Java StreamTokenizer for non-character input?

Question

I have a java binary search tree and I want to create a menu.

To this day I used StreamTokenizer to get the user input,

But now it doesn't seem to work with "+", "-", "?".

My code:

public void listen() throws IOException {
    boolean stay = true;
    System.out.println("Give me commands .. ");

    while(stay) {
        tokens.nextToken();
        if(tokens.sval.equals("+")) {
            tree.insert(new PositiveInt((int) tokens.nval));                
        } else if(tokens.sval.equals("?")) {
            System.out.println(
                    tree.retrieve(new PositiveInt((int) tokens.nval)) == null ? "Not exist" : "exist");
        } else if(tokens.sval.equals("-")) {
            tree.remove(new PositiveInt((int) tokens.nval));                
        } else if(tokens.sval.equalsIgnoreCase("K")) {
            tree.writeKeys();
        } else if(tokens.sval.equalsIgnoreCase("E")) {
            System.out.println("Empty = " + tree.isEmpty());                                
        } else if(tokens.sval.equalsIgnoreCase("F")) {
            System.out.println("Full = " + tree.isFull());              
        } else if(tokens.sval.equalsIgnoreCase("C")) {
            tree.clear();               
        } else if(tokens.sval.equalsIgnoreCase("P")) {
            tree.showStructure();               
        } else if(tokens.sval.equalsIgnoreCase("Q")) {
            stay = false;               
        } else {
            System.out.println("Unaccaptable input.");              
        }
    }
}

When I enter "P" , for example, or any other character, everything's alright.

When I enter "?", "+", "-", I'm getting:

Exception in thread "main" java.lang.NullPointerException
    at TestBSTree.listen(TestBSTree.java:27)
    at TestBSTree.main(TestBSTree.java:54)

As Line 27 is :

        if(tokens.sval.equals("+")) {

In other words, a non-charater is not accaptable with the tokenizer.

Why and how can I fix it?

Whole code:

import java.io.IOException;
import java.io.InputStreamReader;
import java.io.StreamTokenizer;


public class TestBSTree {
    // Test class variables
    BSTree<PositiveInt> tree;
    InputStreamReader reader;
    StreamTokenizer tokens;
    PositiveInt key;
    int in;

public TestBSTree(PositiveInt root) {
    tree = new BSTree<PositiveInt>(new BSTreeNode<>(root, null, null));
    reader = new InputStreamReader(System.in);
    tokens = new StreamTokenizer(reader);
    key = null;
}

public void listen() throws IOException {
    boolean stay = true;
    System.out.println("Give me commands .. ");

    while(stay) {
        tokens.nextToken();
        if(tokens.sval.equals("+")) {
            tree.insert(new PositiveInt((int) tokens.nval));                
        } else if(tokens.sval.equals("?")) {
            System.out.println(
                    tree.retrieve(new PositiveInt((int) tokens.nval)) == null ? "Not exist" : "exist");
        } else if(tokens.sval.equals("-")) {
            tree.remove(new PositiveInt((int) tokens.nval));                
        } else if(tokens.sval.equalsIgnoreCase("K")) {
            tree.writeKeys();
        } else if(tokens.sval.equalsIgnoreCase("E")) {
            System.out.println("Empty = " + tree.isEmpty());                                
        } else if(tokens.sval.equalsIgnoreCase("F")) {
            System.out.println("Full = " + tree.isFull());              
        } else if(tokens.sval.equalsIgnoreCase("C")) {
            tree.clear();               
        } else if(tokens.sval.equalsIgnoreCase("P")) {
            tree.showStructure();               
        } else if(tokens.sval.equalsIgnoreCase("Q")) {
            stay = false;               
        } else {
            System.out.println("Unaccaptable input.");              
        }
    }
}

public static void main(String[] args) throws IOException {
    TestBSTree test = new TestBSTree(new PositiveInt(0));
    test.listen();
}

}

It doesn't matter how does the tree or PositiveInt implemented, the main issue is the tokenizer.

@LutzHorn `if(tokens.sval.equals("+")) { ` , tokens.sval is the null value — Billie, Dec 17 '13 at 07:56

score 0 · Answer 1 · answered Dec 17 '13 at 09:30

if you want so split a string containg a '?' or a plus ('+'), you cannot simply use this symbol to split this String; they are reserved 'words' and need a exclude sign '\' which itself needs an exclusion sign ^^ (so you need two '\\' and the special sign)

try to use something like that:

StringTokenizer tokenizer = new StringTokenizer("ghj?klm", "\\?");
System.out.println(tokenizer.countTokens() );

-> result: the count is 2 !

you can also apply this method for String.split("\+");

//wont work!!
String str = "ghj?klm";
String[] s = str.split("?");
System.out.println(s.length );

but this code will work!

String str = "ghj?klm";
String[] s = str.split("\\?");
System.out.println(s.length );

it's the same 'problem' ^^ i hope this helped!

unfortunaltely i don't know which other symbols require a slahs... :-(

Java StreamTokenizer for non-character input?

1 Answers1