Path dbDir = Files.createTempDirectory("comparator_db_test"); + try(final RocksDB db = openDatabase(dbDir, + BuiltinComparator.BYTEWISE_COMPARATOR)) { + final Random rnd = new Random(rand_seed); + doRandomIterationTest( + db, + toJavaComparator(new BytewiseComparator(new ComparatorOptions())), + Arrays.asList("a", "b", "c", "d", "e", "f", "g", "h", "i"), + rnd, + 8, 100, 3 + ); + } finally { + removeData(dbDir); + } + } + } + + /** + * Open the database using the Java BytewiseComparator + * and test the results against another Java BytewiseComparator + */ + @Test + public void java_vs_java_bytewiseComparator() + throws IOException, RocksDBException { + for(int rand_seed = 301; rand_seed < 306; rand_seed++) { + final Path dbDir = Files.createTempDirectory("comparator_db_test"); + try(final RocksDB db = openDatabase(dbDir, new BytewiseComparator( + new ComparatorOptions()))) { + final Random rnd = new Random(rand_seed); + doRandomIterationTest( + db, + toJavaComparator(new BytewiseComparator(new ComparatorOptions())), + Arrays.asList("a", "b", "c", "d", "e", "f", "g", "h", "i"), + rnd, + 8, 100, 3 + ); + } finally { + removeData(dbDir); + } + } + } + + /** + * Open the database using the C++ BytewiseComparatorImpl + * and test the results against our Java DirectBytewiseComparator + */ + @Test + public void java_vs_cpp_directBytewiseComparator() + throws IOException, RocksDBException { + for(int rand_seed = 301; rand_seed < 306; rand_seed++) { + final Path dbDir = Files.createTempDirectory("comparator_db_test"); + try(final RocksDB db = openDatabase(dbDir, + BuiltinComparator.BYTEWISE_COMPARATOR)) { + final Random rnd = new Random(rand_seed); + doRandomIterationTest( + db, + toJavaComparator(new DirectBytewiseComparator( + new ComparatorOptions()) + ), + Arrays.asList("a", "b", "c", "d", "e", "f", "g", "h", "i"), + rnd, + 8, 100, 3 + ); + } finally { + removeData(dbDir); + } + } + } + + /** + * Open the database using the Java DirectBytewiseComparator + * and test the results against another Java DirectBytewiseComparator + */ + @Test + public void java_vs_java_directBytewiseComparator() + throws IOException, RocksDBException { + for(int rand_seed = 301; rand_seed < 306; rand_seed++) { + final Path dbDir = Files.createTempDirectory("comparator_db_test"); + try(final RocksDB db = openDatabase(dbDir, new DirectBytewiseComparator( + new ComparatorOptions()))) { + final Random rnd = new Random(rand_seed); + doRandomIterationTest( + db, + toJavaComparator(new DirectBytewiseComparator( + new ComparatorOptions()) + ), + Arrays.asList("a", "b", "c", "d", "e", "f", "g", "h", "i"), + rnd, + 8, 100, 3 + ); + } finally { + removeData(dbDir); + } + } + } + + /** + * Open the database using the C++ ReverseBytewiseComparatorImpl + * and test the results against our Java ReverseBytewiseComparator + */ + @Test + public void java_vs_cpp_reverseBytewiseComparator() + throws IOException, RocksDBException { + for(int rand_seed = 301; rand_seed < 306; rand_seed++) { + final Path dbDir = Files.createTempDirectory("comparator_db_test"); + try(final RocksDB db = openDatabase(dbDir, + BuiltinComparator.REVERSE_BYTEWISE_COMPARATOR)) { + final Random rnd = new Random(rand_seed); + doRandomIterationTest( + db, + toJavaComparator( + new ReverseBytewiseComparator(new ComparatorOptions()) + ), + Arrays.asList("a", "b", "c", "d", "e", "f", "g", "h", "i"), + rnd, + 8, 100, 3 + ); + } finally { + removeData(dbDir); + } + } + } + + /** + * Open the database using the Java ReverseBytewiseComparator + * and test the results against another Java ReverseBytewiseComparator + */ + @Test + public void