java.lang.NullPointerException in two related java class

Question

I implemented two java classes to solve the percolation problem but it throws the following exception

java.lang.NullPointerException
    at PercolationStats.<init>(PercolationStats.java:24)
    at PercolationStats.main(PercolationStats.java:54)

Program:

public class PercolationStats {
    int t=0;
    double[] sample_threshold;
    Percolation B;
    int N1;
    public PercolationStats(int N, int T) {
        t=T;
        N1 = N;
        int number_of_open=0;
        for(int i=0;i<T;i++) {
            B=new Percolation(N1);
            while(!B.percolates()) {
                double r1 = Math.random();
                int open_i = (int)(r1*N1);
                double r2 = Math.random();
                int open_j = (int)(r2*N1);
                B.open(open_i,open_j);
            }
            for(int k=0;k<N1;k++) {
            for(int j=0;j<N1;j++) {
                if(B.isOpen(k, j))
                    number_of_open++;
            }
            sample_threshold[i] = (number_of_open*1.0)/N1;
            }

        }
    }
    public double mean() {
        double sum = 0.0;
        for(int i=0;i<N1;i++) {
            sum += sample_threshold[i];
        }
        return sum/t;
    }
    public double stddev() {
        double sum = 0.0;
        double u = mean();
        for(int i=0;i<N1;i++) {
            sum += (sample_threshold[i]-u)*(sample_threshold[i]-u);
        }
        return sum/(t-1);
    }
    public double confidenceLo() {
        return mean()-((1.96*Math.sqrt(stddev()))/(Math.sqrt(t)));
    }
    public double confidenceHi() {
        return mean()+((1.96*Math.sqrt(stddev()))/(Math.sqrt(t)));
    }

    public static void main(String[] args) {
        int N = Integer.parseInt(args[0]);
        int T = Integer.parseInt(args[1]);
        PercolationStats C=new PercolationStats(N,T);
        double mean=C.mean();
        double stddev = C.stddev();
        double confidenceLo = C.confidenceLo();
        double confidenceHi = C.confidenceHi();
        System.out.println("mean                       = "+mean);
        System.out.println("stddev                     = "+stddev);
        System.out.println("95% confidence interval    = "+confidenceLo+", "+confidenceHi);
    }
}

What IDE do you use ? Activate line numbers and look at indicated line. — Christophe Roussy, Mar 11 '14 at 10:24
possible duplicate of [What is a Null Pointer Exception?](http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception) — Bernhard Barker, Mar 11 '14 at 10:32

score 6 · Answer 1 · answered Mar 11 '14 at 10:23

6

You never initialized double[] sample_threshold;. Hence it is null.

answered Mar 11 '14 at 10:23

Harmlezz

7,972
27
35

Doesn't java automated to initialized it to all 0.0? – luojiebin Mar 11 '14 at 10:25
2

Java doesn't know how big the array is. – Hayden Mar 11 '14 at 10:26
@luojiebin Refer to my answer, it shows the declaration (I can only assume the size is T) – nanofarad Mar 11 '14 at 10:27
Then how to fix it please?I am just a novice – luojiebin Mar 11 '14 at 10:27
@luojiebin If you have a question, please state it more precisely. What part of initializing the array do you need help with? My answer shows the line that is to be added, if you have a specific question address it there – nanofarad Mar 11 '14 at 10:29
I am sorry but you said you implemented that code and you need help with initializing array ? Both the answers clearly mentions that you need to initialize the array – Abdullah Shaikh Mar 11 '14 at 10:31

nanofarad · Accepted Answer · 2014-03-11T10:29:47.267

4

Java will indeed fill a double[] with 0.0 once it is initialized to a known size. You must initialize the array first:

public PercolationStats(int N, int T) {
    t=T;
    N1 = N;
    sample_threshold[i] = new double[T]; // add this line
    int number_of_open=0;
    for(int i=0;i<T;i++) {
        B=new Percolation(N1);
        while(!B.percolates()) {
            double r1 = Math.random();
            int open_i = (int)(r1*N1);
            double r2 = Math.random();
            int open_j = (int)(r2*N1);
            B.open(open_i,open_j);
        }
        for(int k=0;k<N1;k++) {
        for(int j=0;j<N1;j++) {
            if(B.isOpen(k, j))
                number_of_open++;
        }
        sample_threshold[i] = (number_of_open*1.0)/N1;
        }

    }
}

edited Mar 11 '14 at 10:29

answered Mar 11 '14 at 10:24

nanofarad

40,330
4
86
117

2 errors found: File: /home/luojiebin/introcs/hello/PercolationStats.java [line: 9] Error: cannot find symbol symbol: variable i location: class PercolationStats File: /home/luojiebin/introcs/hello/PercolationStats.java [line: 9] Error: incompatible types required: double found: double[] – luojiebin Mar 11 '14 at 10:33
I add this line sample_threshold = new double[3*T];and the problem solved.Thank you. – luojiebin Mar 11 '14 at 10:47

score 0 · Answer 3 · answered Mar 11 '14 at 10:55

here at 3rd line where you've written double[] sample_threshold; instead just write double[] sample_threshold= new double[5000]; meaning just initalize the array. Then when you use it in for loop java will only consider the arrays for the times your for loop loops.

java.lang.NullPointerException in two related java class

3 Answers3