Read each existing row in StringBuffer or String

Using StringTokenizer (assuming texto is a String ):

StringTokenizer st = new StringTokenizer(texto, "\n");
while ( st.hasMoreTokens() ) {
    String linha = st.nextToken();
    ...
}

The original text will be kept intact (as opposed to a texto.split("\n") , for example, that would double the amount of memory used), and only small strings would be created - one per line - so that in total double the memory original would be used.

Using CharBuffer . Unfortunately my knowledge of nio is quite limited, I do not know if I can set a good example. But the potential of CharBuffer over StringTokenizer is that it would not be necessary to create a new String for each line of text - one could simply adjust the position ( position ) and limit ( limit ) of buffer to designate the "current line", and use the buffer itself as CharSequence (ie as if it were a String ).

The example below worked on ideone , just do not guarantee - as I said - that is a good way to implement (assuming that texto is any CharSequence , including String , StringBuffer and StringBuilder ):

CharBuffer buffer = CharBuffer.wrap(texto);
int inicio = 0, fim = 0;
while ( fim < buffer.capacity() ) {
    if ( buffer.get(fim) == '\n' ) {
        buffer.position(inicio).limit(fim);
        // Usa-se buffer como se fosse uma String (i.e. a "próxima linha")
        ...
        buffer.position(0).limit(buffer.capacity());
        inicio = fim+1;
    }
    fim++;
}

If I'm not mistaken, the initial wrap creates a copy of the entire text, but once done that can be discarded and no additional object creation operation will be performed. Or better yet, have texto already start being a CharBuffer - for example by reading it from the input file directly in that format.