java_vs_java_reverseBytewiseComparator() + throws IOException, RocksDBException { + + for(int rand_seed = 301; rand_seed < 306; rand_seed++) { + final Path dbDir = Files.createTempDirectory("comparator_db_test"); + try(final RocksDB db = openDatabase(dbDir, new ReverseBytewiseComparator( + new ComparatorOptions()))) { + final Random rnd = new Random(rand_seed); + doRandomIterationTest( + db, + toJavaComparator( + new ReverseBytewiseComparator(new ComparatorOptions()) + ), + Arrays.asList("a", "b", "c", "d", "e", "f", "g", "h", "i"), + rnd, + 8, 100, 3 + ); + } finally { + removeData(dbDir); + } + } + } + + private void doRandomIterationTest( + final RocksDB db, final java.util.Comparator javaComparator, + final List source_strings, final Random rnd, + final int num_writes, final int num_iter_ops, + final int num_trigger_flush) throws RocksDBException { + + final TreeMap map = new TreeMap<>(javaComparator); + + for (int i = 0; i < num_writes; i++) { + if (num_trigger_flush > 0 && i != 0 && i % num_trigger_flush == 0) { + db.flush(new FlushOptions()); + } + + final int type = rnd.nextInt(2); + final int index = rnd.nextInt(source_strings.size()); + final String key = source_strings.get(index); + switch (type) { + case 0: + // put + map.put(key, key); + db.put(new WriteOptions(), bytes(key), bytes(key)); + break; + case 1: + // delete + if (map.containsKey(key)) { + map.remove(key); + } + db.remove(new WriteOptions(), bytes(key)); + break; + + default: + fail("Should not be able to generate random outside range 1..2"); + } + } + + try(final RocksIterator iter = db.newIterator(new ReadOptions())) { + final KVIter result_iter = new KVIter(map); + + boolean is_valid = false; + for (int i = 0; i < num_iter_ops; i++) { + // Random walk and make sure iter and result_iter returns the + // same key and value + final int type = rnd.nextInt(6); + iter.status(); + switch (type) { + case 0: + // Seek to First + iter.seekToFirst(); + result_iter.seekToFirst(); + break; + case 1: + // Seek to last + iter.seekToLast(); + result_iter.seekToLast(); + break; + case 2: { + // Seek to random key + final int key_idx = rnd.nextInt(source_strings.size()); + final String key = source_strings.get(key_idx); + iter.seek(bytes(key)); + result_iter.seek(bytes(key)); + break; + } + case 3: + // Next + if (is_valid) { + iter.next(); + result_iter.next(); + } else { + continue; + } + break; + case 4: + // Prev + if (is_valid) { + iter.prev(); + result_iter.prev(); + } else { + continue; + } + break; + default: { + assert (type == 5); + final int key_idx = rnd.nextInt(source_strings.size()); + final String key = source_strings.get(key_idx); + final byte[] result = db.get(new ReadOptions(), bytes(key)); + if (!map.containsKey(key)) { + assertNull(result); + } else { + assertArrayEquals(bytes(map.get(key)), result); + } + break; + } + } + + assertEquals(result_iter.isValid(), iter.isValid()); + + is_valid = iter.isValid(); + + if (is_valid) { + assertArrayEquals(bytes(result_iter.key()), iter.key()); + + //note that calling value on a non-valid iterator from the Java API + //results in a SIGSEGV + assertArrayEquals(bytes(result_iter.value()), iter.value()); + } + } + } + } + + /** + * Open the database using a C++ Comparator + */ + private RocksDB openDatabase( + final Path dbDir, final BuiltinComparator cppComparator) + throws IOException, RocksDBException { + final Options options = new Options() + .setCreateIfMissing(true) + .setComparator(cppComparator); + return RocksDB.open(options, dbDir.toAbsolutePath().toString()); + } + + /** + * Open the database using a Java Comparator + */ + private RocksDB openDatabase( + final