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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/licenses/publicdomain |
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*/ |
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|
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package jsr166y; |
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|
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import java.util.Random; |
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|
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/** |
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* A random number generator isolated to the current Thread. Like the |
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* global {@link java.util.Random} generator used by the {@link |
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* java.lang.Math} class, a ThreadLocalRandom is initialized with an |
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* internally generated seed that may not otherwise be modified. When |
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* applicable, use of ThreadLocalRandom rather than shared Random |
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* objects in concurrent programs will typically encounter much less |
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* overhead and contention. ThreadLocalRandoms are particularly |
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* appropriate when multiple tasks (for example, each a {@link |
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* ForkJoinTask}), use random numbers in parallel in thread pools. |
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* |
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* <p>Usages of this class should typically be of the form: |
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* {@code ThreadLocalRandom.current().nextX(...)} (where |
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* {@code X} is {@code Int}, {@code Long}, etc). |
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* When all usages are of this form, it is never possible to |
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* accidently share ThreadLocalRandoms across multiple threads. |
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* |
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* <p>This class also provides additional commonly used bounded random |
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* generation methods. |
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* |
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* @since 1.7 |
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* @author Doug Lea |
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*/ |
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public class ThreadLocalRandom extends Random { |
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// same constants as Random, but must be redeclared because private |
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private final static long multiplier = 0x5DEECE66DL; |
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private final static long addend = 0xBL; |
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private final static long mask = (1L << 48) - 1; |
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|
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/** |
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* The random seed. We can't use super.seed. |
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*/ |
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private long rnd; |
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|
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/** |
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* Initialization flag to permit the first and only allowed call |
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* to setSeed (inside Random constructor) to succeed. We can't |
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* allow others since it would cause setting seed in one part of a |
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* program to unintentionally impact other usages by the thread. |
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*/ |
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boolean initialized; |
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|
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// Padding to help avoid memory contention among seed updates in |
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// different TLRs in the common case that they are located near |
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// each other. |
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private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7; |
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|
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/** |
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* The actual ThreadLocal |
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*/ |
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private static final ThreadLocal<ThreadLocalRandom> localRandom = |
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new ThreadLocal<ThreadLocalRandom>() { |
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protected ThreadLocalRandom initialValue() { |
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return new ThreadLocalRandom(); |
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} |
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}; |
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|
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|
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/** |
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* Constructor called only by localRandom.initialValue. |
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* We rely on the fact that the superclass no-arg constructor |
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* invokes setSeed exactly once to initialize. |
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*/ |
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ThreadLocalRandom() { |
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super(); |
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} |
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|
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/** |
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* Returns the current Thread's ThreadLocalRandom. |
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* |
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* @return the current Thread's ThreadLocalRandom |
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*/ |
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public static ThreadLocalRandom current() { |
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return localRandom.get(); |
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} |
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|
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/** |
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* Throws UnsupportedOperationException. Setting seeds in this |
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* generator is unsupported. |
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* |
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* @throws UnsupportedOperationException always |
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*/ |
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public void setSeed(long seed) { |
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if (initialized) |
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throw new UnsupportedOperationException(); |
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initialized = true; |
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rnd = (seed ^ multiplier) & mask; |
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} |
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|
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protected int next(int bits) { |
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rnd = (rnd * multiplier + addend) & mask; |
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return (int) (rnd >>> (48-bits)); |
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} |
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|
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/** |
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* Returns a pseudorandom, uniformly distributed value between the |
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* given least value (inclusive) and bound (exclusive). |
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* |
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* @param least the least value returned |
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* @param bound the upper bound (exclusive) |
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* @throws IllegalArgumentException if least greater than or equal |
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* to bound |
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* @return the next value |
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*/ |
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public int nextInt(int least, int bound) { |
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if (least >= bound) |
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throw new IllegalArgumentException(); |
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return nextInt(bound - least) + least; |
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} |
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|
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/** |
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* Returns a pseudorandom, uniformly distributed value |
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* between 0 (inclusive) and the specified value (exclusive). |
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* |
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* @param n the bound on the random number to be returned. Must be |
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* positive. |
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* @return the next value |
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* @throws IllegalArgumentException if n is not positive |
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*/ |
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public long nextLong(long n) { |
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if (n <= 0) |
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throw new IllegalArgumentException("n must be positive"); |
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// Divide n by two until small enough for nextInt. On each |
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// iteration (at most 31 of them but usually much less), |
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// randomly choose both whether to include high bit in result |
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// (offset) and whether to continue with the lower vs upper |
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// half (which makes a difference only if odd). |
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long offset = 0; |
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while (n >= Integer.MAX_VALUE) { |
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int bits = next(2); |
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long half = n >>> 1; |
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long nextn = ((bits & 2) == 0) ? half : n - half; |
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if ((bits & 1) == 0) |
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offset += n - nextn; |
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n = nextn; |
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} |
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return offset + nextInt((int) n); |
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} |
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|
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/** |
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* Returns a pseudorandom, uniformly distributed value between the |
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* given least value (inclusive) and bound (exclusive). |
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* |
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* @param least the least value returned |
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* @param bound the upper bound (exclusive) |
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* @return the next value |
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* @throws IllegalArgumentException if least greater than or equal |
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* to bound |
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*/ |
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public long nextLong(long least, long bound) { |
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if (least >= bound) |
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throw new IllegalArgumentException(); |
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return nextLong(bound - least) + least; |
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} |
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|
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/** |
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* Returns a pseudorandom, uniformly distributed {@code double} value |
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* between 0 (inclusive) and the specified value (exclusive). |
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* |
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* @param n the bound on the random number to be returned. Must be |
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* positive. |
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* @return the next value |
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* @throws IllegalArgumentException if n is not positive |
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*/ |
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public double nextDouble(double n) { |
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if (n <= 0) |
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throw new IllegalArgumentException("n must be positive"); |
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return nextDouble() * n; |
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} |
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|
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/** |
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* Returns a pseudorandom, uniformly distributed value between the |
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* given least value (inclusive) and bound (exclusive). |
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* |
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* @param least the least value returned |
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* @param bound the upper bound (exclusive) |
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* @return the next value |
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* @throws IllegalArgumentException if least greater than or equal |
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* to bound |
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*/ |
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public double nextDouble(double least, double bound) { |
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if (least >= bound) |
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throw new IllegalArgumentException(); |
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return nextDouble() * (bound - least) + least; |
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} |
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|
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private static final long serialVersionUID = -5851777807851030925L; |
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} |