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Java StreamTokenizer pushBack() Method with Example

StreamTokenizer Class pushBack() method: Here, we are going to learn about the pushBack() method of StreamTokenizer Class with its syntax and example.
Submitted by Preeti Jain, on April 21, 2020

StreamTokenizer Class pushBack() method

• pushBack() method is available in java.io package.
• pushBack() method is used to cause the next call of this StreamTokenizer to return the present value in the ttype field and not to update nval or sval field value.
• pushBack() method is a non-static method, it is accessible with the class object only and if we try to access the method with the class name then we will get an error.
• pushBack() method does not throw an exception at the time of pushing back.

Syntax:

    Public void pushBack();

Parameter(s):

• It does not accept any parameter.

Return value:

The return type of the method is void, it returns nothing.

Example:

// Java program to demonstrate the example 
// of void pushBack() method of StreamTokenizer

import java.io.*;

public class PushBack {
    public static void main(String[] args) {
        String str = "Hi, This is \n a mathematical expression :\n " +
            " 2 * 4 = 8 " + "8 + 5 = 13";

        try {
            // Instantiates FileOutputStream  and ObjectOutputStream 
            FileOutputStream fos_stm = new FileOutputStream("D:\\includehelp.txt");
            ObjectOutputStream obj_out_stm = new ObjectOutputStream(fos_stm);

            // By using writeUTF() method is to
            // write the given string in the file
            obj_out_stm.writeUTF(str);
            obj_out_stm.flush();

            // Instantiates FileOutputStream  and ObjectOutputStream 
            ObjectInputStream obj_in_stm = new ObjectInputStream(new FileInputStream("D:\\includehelp.txt"));

            // Instantiates StreamTokenizer and Reader
            Reader reader = new BufferedReader(new InputStreamReader(obj_in_stm));
            StreamTokenizer st = new StreamTokenizer(reader);

            // Here, we are considering initially 
            // file is not empty
            boolean end_of_file = false;
            do {
                // By using nextToken() method is to
                // parse the next token from the stream
                int token = st.nextToken();

                switch (token) {

                    case StreamTokenizer.TT_EOF:
                        System.out.println("End of File Found");
                        end_of_file = true;
                        break;

                    case StreamTokenizer.TT_EOL:
                        System.out.println("End of Line Found");
                        break;

                    case StreamTokenizer.TT_WORD:
                        System.out.println("word: " + st.sval);
                        break;

                    case StreamTokenizer.TT_NUMBER:
                        System.out.println("number: " + st.nval);
                        break;

                    default:
                        System.out.println((char) token + " Found.");
                }

                // By using pushBack() method is to
                // push back the last token
                st.pushBack();
                token = st.nextToken();

                switch (token) {
                    case StreamTokenizer.TT_EOF:
                        System.out.println("End of File Found");
                        end_of_file = true;
                        break;

                    case StreamTokenizer.TT_EOL:
                        System.out.println("End of Line Found");
                        break;

                    case StreamTokenizer.TT_WORD:
                        System.out.println("word: " + st.sval);
                        break;

                    case StreamTokenizer.TT_NUMBER:
                        System.out.println("number: " + st.nval);
                        break;

                    default:
                        System.out.println((char) token + " Found.");
                }
            } while (!end_of_file);
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
}

Output

@ Found.
@ Found.
word: Hi
word: Hi
, Found.
, Found.
word: This
word: This
word: is
word: is
word: a
word: a
word: mathematical
word: mathematical
word: expression
word: expression
: Found.
: Found.
number: 2.0
number: 2.0
* Found.
* Found.
number: 4.0
number: 4.0
= Found.
= Found.
number: 8.0
number: 8.0
number: 8.0
number: 8.0
+ Found.
+ Found.
number: 5.0
number: 5.0
= Found.
= Found.
number: 13.0
number: 13.0
End of File Found
End of File Found