Path dbDir, + final AbstractComparator> javaComparator) + throws IOException, RocksDBException { + final Options options = new Options() + .setCreateIfMissing(true) + .setComparator(javaComparator); + return RocksDB.open(options, dbDir.toAbsolutePath().toString()); + } + + private void closeDatabase(final RocksDB db) { + db.close(); + } + + private void removeData(final Path dbDir) throws IOException { + Files.walkFileTree(dbDir, new SimpleFileVisitor() { + @Override + public FileVisitResult visitFile( + final Path file, final BasicFileAttributes attrs) + throws IOException { + Files.delete(file); + return FileVisitResult.CONTINUE; + } + + @Override + public FileVisitResult postVisitDirectory( + final Path dir, final IOException exc) throws IOException { + Files.delete(dir); + return FileVisitResult.CONTINUE; + } + }); + } + + private byte[] bytes(final String s) { + return s.getBytes(StandardCharsets.UTF_8); + } + + private java.util.Comparator toJavaComparator( + final Comparator rocksComparator) { + return new java.util.Comparator() { + @Override + public int compare(final String s1, final String s2) { + return rocksComparator.compare(new Slice(s1), new Slice(s2)); + } + }; + } + + private java.util.Comparator toJavaComparator( + final DirectComparator rocksComparator) { + return new java.util.Comparator() { + @Override + public int compare(final String s1, final String s2) { + return rocksComparator.compare(new DirectSlice(s1), + new DirectSlice(s2)); + } + }; + } + + private class KVIter implements RocksIteratorInterface { + + private final List> entries; + private final java.util.Comparator comparator; + private int offset = -1; + + private int lastPrefixMatchIdx = -1; + private int lastPrefixMatch = 0; + + public KVIter(final TreeMap map) { + this.entries = new ArrayList<>(); + final Iterator> iterator = map.entrySet().iterator(); + while(iterator.hasNext()) { + entries.add(iterator.next()); + } + this.comparator = map.comparator(); + } + + + @Override + public boolean isValid() { + return offset > -1 && offset < entries.size(); + } + + @Override + public void seekToFirst() { + offset = 0; + } + + @Override + public void seekToLast() { + offset = entries.size() - 1; + } + + @Override + public void seek(final byte[] target) { + for(offset = 0; offset < entries.size(); offset++) { + if(comparator.compare(entries.get(offset).getKey(), + (K)new String(target, StandardCharsets.UTF_8)) >= 0) { + return; + } + } + } + + /** + * Is `a` a prefix of `b` + * + * @return The length of the matching prefix, or 0 if it is not a prefix + */ + private int isPrefix(final byte[] a, final byte[] b) { + if(b.length >= a.length) { + for(int i = 0; i < a.length; i++) { + if(a[i] != b[i]) { + return i; + } + } + return a.length; + } else { + return 0; + } + } + + @Override + public void next() { + if(offset < entries.size()) { + offset++; + } + } + + @Override + public void prev() { + if(offset >= 0) { + offset--; + } + } + + @Override + public void status() throws RocksDBException { + if(offset < 0 || offset >= entries.size()) { + throw new RocksDBException("Index out of bounds. Size is: " + + entries.size() + ", offset is: " + offset); + } + } + + public K key() { + if(!isValid()) { + if(entries.isEmpty()) { + return (K)""; + } else if(offset == -1){ + return entries.get(0).getKey(); + } else if(offset == entries.size()) { + return entries.get(offset - 1).getKey(); + } else { + return (K)""; + } + } else { + return entries.get(offset).getKey(); + } + } + + public V value() { + if(!isValid()) { + return (V)""; + } else { + return entries.get(offset).getValue(); + } + } + } +}