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Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more.

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/*
 * Copyright (C) 2008 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.io;

import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Preconditions;
import java.io.IOException;
import java.io.Reader;
import java.util.Iterator;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * A {@link Reader} that concatenates multiple readers.
 *
 * @author Bin Zhu
 * @since 1.0
 */
@GwtIncompatible
class MultiReader extends Reader {
  private final Iterator it;
  private @Nullable Reader current;

  MultiReader(Iterator readers) throws IOException {
    this.it = readers;
    advance();
  }

  /** Closes the current reader and opens the next one, if any. */
  private void advance() throws IOException {
    close();
    if (it.hasNext()) {
      current = it.next().openStream();
    }
  }

  @Override
  public int read(char @Nullable [] cbuf, int off, int len) throws IOException {
    if (current == null) {
      return -1;
    }
    int result = current.read(cbuf, off, len);
    if (result == -1) {
      advance();
      return read(cbuf, off, len);
    }
    return result;
  }

  @Override
  public long skip(long n) throws IOException {
    Preconditions.checkArgument(n >= 0, "n is negative");
    if (n > 0) {
      while (current != null) {
        long result = current.skip(n);
        if (result > 0) {
          return result;
        }
        advance();
      }
    }
    return 0;
  }

  @Override
  public boolean ready() throws IOException {
    return (current != null) && current.ready();
  }

  @Override
  public void close() throws IOException {
    if (current != null) {
      try {
        current.close();
      } finally {
        current = null;
      }
    }
  }
}