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Guava IO contains a variety of types and utilities to ease IO handling in Java.

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/*
 * Copyright (C) 2008 Google Inc.
 *
 * 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.base.Preconditions;

import java.io.IOException;
import java.io.Reader;
import java.util.Iterator;

/**
 * A {@link Reader} that will concatenates multiple readers
 *
 * @author Bin Zhu
 * @since 1
 */
class MultiReader extends Reader {
  private final Iterator> it;
  private 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().getInput();
    }
  }

  @Override public int read(char 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;
      }
    }
  }
}