NullPointerException using stacks in RPN example in Java

Question

What I want the program to do: create a stack and manipulate it with the calculate method in HP12C class.

I'm kinda lost here becouse it's been a while since I used Java and I'm having troubles creating and using objects, if you could help me with that...

Since the last time I posted here people were acusing me of posting my homework for you to do, I don't wan't that. If you feel like what I'm asking is too much, just give me a tip on what I'm doing wrong, maybe I can go from there. Thanks.

public interface Stack {

    public void push(String element);

    public String pop();

    public int size();

    public String peek();
}


public class LinkedStack implements Stack {

    String elements[];
    int top;

    @Override
    public void push(String element) {
        top++;
        elements[top] = element;
    }

    @Override
    public String pop() {
        String element;
        element = elements[top];
        top--;
        return element;
    }

    @Override
    public int size() {
        return 0;
    }

    @Override
    public String peek() {
        return null;
    }

}



public class HP12C {
    public Stack stack;

    public HP12C() {
        stack = new LinkedStack();
    }

    public double calculate(String line) {
        String n, n1, n2;
        double d = 0, d1 = 0, d2 = 0;
        // Quebrar a linha nos parametros
        String elements[] = line.split(" ");

        // Empilhar e calcular
        for (String e : elements) {
            // Distinguir o que eh operador e o que nao eh
            switch (e) {
            case "+":
                n1 = stack.pop();
                d1 = Double.parseDouble(n1);
                n2 = stack.pop();
                d2 = Double.parseDouble(n2);
                d = d1 + d2;
                n = String.valueOf(d);
                stack.push(n);
                break;
            case "-":
                n1 = stack.pop();
                d1 = Double.parseDouble(n1);
                n2 = stack.pop();
                d2 = Double.parseDouble(n2);
                d = d1 - d2;
                n = String.valueOf(d);
                stack.push(n);
                break;
            case "*":
                n1 = stack.pop();
                d1 = Double.parseDouble(n1);
                n2 = stack.pop();
                d2 = Double.parseDouble(n2);
                d = d1 * d2;
                n = String.valueOf(d);
                stack.push(n);
                break;
            case "/":
                n1 = stack.pop();
                d1 = Double.parseDouble(n1);
                n2 = stack.pop();
                d2 = Double.parseDouble(n2);
                d = d1 / d2;
                n = String.valueOf(d);
                stack.push(n);
                break;
            default:
                stack.push(e);
                break;
            }
            stack.push(e);
        }

        return -1;
    }
}

import static org.junit.Assert.*;

import org.junit.Test;

public class HP12CTest {

    @Test
    public void testSimple() {
        HP12C hp = new HP12C();
        double result = hp.calculate("10 15 +");

        assertEquals(25, result, 0);
    }

}

trace of the error:

java.lang.NullPointerException
    at LinkedStack.push(LinkedStack.java:9)
    at HP12C.calculate(HP12C.java:55)
    at HP12CTest.testSimple(HP12CTest.java:10)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:678)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)

Possible duplicate of [What is a NullPointerException, and how do I fix it?](http://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) — Logan, May 07 '17 at 17:34

score 2 · Accepted Answer · edited May 23 '17 at 12:26

2

Within LinkedStack you have a field which is never initialised:

String elements[];

This is equivalent to

String elements[] = null;

When you try to access elements[top], you are doing so with a null reference, resulting in your exception.

See: How to initialize an array in Java?

Be aware that if you initialise your stack to, for example, 20 elements then you will need to handle the situation where a user of the class tries to push more than 20 elements to the stack.

You could do this by copying the underlying array and doubling the size or alternatively you could use an auto-resizing datastructure like one of Java's List implementations instead of a primitive array.

edited May 23 '17 at 12:26

Community

1
1

answered May 07 '17 at 17:46

Michael

41,989
11
82
128

How do I remove an element from the stack? like, I have {5, 10, +}, call calculate that results in 25, how do I replace the 3 elements for 25? edit: I don't actualy remove it from the vector? Just change the "top"? – Eduardo Vieira May 07 '17 at 18:09
@EduardoVieira Fix the problems you've identified in this question and post a new question with your updated code. :) – Michael May 07 '17 at 18:12

score 0 · Answer 2 · answered May 07 '17 at 17:49

0

You have to initialize your array:

String[] elements = new String[lengthOfYourArray];

answered May 07 '17 at 17:49

SF23

174
1
12

NullPointerException using stacks in RPN example in Java

2 Answers2