Java I/O How to - Compare text file line by line

Question

We would like to know how to compare text file line by line.
Answer

  /*from   ww  w.ja  v  a 2s.  c o m*/


/*   
* Copyright 2004 The Apache Software Foundation
*
*   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.
*/
//based on Apache technology http://www.apache.org

import java.io.IOException;
import java.io.LineNumberReader;
import java.io.Reader;
import java.util.List;


public class Diff
{
    public static void readersAsText(Reader r1, String name1, Reader r2, String name2,
        List diffs)
        throws IOException
    {
        LineNumberReader reader1 = new LineNumberReader(r1);
        LineNumberReader reader2 = new LineNumberReader(r2);
        String line1 = reader1.readLine();
        String line2 = reader2.readLine();
        while (line1 != null && line2 != null)
        {
            if (!line1.equals(line2))
            {
                diffs.add("File \"" + name1 + "\" and file \"" +
                    name2 + "\" differ at line " + reader1.getLineNumber() +
                    ":" + "\n" + line1 + "\n" + line2);
                break;
            }
            line1 = reader1.readLine();
            line2 = reader2.readLine();
        }
        if (line1 == null && line2 != null)
            diffs.add("File \"" + name2 + "\" has extra lines at line " +
                reader2.getLineNumber() + ":\n" + line2);
        if (line1 != null && line2 == null)
            diffs.add("File \"" + name1 + "\" has extra lines at line " +
                reader1.getLineNumber() + ":\n" + line1);
    }
}